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Some early printed books are hard to OCR-process correctly and the text may contain errors, so one should always visually compare it with the ima- ges to determine what is correct. 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 C M FISHERY BOARD OF SWEDEN Institute of Marine Research, Report No. 1 PHYSICAL AND CHEMICAL OCEANOGRAPHY OF THE SKAGERRAK AND THE KATTEGAT I. Open Sea Conditions by ARTUR SVANSSON 1975 FISHERY BOARD OF SWEDEN Institute of Marine Research, Report No. 1 PHYSICAL AND CHEMICAL OCEANOGRAPHY OF THE SKAGERRAK AND THE KATTEGAT I. Open Sea Conditions by ARTUR SVANSSON Uddevalla 1975 Bohusläningens AB The vignette on the title-page represents Bronze Age fishermen; from a rock-carving at Ödsmål, parish of Kville, Bohuslän. Received for publication, October 1974 Contents 1. INTRODUCTION ................................................................... 5 2. BOUNDARIES. TOPOGRAPHY................................................................................. 7 2.1. Boundaries ....................................................................................................................... 7 2.2. Topography ................................................................................................................... 7 3. FRESH WATER SUPPLY............................................................................................ 9 4. POSITIONS OF SOME PERMANENT POINTS OF OBSERVATION................. 10 5. CURRENTS. WAVES .................................................................................................... 11 5.1. Current Measurements .................................................................................................... 11 5.2. Standing Waves. Characteristic Periods......................................................................... 11 5.3. Tides ........................................................................................ 12 5.4. Wind Currents and other Currents Generated by the Effects of Atmospheric Pressure 13 5.4.1. Wind Currents ............................................................................................................... 13 5.4.2. The Direct Effect of Atmospheric Pressure.................................................................. 14 5.4.3. Indirect Wind Effects .................................................................................................... 15 5.5. Permanent (Residual) Currents ..................................................................................... 16 5.5.1. The Water Exchange of the Baltic................................................................................. 17 5.5.2. The Skagerrak and the North Sea Proper .................................................................. 20 5.6. Surface Waves ............................................................................................................... 21 6. SALINITY ....................................................................................................................... 22 6.1. General .......................................................................................................................... 22 6.2. Long-Term Variations .................................................................................................... 23 7. TEMPERATURE ........................................................................................................... 24 7.1. General .......................................................................................................................... 24 7.2. The Upper Layers of the Skagerrak............................................................................. 24 7.3. The Deep Water of the Skagerrak................................................................................. 24 7.4. The Kattegat and the Belt Sea ..................................................................................... 25 7.5. Long-Term Variations .................................................................................................... 25 8. CHEMICAL PARAMETERS. PRIMARY PRODUCTION. OPTICAL CONDITIONS ............................................................................................................... 26 8.1. Oxygen.............................................................................................................................. 26 8.2. Phosphorus ......................................... 26 8.3. Pollution .......................................................................................................................... 27 8.4. Optical Conditions........................................................................................................... 28 8.5. Primary Production ........................................................................................................ 29 9. DECADE MEAN VALUES OF WATER PARAMETERS........................................ 30 10. SEDIMENTS ................................................................................................................... 31 3 11. FISHERIES HYDROGRAPHY ............................................................................. 32 11.1. Herring (Clupea harengus) ............................................................................................ 32 11.2. Sprat (Sprattus sprattus) ................................................................................................ 33 11.3. Deepwater Prawn (Pandahts borealis) .......................................................................... 33 11.4. Cod (Gadus morhua) 33 11.5. Mackerel (Scomber scombrus)........................................................................................ 34 11.6. Haddock (Melanogrammus aeglefinus) .......................................................................... 34 11.7. Other Fishes ................................................................................................................... 34 12. HEAVY METALS, ORGANOCHLORINE PESTICIDE RESIDUES AND PCBs IN FISH .......................................................................................................................... 35 12.1. Mercury .......................................................................................................................... 35 12.2. Cadmium and Lead ........................................................................................................ 36 12.3. Zinc and Copper ........................................................................................................... 36 12.4. Organochlorine Pesticide Residues and PCBs................................................................ 36 12.5. Comments on Geographical Differences ...................................................................... 36 13. Acknowledgements ........................................................................................................ 37 References .................................................................................................................................. 38 Tables ......................................................................................................................................... 45 Figures ...................................................................................................................................... 57 This compilation of the Hydrographical (Physical and Chemical) Conditions in the Skagerrak and the Kattegat covers mainly the open sea. A second volume on Coastal Conditions is planned but will probably be delayed for a time, as the preparatory work so far is rather scarce. Author’s address: Institute of Marine Research Hydrographic Department Box 4031 S-400 40 GÖTEBORG 4, SWEDEN 4 1. Introduction The first hydrographic investigation of the Skagerrak started by F. L. Ekman in 1868 and continued in 1869, was restricted to the coast of Bohuslän (Ekman, 1870). Even then at that time F. L. Ekman showed that the salinity of the Koster fjord was that of nearly unmixed ocean water. During the summer of 1872 an expedition was carried out in the North Sea by the German ship POMMERANIA (Meyer, 1875). In their big survey of the hydro­ graphy of the Skagerrak and the Kattegat, “Grunddragen av Skageracks och Kattegatts hydrografi”, Otto Pettersson and Gustaf Ekman (1891) regarded these results as very important together with those of the German ship DRACHE during the summers of 1882 and 1884 (Anon. 1886), because “in the hydrography of the North Sea we must find the explanation of the conditions that were found in the Skagerrak and the Kattegat” (Translation from Swedish, Pettersson and Ekman, 1891, referred to in the following as “Grunddragen”). During the summer of 1877 F. L. Ekman surveyed the Baltic, the Kattegat and the Skagerrak extensively. The results were edited by O. Pettersson after F. L. Ekman’s death (Ekman and Pettersson, 1893). In the winter of 1878—79 G. Ekman took measurements in those parts of the skerries of Bohuslän, where there was extensive herring fishery (G. Ekman, 1880). This started in the winter of 1877—78, when, for the first time since 1808, the winter herring invaded the coast and skerries of Bohus­ län. During the period 13—19 February 1890, five ships were sent out on expeditions into the Kattegat and the Skagerrak (“Grunddragen”). In 1897 O. Pettersson and G. Ekman published a further paper, emphasizing especially the connection between the hydrographic factors and the decline of the herring fishery off the coast of Bohuslän in the winter of 1896—97. This problem became more and more acute in the Swedish oceanographic investigations, which after the establishment of the International Council for the Exploration of the Sea in 1902 became part of the international cooperative work. The authors of “Grunddragen” are of the opinion that “on the bottom of the deep parts of the Skagerrak there is a mighty layer of water, which because of its salinity of a little more than 35 %0 must originate from the Atlantic Ocean. It does not have the same salinity 35.5 %0 as the surface water of the Atlantic Ocean around the Faroes and Shetlands during the summer, but is, in respect of salinity, more like the water which enters from the North Atlantic over the north plateau and the western edge of the Norwegian channel” (Translation from Swedish of p. 132 in “Grunddragen”). The last conclusion was made by the authors after their study of the sections measured by the R/V DRACHE 1884 (Fig. 1, stations Dr). The high salinity of 5 35.8 %o may be too high, but the relative picture is very informative (see also Eggvin, 1940). Current measurements carried out in June 1961 (Lj0en, 1962) confirm the old theory that water flows southwards in the outer part of the Norwegian channel (vicinity of D6, at 50, 100 and 145 m, while at 10m it is variable). “Grunddragen” gives an account of a winter expedition (February 1890), when the isohaline of 35 %0 was located deeper than it was in the summer of 1877. “Between winter and sum­ mer there must be an inflow of water that is more salty so that its mass increases” (Translation from Swedish of p. 133 in “Grunddragen”). This remark also refers to water of a salinity of between 34 and 35 %0 (here called 34—35 Water). We continue the quotation “Above the 34—35 Water we find the Bank Water, 32—34 %0 S. This water is found especially outside the west coast of Norway and on the Norwegian banks.” This type of water plays a very important role in the discussion of the herring fishery. In 1880 a large number of Danish and some Swedish lightvessels started daily observations of temperature and salinity but also of surface currents many times a day. Except for L/V Grisbådarna, which made observations during 1923—1928, and L/V Skagens Rev, the lightvessels were and are still not situated in the Skagerrak (see further Ch. 4). 6 2. Boundaries. Topography 2.1. Boundaries In this review of the physical and chemical conditions in the Skagerrak and the Kat­ tegat also the adjacent seas, the North Sea, the Belt Sea and the Baltic will be men­ tioned when deemed necessary. Oceanographic limits as they are defined by Wattenberg (1949) and others have been preferred. These differ sometimes from those approved by the 1952 Internatio­ nal Hydrographic Conference (Anon. 1953). The border between the North Sea and the Norwegian Sea is drawn along the latitude of 61° N from Norway to the Shetland Isles continuing to the British Main­ land (Anon. 1953). The southern border of the North Sea is preferably South Fore­ land—Cap Griz Nez in the Strait of Dover (Lee 1970). The border between the Ska­ gerrak and the North Sea (Anon. 1953) is a line drawn between Hanstholm (Den­ mark) and Lindesnes (vicinity of Mandai, Norway). In Fig. 2 this line approximately coincides with section 0:0. The border between the Skagerrak and the Kattegat (Wattenberg 1949) is a line drawn between Skagen and Marstrand (Fig. 2 approximately section 0:11). The border between the Kattegat and the Belt Sea (Wattenberg 1949) consists of two lines : Hassenspr ( SSE of Ebeltoft)—Sjaellands odde (section 1:2, 3) to that part of the Belt Sea which is called Sam so Baelt and Gilleleje—Kullen (section 2:0) to that part of the Belt Sea, which is named Öresund. The border between the Baltic and the Belt Sea (Wattenberg 1949) also consists of two lines: a) Gedser Rev—Darsser Ort (passing the Darsser Sill (section 4:7, 8)) to that part of the Belt Sea which is named the Bay of Mecklenburg and b) DragOr— Saltholm—Limhamn to the Öresund (approximately section 2: 6). Note, however, that Anon. (1953) puts the southern boundary line of Öresund along Stevns Klint— Falsterbo, section 2: 8. This line is always used in conventions of fishing and pollu­ tion. Sometimes we speak only of the North Sea and the Baltic. The Skagerrak is then in­ cluded in the North Sea and the Kattegat in the Baltic. The border is a line drawn between Skagen and Marstrand. The Convention on the protection of the marine en­ vironment of the Baltic Sea Area defines this boundary to be the parallel of the Skaw at 57° 44' 8" N. 2.2. Topography The North Sea is usually described as a shallow sea with a mean depth of 94 m. Along the coast of Norway there is, however, a deep trench, the Norwegian Trench 7 (the German name Rinne is often used by British fishermen: the Norwegian Rinne) with a maximum depth of 700 m in the Skagerrak. The sill depth of this “Skagerrak Deep” is 270 m and is situated off Utsira in Norway (approximately N 59°20')- From the Norwegian Trench in the Skagerrak a narrow trench (named the Deep Trench) penetrates down into the Kattegat along the Swedish coast. The depth decreases from approximately 100 m in the North to about 75 m SW Vinga but increases again to approximately 100 m in isolated deeps down to Anholt. Generally speaking, however, the Kattegat is very shallow with a mean depth of 23 m. The sill depth between the Baltic and the North Sea is situated at Darsser Sill (D. Schwelle, see above) and is 18 m. The sill depth does not increase above ap­ proximately 23 m in the Belt Sea and the southern Kattegat until we come to the An- holt area but also in the Belt Sea there are isolated deeps of up to 80 m in depths. At the border line between Öresund and the Baltic the sill depth is only 8 m. The maximum depth (50 m) of the Öresund is off Landskrona. Table 1 presents volumes, areas and mean depths of the areas concerned. The values for the Baltic have been slightly revised by Dahlin (1973) and Ehlin, Mattisson and Zachrisson (1974). A very thorough study of the late quaternary history was made by Mörner (1969). This work contains among other things, a very detailed geological map of the (present) seabed of the Kattegat. See also Flodén (1973). 8 3. Fresh Water Supply While relatively much is known about the fresh water supply to the Baltic (Brogmus 1952, Mikulski 1970, 1972), we do not have these figures for the Kattegat and the Skagerrak published in summary form. Here an attempt will be made to present some rough figures. Whereas Swedish and Norwegian data of river discharge are published, this is not the case with Danish discharges. Instead such figures are roughly computed from net precipitation figures published by Anon. (1971). The discharge of Danish rivers to the Kattegat may be subdivided into 3 parts: a) from the island of Sjaelland with a catchment area of 2460 km2. With a net precipi­ tation figure of 180 mm/year we arrive at 14 m 3/s. b) from Jylland, except Limfjord, a catchment area of 5815 km2. With a net precipi­ tation figure of 300 mm/year we get 55 m3/s. c) from the Limfjord. There is a discharge from Jylland to the Limfjord corres­ ponding to a catchment area of 7200 km2. With a net precipitation figure of 350 mm/year we arrive at a discharge of 80 m3/s. Assuming that most of this fresh water is drained to the North Sea, we take 30 m3/s as a very rough discharge figure from this area to the Kattegat. In this way we have a total discharge of 99 m3/s from the Danish rivers discharging to the Kattegat. Adding the Swedish river contribution (Table 2) we arrive at 885 m3/s to the Kattegat. The total catchment area is 81,115 km2. The volume water discharged by Swedish rivers to the Skagerrak is small, 45 m3/s (Table 2). The figure is taken from Melin (1955) and represents the period 1909—50. The water discharged by Danish rivers to the Skagerrak is still smaller. We assume the catchment area to be 1000 km2 and the net precipitation to be 300 mm/year. We then get 10 m3/s. The Norwegian figures were taken from Anon. (1958). They represent the period 1911—1950. Tollan (pers. comm.) is of the opinion that the figures also represent the period 1931—1960. Tollan gives a total discharge of 2190 m3/s to the Skagerrak from Norwegian rivers 1931—1960, and it seems permissible to fill up the difference by a post “others” of 249 m3/s in Table 2. Adding all contributions from the three countries we get a total discharge of 2245 m3/s to the Skagerrak. 9 4. Positions of Some Permanent Points of Observation The Kattegat and Belt Sea area is characterized by a very great density of observing lightvessels from which measurements have been carried out daily, with regard to currents even several times a day. Fig. 2 gives the positions of the points of observa­ tions. Svansson (1971) contains information as to where and how the data is stored. This type of observation platform, however, hardly exists in the Skagerrak. Hence the research vessels are of special importance in this area. As part of the international investigation directed by the International Council for the Exploration of the Sea, Sweden undertook measurements in the Eastern part of the Skagerrak (Fig. 1, stations S) and Germany in the Western part (stations D) during the years 1902—1914 in February, May, August and November. Mean values were computed and presented as sections by Kobe (1934). Whereas there are very few measurements in the open Skagerrak in the period 1915—1946, both Norwegian and Swedish research vessels started work in this area in 1947, the Norwegian on a section Arendal—Hirtshals, which is still under survey, the Swedish on section M between Arendal and Skagen (1947—1960), section Å per­ pendicular to the Swedish coast of Smögen (from 1962) and section P between Mar- strand and Skagen (from 1947). Ten year means of temperature, salinity and che­ mical parameters at the Å section are presented in Ch. 9. Swedish measurements of chemical parameters in the Kattegat started 1965 at 4 positions: Fladen and Kullen (see Table 15), Lilla Middelgrund (N 56° 57.5' E 11° 45.5') and Stora Middelgrund (N 56° 34', E 12° 13'). They are usually made from research vessels 4 times a year but at Fladen and some coastal positions since 1971 measurements are being made on a monthly basis by the Swedish Coast Guard. Recently (1974) a Danish 5 year project started to investigate the transports of water and material through the Belt Sea and the Kattegat. At the same time the Fishery Board of Sweden commenced to survey 10 stations at a section Frederiks- havn—Göteborg twice a month. Measurements with automatically recording instru­ ments is an integral part of the projects. 10 5. Currents. Waves In the following account the least important currents, those generated by the tides, will be dealt with first, then follow wind currents and, lastly, the permanent currents. This division has been made because stratification has little effect on the tidal cur­ rents, somewhat more on the wind currents, but determines the permanent currents. 5.1. Current Measurements The lightvessel observations mentioned above comprise mostly surface currents de­ termined up to 8 times a day with a current cross. On Swedish lightvessels measure­ ments were also made at a depth near the bottom, but the method was probably not quite reliable. On Danish lightvessels measurements at many depths have been carried out on some occasions with J. P. Jacobsen’s level-current-meter, see Table 4. Measurements with automatically recording current-meters have been carried out on different occasions since 1911, see Table 3. Scattered observations with Ekman’s current meter or with drifting parachutes will be referred to below. 5.2. Standing Waves. Characteristic Periods Like, for example, organ pipes sea basins have their characteristic periods. In a closed channel of length 1 and depth h this period is T = 21/1/gh (frictionless conditions), where g is the acceleration of gravity and ]/gh the velocity of long barotropic waves. (Barotropic means conditions, when stratification is neglected.) If the channel is closed only at one end, the period is double that magnitude. These are the lowest modes of oscillation with one nodal line but there may also be higher modes with two, three or more nodal lines (overtones). Tomczak (1968) gives a period of 5 hours for the semienclosed Skagerrak. Koltermann (1968) worked with a barotropic x-y model North Sea—Skagerrak — Kattegat with open boundaries (with sea level varia­ tions) toward the Norwegian Sea, the Channel and the Baltic. He found three impor­ tant characteristic periods: 21.8, 10.7 and 4.3 hours (friction included). Svansson (1972) found a period of 11 days in a multichannelled system (friction excluded) Baltic—Skagerrak. Further modes for this system were 1.65 days (nodal line in the southern Gulf of Bothnia), 1.25 days (nodal lines in the Gulf of Bothnia and the northern Baltic proper) and 0.99 days (nodal lines in southern Öresund, Darsser Sill region, northern Baltic proper and middle Gulf of Bothnia). As friction may be decisive, these figures can only be used as guidelines. The peak of 5 days found by Magaard and Krauss (1966) in frequency analyses of the Baltic sea level data, 11 may be a nonlinear characteristic period. In stratified seas there are also internal (baroclinie) waves. These travel nearly 2 orders of magnitude slower than barotropic waves (1:50 is an often used ratio). Whereas barotropic waves in enclosed seas are fast enough to hit the boundary coast, be reflected and together with the original wave form standing waves (barotropic seiches, see above), the internal waves need much more time before they can build up corresponding features. But in the open sea there may also exist so-called Poin­ caré waves; they consist of a cellular pattern of standing waves with near-inertial period. This period depends upon the Coriolis parameter, which means that it is latitude dependent (latitude 30° : inertial period = 24 hours, 55° : 14.7 h„ 60° : 13.9 h., 65° : 13.3 h., 90° : 12 h.). Mortimer (1967) considered the internal wave pattern in the Great Lakes to consist of nearshore Kelvin waves (Cf. Ch. 5.3.) but in open sea of standing Poincaré waves. Kullenberg (1935) in his investigation of internal waves in the Kattegat, also found periods which he supposed to have the inertial period (see also 5.3 Tides). Also in the Skagerrak inertial periods were found (Tomczak 1968). 5.3. Tides In the areas concerned, the local tide generated by the gravity of the sun and the moon can be neglected. Only the tidal waves originating from the ocean have to be considered. The tidal variations are usually looked upon as composed of several (harmonic) tidal components, each with its period. These components have in any given place the very same (but local) sinusoidal tidal variation, determined by a cer­ tain amplitude (= half of the range between ebb and flow) and a certain phase (can be expressed as the delay in hours of the high water after the meridional passage of the corresponding period “moon”). In the North Sea, Skagerrak, Kattegat and the Belt Sea the most important components are semidiurnal: M2, 12.42 hours, S2, 12.00 hours, N2, 12.66 hours and p2, 12.87 hours, whereas diurnal components, e.g. Ol and K1 are smaller. During full moon and new moon (the syzygies) the contributions of M2 and S2 are added at the most, we have spring tides. Fig. 154 A (Fig. 3) in Defant (1961) shows lines connecting places with the same interval in hours after the upper culmination of the moon in Greenwich and the high water for the North Sea and the western Skagerrak. In Fig. 154 B (Fig. 4) in the same work one can see that, while sea level differences (not amplitudes) of more than 4 meters are common along the east coast of Great Britain, the corresponding figures at the mouth of Skagerrak are less than 25 cm. The reason for this asymmetry, as assumed by Defant, is that the tidal wave entering the North Sea from the north, which, on account of the rotation of the earth, has a larger amplitude “to the right” along the coast of Great Britain (Kelvin wave), looses a great deal of energy in the shallow southern part of the North Sea. The reflection of the wave, which should make the picture symmetrical in case of no friction, would thereby be weak. Fur­ thermore it is assumed that a small portion of the incoming wave goes directly 12 through the Skagerrak to the Baltic and is lost there. For the tide in the area dealt with in this paper, Fig. 5 has been composed with the help of Defant (1934 and 1961). A few smaller changes have been made from Svansson (1962); a more re­ liable figure has been obtained for Smögen after the Tidal Institute in Liverpool (present Institute of Oceanographic Sciences) processed data from one year (1959). Also some corrections have been made for Bornö and Göteborg. In spite of the fact that in later years a fair number of current measurements have been carried out in the Skagerrak no analyses of the tides have been made of the material, the reason of course being that no sizeable figures are to be expected. The few cm/s which appeared in the material from 1913 (Table 3) indicates the small order. In the spectral-analyses which have been performed (Tomczak 1968) the semidiurnal tide seems weak and sometimes difficult to distinguish from the inertial periods. The M2-tide in the Kattegat has been summarized best by Defant (1934). One gets the impression of a wave entering from the north and loosing all its energy in the Danish Straits, so that it neither enters the Baltic nor is reflected. It is largely the deflecting force of the rotation of the earth (the Coriolis force) that makes the ampli­ tude larger on the Danish side than on the Swedish (Kelvin waves). See Fig. 6 for an explanation where the Coriolis effect has been computed for a current of 10 cm/s. As appears in Svansson (1962) this is the correct order of magnitude according to the results of processing of measurements from L/V Anholt Knob and other sources (Jacobsen 1913). In later years also measurements from L/V Läsö Rende (Table 4) have been processed (Rossiter 1968). Tidal currents are reasonably uni­ form at all the horizons, 2.5 m, 5, 10, 15 and 20 m. M2-amplitudes were 20, 23, 26, 26 and 23 cm/s respectively. It is to observe that L/V Läsö Rende was situated in a narrow passage, and that bottom depth was 22 m. The to and fro movements in two opposite directions is a simplification. In reality the current rotates, usually clockwise. The current vector end points describe an ellipse with its major axis in the longitudinal direction and its minor axis, in the Kat­ tegat 3—4 times shorter, in the transversal direction. Internal waves with, among other things tidal periods appear in the halocline. Kullenberg (1935) found in his investigations with a submerged float in the Fladen area that the halocline could attain an amplitude of 1.3 m at springtide. Internal tidal waves may disturb the vertical distribution of tidal currents (Schott 1971). 5,4. Wind Currents and other Currents Generated by the Effects of Atmospheric Pressure 5.4.1. Wind Currents When a wind blows over a sea surface it exerts on it a drag, the wind stress, which causes the water to move. According to theory the surface current is deviated to the right of the wind. The angle of deflection increases regularly with depth, so that at 13 a depth D, the so-called Ekman Depth, the current is directed opposite to the surface current. The velocity decreases regularly with increasing depth and is at depth D only one twenty-third of the value at the surface. The depth D depends upon the value of the vertical eddy viscosity coefficient Kvz. In relatively homogeneous water Kvz is of the order 0.1 m2/s with a corresponding Ekman Depth of 125 m. But in the Kattegat Kvz is probably much smaller. Jacobsen (1913) computed Kvz by means of tidal observations at various depths on board Danish lightvessels and obtained figures between 0.00003 and 0.0011 nr/s. Defant (1934), however, showed that similar measurements made during a few weeks 1931, were in accordance with a theoretical approach with Kyz = 0.01 m2/s. The vertical eddy viscosity coefficient is probably not a constant. Near bottom smaller values are expected (Rodhe 1973), near sea surface the coefficient is probably a function of the wind stress. G. Kullenberg (1971) has shown that the vertical eddy diffusion coefficient K,iZ usually is very low in upper layers of the Kattegat and that it is a function of stratification, wind stress and the absolute value of the velocity gradient. There seems further to be a relation between Kdz and Kvz. The surface current is often supposed to have a velocity of 1 % of the velocity of the wind. This factor has usually been obtained when measuring the surface current with a current cross, 0.5 m high. For more shallow objects the factor is greater. Olsson (1968) used 3 % for drift bottles. Theoretically the velocity of the surface current is inversely proportional to the magnitude D of the Ekman Depth. As D may be smaller in the Kattegat the surface current may accordingly be larger than usually assumed. In Gustafson och Otterstedt (1931) there is an interesting theoretical attempt to account for the way, in which the spreading downward of a drift-current is modified, when Kvz is no longer constant but suffers a diminuation in the vicinity of a discontinuity surface. 5.4.2. The Direct Effect of Atmospheric Pressure In the open ocean the adjustment to atmospheric pressure is usually very rapid, the long wave velocity being much higher than the velocity of low and high pressures. In this case sea levels adjust to normal atmospheric pressures implying that statically a change of the atmospheric pressure of one millibar is giving a change of the sea level of 1 cm. In the vicinity of coasts and in bays with large characteristic periods this is no longer true. If the sea levels at Smögen are compared with the atmospheric pressures a re­ markably good correlation is obtained (Fig. 7). Furthermore it is clear that in this area a change of the atmospheric pressure of 1 mb brings about a change of the sea level of 2 cm rather than the 1 cm expected. A low atmospheric pressure is usually related to westerly winds, which raise the sea level in the North Sea and the Skagerrak. In Mandai the static response is more correct according to theory (Svansson and Szaron 1975). That the correlation between atmospheric pressures and levels of the Baltic is very low is quite evident due to the long characteristic period. 14 Finally we note that the variations of atmospheric pressure hardly ever exist with­ out winds. Witting (1918) introduced the concept of an anemo—baric effect ex­ pressing the two-fold influence of atmospheric pressure. 5.4.3. Indirect Wind Effects The presence of coasts creates indirect wind currents, sea level currents. For the sake of simplicity, let us first consider the effects of a wind blowing longitudinally over a narrow lake with a pycnocline (discontinuity of density). The surface layer is brought to the downwind end of the lake, where the water level rises sufficiently to create an excess pressure which will force the water back, mainly below the pyc­ nocline. After initial oscillations (seiches) steady state conditions prevail and practi­ cally the same amount of water is transported back (Fig. 8, Stage 1). Gradually (but slowly) also the pycnocline starts to incline and at (a second) steady state condition the inclination is just large enough for the current to cease below the pycnocline and then also the transport back must take place above the pycnocline (Fig. 8, Stage 2). For our waters these latter effects can be assumed to be unusual, since the winds hardly ever display the constancy required, close to a coast, however, changes in the stratification can occur fairly rapidly. The Kattegat and the Belt Sea are far more influenced by winds and changes in the atmospheric pressure over the North Sea and the Baltic than by the direct effects of the corresponding local forces. This has been shown by Dietrich (1951) and others, whose maps of the currents'in the Kattegat under different wind conditions are re­ produced here as Figs 9—12. It is evident that tidal waves entering the Belt Sea are practically extinguished in the straits, but longer waves, which are probably damped less (Lamb 1953), may enter the Baltic more easily. Large long-term oscillations (order of magnitude 14 days) cause the Kattegat water to be drawn alternately into the Baltic or out into the Ska­ gerrak, which creates large salinity variations in the Kattegat and the Belt Sea and also along the coast of Bohuslän (Fig. 16) due to the large horizontal transports (see also below under sections “Salinity”). Fig. 13 shows daily means of currents measured 1967 partly SW Hållö at a depth of 50 m (see Table 3) partly at L/V Halsskov Rev. Oscillations of 5-day type are apparently dominant. There is also a clear negative correlation between the two series. Fig, 14 shows the daily means of the N- and E-components of the German current meter records during the cooperation 1966 at two stations, one at the entrance into the Skagerrak of the Jutland Current (Stn. 41) and the other on the border be­ tween the Skagerrak and the Kattegat (44). The figure also shows measurements of currents from two Danish lightvessels. The similarity between the E-components of Stn. 44, at 40 m, and of L/V Skagens Rev, at the surface, is quite evident. There seems, however, to be hardly any similarity between the record of Stn. 41 and the remaining records. The data period is short but the comparison gives some support to the idea, that the strong variations on the border between the Skagerrak and the 15 Kattegat are caused mainly by the Baltic oscillations and not by Jutland Current variations. Remembering Fig. 13 we now see that the phase seems to be the same for the current (N-comp.) at 50 m depth off Smögen and for L/V Skagens Rev (E-comp.). An explanation may be this: when the sea level of the Baltic sinks and the water trans­ port is outwards, the Jutland Current is forced to take another direction. The E-com- ponent at L/V Skagens Rev and the N-component at Hållö are both weakened. When on the other hand the transport is flowing inwards to increase the level of the Baltic, the conditions may be “normal” with the Jutland Current flowing eastward at L/V Skagens Rev and northward at Hållö. Another interesting relation is seen in Fig. 15. In connection with the water trans­ ports created by changes of wind and atmospheric pressure, the sea level of the entire Baltic oscillates. In this process Kattegat water is drawn alternately into the Baltic or out into the Skagerrak, which creates large salinity variations in the Kattegat and the Belt Sea and also along the coast of Bohuslän (Fig. 16) due to large horizontal transports (discussed further in Ch. 6). Kelvin waves have been mentioned earlier. By this we mean a long wave influenced by the deflecting Coriolis force but nevertheless without transversal velocities. The width of the barotropic (no influence from stratification) Kelvin wave is of the order of some hundreds of kilometers. The tidal wave in the Kattegat seems to be a good example of this category. It travels in a longitudinal direction with the velocity of a barotropic long wave (see Fig. 6). Due to their much lower speed internal Kelvin waves should be restricted to a much narrower strip along the coast. Actually the strip is of the order 5 km or again 1/50 (5.2) of the width a barotropic Kelvin wave (Walin 1972). At those coasts, how­ ever, which in relation to the wind have a favourable direction for (Ekman) upwelling this may be so strong that there are no longer two layers of water but only upwelled deep water. A few words about upwelling. When the wind blows in some relation to a coast, the winddriven transport will cause sea level differences perpendicular to the coast. These differences will give rise to a gradient current along the coast. In a bottom friction layer there is a compensation for the winddriven transport perpendicular to the coast. If the depth is large in relation to the Ekman depth (see 5.4.1.) the most effective wind is an alongshore one. For the Swedish West coast a northerly wind would give most upwelling. As the Ekman depth is probably rather small this would also hold true in nature. Unfortunately it is difficult to distinguish separate local up­ welling effects from large scale effects of the Baltic. 5.5. Permanent (Residual) Currents Actually nothing is more permanent than the tide, which with great punctuality surges back and forth. Tidal currents are not usually, however, referred to as permanent currents. The Gulf Stream is a typical permanent current even though it varies (at 16 least) with the season. If, on the other hand, the period of the variations is less than a few weeks the current should not be considered permanent. The tide enters the Skagerrak and the Kattegat like in a narrow channel without large variations across the channel. Also the tide has the same direction from surface to bottom. This is, however, not always the case with the permanent currents, which consist mainly of the Baltic Current (influenced by the water exchange of the Baltic) in all of this region and the offshoot of the permanent current system of the North Sea into the Skagerrak. 5.5.1. The Water Exchange of the Baltic In many respects the Kattegat and the Belt Sea can be regarded as a big river mouth, where usually there is a tongue of saline bottom water, which is pressed towards the sea by an increase of the runoff. The problem of the water exchange is rather complicated and it is not astonishing that there is more than one approach to the problem. First are presented the classical ideas of Martin Knudsen (1899 and 1900). It is assumed that in the strait between the ocean and an enclosed sea filled with brackish water there are two layers, a top one consisting of outflowing brackish water (Qi m3/s) and a bottom one (Qg m3/s) of much higher salinity flowing inwards (Fig. 17). It is furthermore assumed that at a certain section we can distinguish be­ tween the two regimes and also determine their respective salinities. Finally assuming the salt transport to be zero we obtain the Knudsen relations Oi Sg Sr -S, • Qo where Q0 is the fresh water supply. Knudsen applied the formulae at many sections. Most interesting is the Darsser Sill section at the smallest depth (the sill depth) between the Baltic and the ocean. For the period 1877—1897 Knudsen found in the literature 19 measurements of the salinity at 19 m depth. Of these he kept 13 values disregarding all salinities below 15.5 %0 because “these salinities cannot renew the deep water of the Baltic”. So for S2 he got 17.4 %0 and without going much into detail Si was taken as 8.7 %0. Thereby the compensating inflowing current would be of the same magnitude as the fresh water supply Qo- The two salinities 8.7 %0 and 17.4 %0 are thereafter found in the literature over and over again, e.g. in Schultz (1930) and Brogmus (1952), as well as in a paper by Kullenberg (1967). Kullenberg computes a factor by which the annual supply into the Baltic of a (conservative) pollutant is to be multiplied. Kullenberg found this factor to be 35.1 and it means that if e.g. 10,000 tons a year are supplied, in a steady state there will be an accumulation of 351,000 tons. We derive this factor by 2 — Physical and chemical 17 dividing the volume of the Baltic (Vb) by that part of the outflow which does not re-enter. The Kattegat water (salinity Sk) consists of y parts of Baltic water (salinity SB) and (1-y) parts of ocean (Skagerrak) water (salinity S). When Q2 km3 of the Kat­ tegat water flows back into the Baltic, y parts of it are therefore of Baltic origin and it is only Qi — y Q2 that really leaves the Baltic ultimately. Our factor f is derived from f = VB Qi—y Q2 Qi = Sk Sk—SB • Qo Qa- SB Sk —Sb Qo Sk — (1-y) S + y SB With these assumptions the factor f turns out to be independant of the conditions in the Kattegat: Y*. (i_5l) Qo s ; If we use the value of Q0, from Table 2 a, 439 km8/year, VB = 20,920 km3 and S = 34.8 %0 we derive at f = 47.7 — 1.37 SB Knudsen’s relation was applied to the northern part of the Kattegat by Schulz (1930). He found Qi = 5.3 Qo and f = 0.2 for the Kattegat. It is quite clear that there are difficulties to find the right salinities to enter into the Knudsen equations. Furthermore there seem to be a few cases when there are cur­ rents in the opposite directions on top of the other. Table 4 shows mean values of Danish current measurements determined at non­ surface horizons. (The mean values for the surface measurements are presented in Table 5). While the data of L/V:s Läsö Rende and Lappegrund clearly reveal out­ going (in the surface layer) and ingoing (in the deep) currents, the outgoing currents are remarkably weak at L/V Anholt Knob and L/V Schultz’s Grund. L/V Anholt Knob is often assumed to be situated in some kind of “countercurrent” in Kattegat (Dietrich 1951, Svansson 1968); the data from L/V Schultz’s Grund is more diffi­ cult to interpret. Stommel and Farmer (1953) and, in a slightly different manner, Kullenberg (1955) derived a relation between the transports Qi and Q2 as functions of the fresh water supply Qo for an estuary assumed to contain well mixed water. The solution of the problem is such that Q2 as function of Q0 increases from zero (Qo = 0) up to a maximum, thereafter decreases steadily to zero for Qo = Qo max.» when the fresh water fills the sill area completely. Svansson (1972) presented arguments that the Baltic 18 may be assumed to be well-mixed in this respect with a maximum Q2 at Q0 ~ 30 km3/month and Qmax. ~ 100 km3/month. These figures are, however, very uncertain and must be checked. By means of his investigations Jacobsen (1925) established a formula, by which the water transport (total from surface to bottom) can be computed if only the magni­ tude of the surface current at L/V Drogden is known. Wyrtki (1954) kept Jacob­ sen’s formula for the Öresund Ms = 1.5 X VD where Ms is the net transport in km3/month and Vd is the magnitude (with its sign) in cm/s at L/V Drogden, but used for the Belts Mb = 4.1 X VH where Vh is the value of the surface current at L/V Halsskov Rev and MB is the net transport through the Belts in km3/month. By means of such formulae Hermann (1967) derived a northgoing transport through the Öresund of 460 km3/year and a southgoing one of 350 km3/year. Note that we are no longer talking of a 2-layer system; the transport is in every case either outgoing or incoming. Soskin (1963) improved the formulae of Jacobsen and Wyrtki mostly by using one formula for incoming transports and another for outgoing transports. To con­ struct his formulae Soskin also used data for the period 1921—1931, possibly even 1898—1912 for which periods figures for the fresh water supply are available. Then he computed the transports for every year 1898—1944. The difference between out­ going and incoming transports is called water exchange. The fluctuations are really large; one asks if it is possible that some years there is no net outflow at all. It seems to be quite clear that two various types of atmospheric circulation, zonal (with wester­ ly winds) with large amounts of precipitation and meridional with smaller amounts of precipitation are most responsible for the variations. As mentioned above we have data of the total fresh water supply only for short periods but we can study the out­ flow from some large river like Soskin and others have done, see Fig. 18, where the runoff data of the river Vuoksi in Finland are included. Even if the variations are large the river transport hardly goes down to zero. Table 6 shows the various com­ ponents of the water exchange. Lisitzin (1967) used the day-to-day variations in sea level to claim that the average water quantity involved every year in the renewal of the Baltic is Qi — 1754 km3, Q2 would then be 1315 km3 The present author has tried a method of computing the steady state concentra­ tions of an outlet by using the following rough model. The area is subdivided into many boxes, each one extending from surface to bottom. The mean salinities in each box were computed from mean values of Anon. (1933) for the Danish light-vessels and Granquist (1938) for the Northern Baltic, while the remaining salinities were interpolated. Assuming the salt transports to be zero “compensation transports” were calculated for each section. Then these “transports” were used to compute the steady state concentrations of 10,000 tons of a substance released every year 1) in the middle of the area (Fig. 19) and 2) in the Danish sounds (Fig. 20). The substance is, of course, 19 assumed to behave like salinity without going into any biological cycle or sediment. Note that one of the ideas is to proceed out to clean water. 5.5.2. The Skagerrak and the North Sea Proper It seems quite probable that all the inflows of water to the North Sea unite in the Skagerrak and leave the area along the Norwegian coast. Fig. 21 shows the probable surface currents: on the Danish side the incoming Jutland Current, from the Kattegat the Baltic Current and along the coasts of Sweden and Nor­ way the two currents united. At non-surface horizons we know much less but some measurements were made, see Table 3. Furthermore, measurements have been carried out from anchored research vessels. The data show that the currents are usually run­ ning in the same direction from surface to bottom (Helland-Hansen 1907, Svansson 1961, Anon. 1969). Therefore it is less advisable to use the method of “layer of no motion” to compute geostrophic currents out of data of temperature and salinity as Kobe (1934) and Tomczak (1968) did. Svansson and Lybeck (1962) tried to com­ pute the geostrophic transport by referring to measured surface currents in calm weather. They got a transport of approximately 500,000 m3/s (equiv. to 16,000 km3/ year) for both ingoing and outgoing currents. Table 7 shows the mean values during 16 days of German current measurements during the cooperation in 1966 at the section Hanstholm—Mandai (See Fig. 2). Fig. 22 shows the daily mean of July 9 on the same occasion. It is evident from this figure, as well as from the salinity maps in the Atlas of the cooperation 1966 (Anon. 1970), that a great part of the water circulating in the Skagerrak comes from the Norwegian Sea along the 150—200 m isobaths, but in the surface layers there is probably also a transport from, for example, the Southern North Sea. Table 8 shows some kinds of N-component for the L/V Horns Rev (N 55° 34.F E 07° 10.5') as presented by Jacobsen (1913). These figures are im­ portant to consider when we discuss the possible influence of the heavy pollution in the SE corner of the North Sea on the water discussed here (Cf. Ch. 10.3). Kautsky (1973) presents distributions of Cs 137 (See Ch. 8.3.) which are in accordance with a transport from the Strait of Dover to the Skagerrak. There is possibly a closed horizontal circulation in the Skagerrak (e.g. Lindquist 1970). In Engström (1967) there are indications of such closed paths of surface drifters. We do not know, however, whether ordinary horizontal eddy diffusion may be an agency strong enough to transport objects from one strong current to the other. The criticism presented above of Kobe’s (1934) geostrophic computations may be less relevant along the coast of Norway where, contrary to conditions in the Jutland current, bottom friction probably plays a much smaller role and a geostrophic layer of no motion at some great depth may be plausible. A recent calculation of geo­ strophic transports between stations M7 and M8 (Position in Fig. 1) for 13 cases 1948—1959, confirmed the seasonal variations disclosed by Kobe: maximum in November (500 000 m3/s) and minimum in February (200 000 m3/s). The M7— M 8 transports were higher respectively lower than these one’s. 20 In order to further study seasonal variations Tables 8 and 9 are investigated. Table 8 shows mean surface currents measured at L/V Horns Rev, SW of Denmark in the North Sea. One may think that annual variations at this position reflect the same variations that are met with along Norway, i.e. of the North Atlantic current. Whereas a winter maximum is found, the minimum occurs in Summer. — Table 9 shows the monthly mean values of the N-component for 5 months in 1967 of data from the current meter SW of Smögen. There is a summer minimum and a highest value in October. — Finally reference is made to Jacobsen (1925) and Svansson (1965) concerning deep currents (20 m) measured at L/V Schultz’s Grand 1910— 1916. Again is found a winter maximum and a summer minimum. If the variations at L/V Horns Rev are disregarded we may make the following interpretation. The current along Norway is nearly independant of the Baltic outflow, whereas the Kattegat deep current, and the currents in the SE corner of the Skager­ rak (Ch. 5.4.3.) are influenced both by the Baltic outflow and the Atlantic inflow. The surface currents in the Kattegat are usually at their maximum in Spring and at minimum in late Summer (Table 5), facts which would complicate a comparison between the Kattegat and the Skagerrak. An alternative interpretation would be to assume a winter maximum and a sum­ mer minimum to be a more general phenomenon. In this case the geostrophic late winter minimum must be discarded and the Baltic outflow considered to have a small influence on the deep current of the Kattegat. Further work is necessary before this question can be solved. A transport figure of 500,000 m3/s from the North Sea, presented above, would be compared with other figures given in the literature, see Table 10. Apparently they differ considerably. 5.6. Surface Waves Wahl (1973) presented results of wave measurements made by means of an accelero­ meter anchored in the vicinity of L/V Fladen (Position, see Fig. 2). The signi­ ficant wave heights H1/3 m (average of the heights (double amplitudes) of the one- third highest waves) was computed and compared with the wind velocity V m/s. A formula H1/3 — 0.12 V was found for more open sea conditions and H1/3 = 0.075 V + 0.15 for land winds (NE-E). 21 6. Salinity 6.1. General Fig. 23 is a map from Anon. (1927) of the mean distribution of the surface salinity in the Baltic and the North Sea (August). The large river mouth features of the Katte­ gat—Skagerrak area appear quite clear. Looking also on the North Sea proper the map shows low salinity regions along the coast of Norway of Baltic water, in the Ger­ man Bight and (less pronounced) along the British coast. Water of higher salinity comes from north and south. On the maps of salinity at different horizons (Goedecke et al. 1967) the wedge of high salinity from north seems to pass the Fladen Ground area on all horizons down to 40 m. (Already Böhnecke (1922) interpreted currents in the North Sea from salinity distribution maps.) On the maps of the deeper hori­ zons there are indications of two wedges, one along the British coast, and one along the outer edge of the Norwegian Trench. Fig. 24 shows the salinity distribution in the Eastern North Sea and the Skagerrak at 50 m during the summer of 1966 (Interna­ tional Skagerrak Expedition, Anon. 1969). It is here evident how the more saline water enters the Skagerrak along the outer edge of the Norwegian Trench. Fig. 25 shows a salinity section in the Kattegat and the Belt Sea constructed from the mean values of observations made by the Danish lightvessels from L/V Skagens Rev to L/V Gedser Rev during the period 1903—1926 (positions in Fig, 2). It pre­ sents the equilibrium established between the excess of fresh water, the mixing con­ ditions and possibly also the currents in the Skagerrak. The surface salinities vary with, among other things, fresh water supply and sea level variations of the Baltic. The short term variations seem to depend highly on the sea level variations : When the sea level in the Baltic rises the Baltic water is drawn back (to the Baltic) from the surface of the Kattegat. The further north we go in the Kattegat the more often the bottom water is exposed, whereby the vertical water exchange is greatly intensified. During what was probably the largest inflow of saline water into the Baltic—in December 1951 (Wyrtki 1954 b)—the Kattegat was for a long period of time almost void of Baltic water, see Fig. 26. It also seems probable that the “movement of a cold water front” described by Eggvin (1940) is to be explained in the same way: a strong fall of the sea level (110 cm) in the Baltic during January 1937. (Finally, see Ch. 11.1 discussing a connection with herring fishery.) But when we look at monthly means of salinity and sea levels the influence of the variation of fresh water supply is increasing. The connection between surface salinity (Fig. 27) and sea level is no longer so clearcut. Figs. 28, 29 and 30 show the month to month variations at L/V Schultz’s Grund, 22 L/V Läsö Trindel and Bornö Station respectively. The lowest surface salinities occur in May and June concurrently with Sum 1 in Table 6 being at a maximum. It there­ fore looks as if the sea level variations are not of great importance with respect to this phenomenon. The bottom salinities being at their highest at the same time (not for Bornö, however) suggest that the compensation current may have increased simul­ taneously with the outgoing current. This does not fit in, however, with the deep current at L/V Schultz’s Grund being close to its minimum in the early summer. See also Ch. 5.5.2. Figs. 31 and 32 show two quite different pictures of the salinity conditions at the section  in the Skagerrak, one in the spring and one in the late autumn. From what has been said above, we do not know if the difference is due mostly to variations in fresh water supply or sea level variations. Table 11 shows the frequency distribution of salinities measured once a day at L/V Fladen during the decade 1951—1960. 6.2. Long-Term Variations Fig. 18 shows a long series of surface salinities measured at L/V “Schultz’s Grund”. The data were taken from Jensen (1937), Neumann (1940) and Publications of Danish lightvessel data (See References). The top curve shows the variation in fresh water supply of one of the larger rivers to the Baltic. It seems quite feasible that a small amount of precipitation gives high salinity, in the Kattegat nearly immediately, in the Baltic 5—10 years later. Also Schott (1966), who carried out Fourier analysis of series of monthly means of surface salinity in the North Sea, concludes that long­ term fluctuations of surface salinity can be correlated with fluctuations in the dis­ charge of river water and in precipitation, and that these in their turn depend upon the west wind component over central Europe. A different interpretation has been given by Dickson (1971 and 1972). He thinks that one important effect is the increased in­ flux of water from the North Atlantic under the stress of the increased southerly wind. Table 12 contains all monthly means of surface salinities at L/V Anholt Nord (up to 1945 Anholt Knob). Annual means are in parentheses when a month’s value is missing or was based on few original measurements. In the latter case also the monthly mean value is in parentheses. These salinities were recently used in a study by Nilsson and Svansson (1974). 23 7. Temperature 7.1. General Figure 1 (b) in Dietrich (1950) shows the temperature distribution during the year for a station in the English Channel. Due to the strong tidal motions the water is well mixed from surface to bottom and there is never a thermocline. Figure 1 (a) in the same publication is a similar picture from the southern part of the Northern Sea. The annual salinity variations are small (34.9—35.1 %0) but there is a well developed thermocline in the summer. Fig. 1 (c), similar to our Fig. 35, is constructed from data from the Kattegat. Due to the yearround haline stratification the temperature varia­ tions differ somewhat from the conditions mirrored in Dietrich’s Fig. 1 (a). 7.2. The Upper Layers of the Skagerrak Fig. 33 shows a temperature section between Skagen and Risör in the early summer of 1966. On account of the considerable counterclockwise circulation along the edges, ascending motions are created in the horizontally stationary central part. These mo­ tions cause the isolines to appear like a dome. In the summer relatively cold water rises towards the surface; the warming up proceeds from the surface downwards and the result is a very marked thermocline, which is evident in the figures on p. 73 in Anon. (1970). Here the observations have been made with a continuously recording instrument (Delphin). Table 13, taken from Tomczak and Goedecke (1962) shows minima and maxima, partly in the incoming Jutland Current, partly in the dome in the centre and, finally, in the outgoing current along the coast of Norway. The outgoing water differs from the incoming in so far as, in the winter, its temperature is lowered and, in the summer, raised due to the influence of the Baltic water from the Kattegat. 7.3. The Deep Water of the Skagerrak The seasonal variations decrease with increasing depth. Instead, rather large fluora­ tions are created in the bottom water owing to cold winters forming such heavy water in the North Sea that this water sinks to the bottom, see Fig. 34. The phenomenon has been described by Lj0en (1965), Svansson (1966) and by Lj0en and Svansson (1972). 24 7.4. The Kattegat and the Belt Sea If the temperature development at a lightvessel in Kattegat (Fig. 35, L/V Fladen) is compared with that at a Baltic lightvessel (Fig. 36, L/V ölandsrev) rather great dif­ ferences appear. Partly are corresponding temperatures generally higher at L/V Fla­ den except for the surface during winter, partly are the seasonal variations at corres­ ponding depths larger at L/V Fladen. The first mentioned fact probably depends upon the Skagerrak’s temperatures being higher than those of the Baltic. The larger seasonal variations are easy to explain for the surface layer. The great sta­ bility created by the salinity stratification gives higher summer temperatures and lower winter temperatures (which leads to earlier freezing in the Kattegat than at the same latitude in the Baltic). It is easy to understand that the stability prevents a swift cooling of the deeper strata in the autumn, but the question is how the heat reaches down at all. A permanent inflow of warmer water from the Skagerrak is probable. Already in the beginning of this century there was the opinion (Anon. 1903) that this warmer water originated from the southern banks of the North Sea and was therefore called Southern Bank Water. The ideas were supported by temperature studies during the whole year in the Kattegat. Our Figs. 37 and 38 of the temperature conditions at 20 m and 30 m respectively (Anon. 1933) confirm these ideas: maximum in the northern Kattegat in August but occurring continuously later southwards, October in Öresund being the extreme. The theory in Anon. (1903) that the maximum would occur still later in the Baltic is not confirmed by this material, nor by Lenz (1971). The intrusion theory is also supported by the fact that the water is continuously being cooled. — In winter the conditions seem to be opposite in the sense that intruding water with minimum tem­ perature in February in the northern Kattegat is slightly heated towards Öresund, where the minimum occurs in March. 7.5. Long-Term Variations Lee (1970) may here be quoted: “Smed (1963) gives the 5-year running means of sea surface temperature for each month of the year for the northern and central North Sea. In the former area these show a minimum in the early 1920s for all months ex­ cept I uly—September, for which it took place in the late 1910s with a secondary minimum in the late 20s. A break in the records due to the Second World War pre­ vents the exact timing of the maximum from being established, but in general it occurred between 1935 and 1945. In the 1950s all months, except November and December, show a downward trend and all this applies especially to May. Prahm (1958) has examined the bottom temperature record for summer in the central and northern North Sea in the regions of the Great Fisher Bank and the Fladen Ground.” Nilsson and Svansson (1974) investigated annual means of surface temperatures measured at L/V Anholt Nord 1880—1970. 15 year running means show a weak minimum (8.5°C) around 1920 before the rise started toward a maximum around 1950 (9.2°C). The curve of 5 year running means has maxima in 1898, 1912, 1935, 1950 and 1960. 25 8. Chemical Parameters. Primary Production. Optical Conditions Below, in Ch. 9, will be presented 10-year mean values of Swedish measurements of temperature, salinity, oxygen, phosphate-phosphorus and total phosphorus in the Skagerrak. Corresponding results from the Kattegat also include silicate and nitrate (Table 15). There are chemical observations carried out also by other countries, e.g. Denmark, but so far unpublished. Observations made by GDR, mainly in the North Sea proper but also some in the Kattegat were recently published (Frank et al. 1972). They include N03-N, NHLj-N, SiO^-Si, chlorophyll and sometimes seston. 8.1. Oxygen Table 14 shows the variations in oxygen saturation 1966—1973 near the old position of L/V Fladen. In the late summer a minimum occurs in the deep-water probably due to sinking organic matter requiring more oxygen than is supplied. Danish obser­ vations in the Kattegat in September 1968 show that the saturation percentage in­ creases from S to N, but also from W to E in the Northern Kattegat. The results of the international investigation 1966 gave a few low oxygen figures (Svansson 1968). While saturation values in most of the Skagerrak were generally higher than 90 % except for a couple of values close to Denmark and Sweden (80—- 90 %) they were lower in the SE corner of the Skagerrak and the N part of the Kat­ tegat; between Frederikshavn and Göteborg values as low as 40 '% were observed. Fonselius (1969) has shown that the oxygen figures for the deeper parts of the Baltic and the Gulf of Bothnia have dropped since the end of the 19th century. A similar investigation shows that such a decrease has also taken place in the Kattegat (Corin et al. 1969) mostly during July—November. Hermann and Vagn Olsen (1970) have examined all data from a more statistical point of view. They also find a decrease since the observations early this century, the values were, however, as low during the 30’s as they are now. In Hermann and Vagn Olsen (1970 b) is shown the marked decrease (appr. 1 ml/1) of the oxygen content of the bottom-water in the Öresund during the period 1966—1969 as compared with 1956—1964. 8.2. Phosphorus Hermann et al. (Loc. cit.) also show that the PO4-P figures have increased in Öre­ sund during the same time. At a station S of Ven a doubling at the surface and a 40 % increase in the deepwater have taken place. 26 Danmarks Fiskeri- og Havunders0gelser carried out observations of total phos­ phorus (and currents) every day for about 8 months 1969—1970 at 4 depths from L/V Halsskov Rev. The mean values at 0 m and 15 m for the entire period are 0.81 and 0.96 /xgat/1 respectively, but the fluctuations are large. 8,3. Pollution (Biochemical Oxygen Demand, Nitrogen, Phosphorus, Cæsium 137 and Strontium 90) A working group in ICES recently published a comprehensive report of the present knowledge of the pollution of the North Sea (Anon. 1974). This report includes not only the Skagerrak but also the Kattegat to a certain degree. Table 16 which is ex­ tracted from this report, presents values of BOD, N and P in sewage discharged to the Skagerrak from all the surrounding countries but to the Kattegat only from Swe­ den. There is always a great problem in estimating the correct figures as pointed out in the Report (loc. cit.): “The input data were of varying completeness in respect of the discharges to rivers and estuaries. Some countries (e.g. Sweden and England) assumed that the majority of a discharge made directly to an estuary or fjord would ultimately reach the sea, but others provided only data on the discharges entering the sea or the outer reaches of estuaries.” For this reason information on population is included. Further comments to Table 16: a) Dry Weather Flow from Norway is estimated releases to rivers, estuaries and fjords. Dry Weather Flow from Sweden is estimated on the assumption that water use per person is the same as for Norway (0.391 m3/day). b) BOD: Norwegian figures are estimated at 60 g/person and day for raw sewage. Swedish BOD is 7 day (70 g/person and day) for raw sewage. The figures are re­ duced in relation to the degree of treatment. c) Nitrogen and Phosphorus: Norwegian figures are estimated at 12 g N and 2.5 g P per person and day. Swedish figures are estimated from direct measurements of total N and total P concentrations in river mouths and by assuming that discharge is 13 g N and 4 g P per person and day in areas lying between rivers. Figures for Denmark were calculated using 56 mg N and 8.9 mg P per litre for raw, 30 mg N and 5.8 mg P per litre for settled and 20 mg N and 4.5 mg P per litre for biologically treated sewage. The ICES Report also contains input of pollution in industrial wastes. To the Ska­ gerrak there comes from Norway a total flow of 1,030,000 m3/day, mostly originating from pulp paper industry with a BOD of 215 tonnes/day. Sweden presents a figure of 265,000 m3/day (BOD = 60 tonnes/day) for Skagerrak and Kattegat taken to­ gether. Sewage sludge is dumped outside the Oslofjord at 50° 10' N, 10° 40' E. There are no other authorized dumpings in the Skagerrak or the Kattegat. Aarkrog (1974) presented some information on Strontium -90 (Sr 90) and Cae- 27 sium 137 (Cs 137) in “Danish waters” (about Denmark, the Faroe isles and Green­ land). Around the Danish islands there was a Sr 90 maximum in surface water in 1965 of 1.1 picocurie (pCi) pr liter whereas the present values are around 0.7 pCi/1 (0.1 pCi/1 around the Faroes). There seems further to be an inverse relation with salinity. The author (loc. cit) makes the following rough calculation of a Baltic Sr 90 annual budget (mean of 1969—71): from rivers 200 curie, from precipitation 400 and from the North Sea 150 curie, i.e. 750 curie to the North Sea. But the figures change, e.g. of precipitation 1973 only 30 curie and from the North Sea a somewhat higher figure due to a probable somewhat enlarged radioactive pollution of the North Sea. Concerning Cs 137 there is such a heavy uptake by fresh water plants that the ratio Cs 137/Sr 90 is smaller in the brackish Baltic than in the more saline Kattegat water. In the ocean this ratio is approx. 1.5. Comparison of Cs 137 contents in fish tissue shows that there is about 100 pCi/kg in cod from the Danish islands compared to 15 pCi/kg for Faroe Isle cod. The value 100 pCi/kg is low compared to 1965 values for meat, milk etc. but is relatively high compared to recent concentrations in land food. Kautsky (1973) shows the distribution of Cs 137 from nuclear fuel reprocessing plants in France and Scotland. 8.4. Optical Conditions A very large amount of basic optical investigations were made in the Gullmar fjord, particularly by Jerlov (see e.g. Jerlov 1968). These will be reviewed in more detail in Part II. Malmberg (1964) measured transparency by means of a beam transmittance meter in two wave lengths (380 mu and 655 m,u). Computations of dissolved Yellow Sub­ stance (absorption coefficient = ay) and (roughly) particle content could be made. Malmberg (loc. cit.) distinguishes between the following water masses: a) Baltic Water, S < 32 %0, ay > 0.30. b) Continental Coastal Water, S < 34 %0, ay = 0.10—0.30. c) Skagerrak Water, S < 34 %0, ay = 0.05—0.10 and d) Atlantic Water, S > 35 %0, ay < 0.05. H0JERSLEV (1971) made Tyndall (particles) and fluoresence measurements on a cruise from Copenhagen to Bergen in September 1970. Fluorescence like Yellow Sub­ stance was found to be inversely proportional to salinity. There were indications of considerable amounts of Yellow Substance furnished by rivers into the Kattegat in addition to the high concentrations of the water from the Baltic (see also Bladh, 1972). — The particle distribution in the Skagerrak showed among other things (re­ lative) maxima between 25—50 m probably coinciding with minima in vertical eddy viscosity coefficient distributions (Ehricke 1969). In the Kattegatt high values were found near the bottom : “Under the influence of horizontal flow and a bottom consist- 28 ing of loose sediments, this is to be expected.” — Returing to water mass aspects the Baltic water is particle-rich and the Atlantic water particle-poor. Very little is found in literature on irradiance measurements in the Kattegat and the Skagerrak but there is some information in Johnsson and Kullenberg (1946), particularly from the Gullmar fjord. 8.5. Primary Production Recently Steemann Nielsen (1971) published values of the primary production at some lightvessels in the Kattegat as measured by the C 14 method. Table 17 shows the variations from month to month at L/V Anholt Nord, the mean total annual primary production was 71 g C/ma. Table 18 presents the variations of the annual value during many years. From the latter table it is clear that the natural variations from year to year are so large that it is impossible to discern any possible long term trend of increase. There seems to be a correlation between primary production and salinity. 29 9. Decade Mean Values of Water Parameters From 1962 hydrographical data have been punched on ICES punch cards (Anon. 1973), “Hydro Master”, “Hydro Depth” and “Hydro Chemistry”. Since the “Hydro Chemistry” card contains all information from a station where chemical parameters were measured, data on these cards have been used in a processing of data 1962— 1971 from 52 stations in the Kattegat and the Skagerrak (See also Johansson and Svansson 1974). The result is presented both as quarterly and annual means, whereas the data were too scarce for a presentation of monthly means. Figs. 39—44 show conditions (N.B. the logarithmic depth scale) as they appear in the section  (See Fig. 2 for position). In Fig. 39, the salinity section, we see that the less saline Baltic water is at its maximum in Quarter II. Fig. 40 is a presentation of the temperature conditions. In Quarter I the water is coldest in the surface along the Swedish coast. In Quarter II the surface water is now warmest, and there is a mini­ mum of < 5°C at 20 m above the deepest part. In Quarter III the conditions are similar to those of Quarter II. In Quarter IV the Baltic water is again relatively cold. The bubble of water warmer than 10°C is particular. Oxygen, Fig. 41: In the surface layers there is the highest supersaturation in Quar­ ter II (primary production?), whereas conditions in Quarter IV show nearly 100 % saturation. At deeper horizons the conditions are “most favourable” in Quarter II, whereas in Quarter IV there are values lower than 85 %. Phosphate-Phosphorus, Fig. 42: Surface values are highest in Quarter I and IV, approximately 0.4 /xgat/1 and lowest in Quarter II and III, approximately 0.1 figat/1. There are some rather high values at deep horizons of station 18 B. Fig. 43 presents annual means of salinity, density and oxygen saturation percentage, and Fig. 44 shows PO4-P and total phosphorus (Tot.-P). Whereas Tot.-P is some­ what higher than PO4-P in the surface layer, there is no difference in the deep. Table 15 is an extract of the decade mean values computed for two Kattegat sta­ tions. The values of phosphorus at the Fladen station do not differ considerably from those in the Skagerrak, but the Kullen values show influence from the Öresund. Also oxygen is lower at Kullen. Apart from the influence from pollution, the Kullen values may also be explained in terms of vertical stability which is greater at Kullen. 30 10. Sediments In an article by Eisma (1973) there is a map of the distribution of muddy sedi­ ments in the North Sea except the eastern Skagerrak. Mörner (1969) has a rather detailed map of the sea-bed of the Kattegat. The shallow western parts are covered with sand and coarse silt but also till, gravel, stones etc. The deeper eastern parts are covered with clay and fine silt. Also in Anon. (1965) there is detailed information in this respect. Olausson et al. (1972) present determinations of carbon, nitrogen, phosphorus and some heavy metals in Kattegat and Eastern Skagerrak sediments (usually 0—2 cm of the sediment surface): Carbon: Open sea sediments have generally 0.5—1 % carbon of dry substance. Only in the vicinity of the Öresund are there larger values up to 2.5 %. Higher amounts of carbon at sediment surface as compared with depths of 2—4 cm, are only met with in the northern Öresund. Phosphate-Phosphorus: The higher values in the sediment surface as compared with non-surface amounts can be interpreted as an increase from the 1960’s, parti­ cularly in the SE Kattegat and off Göteborg where values up to 1000 ppm were found. Lead: The Pb/C ratio is usually 5 units in the Skagerrak. Higher values probably indicate input of lead from, e.g., gasoline. Kattegat values are usually slightly larger than 5. Mercury values are higher in the sediment surface than deeper down, particularly off Göteborg and in the northern Öresund, where the amounts are approximately 0.4 mg/kg dry weight. The sedimentation process in northern Kattegat may influence bottom conditions according to Rodhe (1973). Suspended matter is carried with the Jutland Current and settles in the Deep Trench. 31 11. Fisheries Hydrography 11.1. Herring (Clupea harengus) Many attempts have been made to find a relation between fishery of North Sea Herring in winter at the West Coast of Sweden and some hydrographic parameters (Andersson I960, Svansson 1965). This herring spawns in the autumn in the North Sea and does not migrate for food in winter. One thing seems to be rather clear: if the amount of Baltic water is considerably larger than normal in the Eastern Skagerrak and the fjords of Bohuslän, the herring will leave these areas (Andersson op. cit.). O. Pettersson and G. Ekman (1897) could draw this conclusion from salinity measurements in a famous example when after a long period of herring winters, the herring disappeared in December 1896 (temporarily, however; the Herring Period did not cease until about 1920). Looking now at sea levels at Landsort for the period concerned, it is quite evident that it was low during December 1896 and January 1897 (See Ch. 6.1.). While, however, a high sea level in the Baltic is a necessary condition for a good herring fishery, it is not at all sufficient. For the first time since 1920, North Sea herring were again caught, although in small quantities, during the period 1942—54, on the coast of Bohuslän (Andersson 1960). Svansson (1965) showed that during most years 1942—54 t-S plots seem to belong to one water mass, while those for 1935—41 and 1955—57 fall more along a straight line. The autumn spawning herring of the Kattegat and the spring spawning herring of the Skagerrak may meet critical environmental conditions for eggs and larvae (See e.g. Ackefors 1970). The eggs of herring are not pelagic but adhere to the bottom. As hatching takes at least one week but usually more the eggs may be exposed to water of very different temperatures. The larvae are pelagic and highly dependent upon surface currents and surface waves. Daily vertical migration of herring larvae is dependent on density stratification of the water (Höglund 1970). Dietrich et al. (1959) write about concentration of herring and the distribution of temperature in the northern North Sea: a. In summer and autumn the herring is concentrated in the core of the cold bottom water. b. The lower the temperature of this cold water, the longer is the duration of the concentration. c. The geographical position of this concentration fluctuates with the dislocation of the centre of the cold water. 32 d. The daily vertical movements of the herring schools are influenced by the structure of the thermocline. 11.2. Sprat (Sprattus sprattus) According to Swedish investigations 1959—63 (Lindquist 1970) the spawning of sprat in the Skagerrak is sharply defined to an area at the border between the Kat­ tegat and the Skagerrak, where Baltic water meets Jutland current water. The spawning takes place in May—June in water warmer than 6°C. Calm wind condi­ tions seem to favour a good year class. Sprat fishing takes place mainly during the period October—March. Water tem­ perature seems to be the main factor regulating the occurence of sprat in Swedish waters (Lindquist 1964). When the surface water layers are cooled, the warmest water will be met with near the bottom in the more shallow areas of the archipelago. In Lindquist (1964) attention is drawn to the changes in the localization of sprat fishing since 1859. These changes are supposed to be due to hydrographical changes (water temperature). Since there are no hydrographical series of such length, obser­ vations of air temperature along the coast of Sweden and Norway were used. It is probable that the larger catches in the northern part of Bohuslän up to 1920 (Mo- lander 1952) can be explained by higher air temperatures and thereby higher sea water temperatures, compared with the southern part where during the period before 1920 the catches were small and the temperatures were lower. The differences of tem­ perature between neighbouring cooled areas are of greater importance for distribution than absolute temperature values. The growth of O-group sprat is dependent on food supply. Increased lengths are supposed to be due to higher precipitation and land-influence (Lindquist 1973). 11.3. Deepwater Prawn (Pandalus borealis) During the period 1963—66 the fishery for deepwater prawn (at 50—500 m depth) in the Skagerrak area showed a marked decline (Rasmussen 1967). This author thinks that probably the abnormal cooling of the bottom water (See Ch. 7.3. and Fig. 34) in the winter 1962—63 was the main factor causing the decline. Höglund and Dybern (1966) claim overfishing to be a more important factor. Mass occurrence of a medusa, Tima bairdi, toward the end of the period was probably related to the cold water masses and may have had an additional effect. 11.4. Cod (Gadus morhua) Studying Andersson (1964) we are thrown into an interesting discussion of spawning migrations. Whereas it is quite evident that eggs and larvae which are “let” free in the water move with the currents, it is more difficult to follow the author when he presupposes along the coast of Norway a southward current by which the cod are 3 — Physical and chemical 33 passively transported from the Barents Sea to spawning grounds in the coastal waters of Mid-Norway. Active spawning migrations rather independent of hydrographical factors seem to be a much more probable alternative not only for cod but for most fish species. Returning to egg and larvae, the author (Andersson op. cit.) argues that favour­ able conditions for a good yearclass of e.g. cod, which spawns in winter, is a calm and cold winter. This would be the case because the vertical convection then pene­ trates deeper and brings more nutrients to the surface. 11.5. Mackerel (Scomber scombrus) The studies referred to above (Ch. 11.2.) in May and June during 1959—1963 (Lind­ quist 1970) also included eggs and larvae of mackerel. The spawning was most in­ tense along the Norwegian coast of the Skagerrak. (The mackerel seems to prefer to migrate into the Skagerrak swimming against the water current.) It is supposed that temperature must reach a magnitude of 12°C before spawning can take place in the surface waters. In the coldest months of winter the mackerel seems to stay in deeper parts of the Norwegian Trench, e.g. the western parts. In the middle of the summer it spreads over the whole North Sea (Lindquist and Hannerz 1974). 11.6. Haddock (Melanogrammus aeglefinus) With the Jutland Current many larvae are transported to the Skagerrak and Kattegat and this seems to be the main source for the haddock stock in this area (Höglund 1930, 1933). In Dietrich et al. (1959) there are interesting relations between catches of haddock and water temperature: “conclude that a correlation between the con­ centration of haddock and the temperature exists in the region of the Dogger Bank as soon as the total number of fish in this area and the differences in temperature reach a sufficient level. 11.7. Other Fishes So far, not much has been written about the hydrographic conditions in relation to such important fishes as whiting, coalfish, Norway pout, plaice, sole, sandeel, etc. 34 12. Heavy Metals, Organochlorine Pesticide Residues and PCBs in Fish In the ICES Report already referred to in Ch. 8.3. (Anon. 1974) there is a compre­ hensive part dealing with a Fish and Shellfish Base-Line Study. This study was car­ ried out largely in 1972 and was concerned with organochlorine pesticide residues, PCBs and certain metals. Cod, plaice, herring, shrimp, and mussels of specified age or size were sampled over the whole North Sea and analysed. The result of the shrimp investigation will be excluded here due to the low sampling rate. Mussels, indicators of coastal conditions, which are not taken up here will also be excluded. From the Skagerrak there is therefore information on cod, plaice and herring only. From the Kattegat, otherwise included in the report, there were no samples except from the northernmost part. The acronym IR will be used below when reference is made to the ICES Report. Westöö and Rydälv (1971) presented information on mercury in fish, and so did also Somer (1972), who particularly pointed out the importance of relating con­ centrations to fish weight (age). In Ch. 8.3. there is some information on Caesium 137 contents in fish tissue. 12.1. Mercury As mentioned already, Somer (loc. cit.) relates concentrations y mg/kg, wet weight, to the weight x kg. The following rough relations were derived from Somer’s drawings: Herring: y = 0.22 x + 0.023 Flounder, (Kattegat): y = 0.073 x + 0.06. In the Öresund the values may be five times higher. Plaice: y = 0.063 x + 0.050 for the North Sea, but 0.025 mg/kg lower for the Kat­ tegat. As usual the concentrations were higher in the Öresund. Porbeagle, Lamna nasus (The North Sea Proper and the Atlantic Ocean): y = 0.05 x Cod: y = 0.025 x + 0.17. In the Öresund the values may be five times higher. Looking a little more carefully into the concentrations reported for the North Sea Proper, a higher concentration, 0.08 mg/kg in a fish weighing one kg, is found in the SE corner (the German Bight) than in the open northern part, where the value is 0.04 mg/kg in a fish weighing one kg. 35 Such a geographical difference is still more emphasized in paper IR. The southern part of the North Sea, particularly the German Bight, has the highest values both for cod, plaice and herring, whereas the Skagerrak values are quite similar to those of the Northern North Sea. (It must be remembered that this synopsis does not include coastal areas, e.g. the Göteborg archipelago, where high mercury concentrations are encountered). 12.2. Cadmium and Lead Contrary to the case of mercury, the ICES Base-Line Study shows that the concentra­ tions of cadmium, ranging between 0.02 and 0.5 mg/kg, and lead, ranging between 0.1 and 3.0 mg/kg, were highest in the Skagerrak. Judging from the intercalibration sample there is no reason to suppose that the values, determined by Sweden, are in­ accurate. It has been noted, however, that high values were reported when small samples with consequently higher detection limits had been used. 12.3. Zinc and Copper Paper IR: There were slightly higher values of zinc in the Southern North Sea. The mean concentrations of zinc and copper in herring were reported to lie between 3 and 17 mg/kg and 0.6 and 3.6 mg/kg respectively. In both cases this is somewhat higher than for cod and plaice. 12.4. Organochlorine Pesticide Residues and PCBs Jensen et al. (1972) determined DDT and PCB in the Baltic but also on some sam­ ples from the sea area here concerned. Whereas the amounts of DDT in the Baltic in herring may come up to 40—50 mg/kg on fat bases, in the northern Kattegat they were only 2—3 mg/kg. Paper IR: The highest total DDT concentrations in cod muscle were reported for the Swedish and Danish west coasts and off Germany. Almost without exception the concentration of PCB was reported to be greater than that of DDT. In no case did the mean concentrations reach 0.1 mg/kg (wet weight basis). Cod liver with its usual­ ly much higher concentrations was not analysed. (Cod liver from the Göteborg archi­ pelago has been declared unfit for human consumption on account of high PCB con­ tent.) In herring the concentrations are higher as expected in the light of the much higher lipid content of herring muscle. — Dieldrin was not analysed on Skagerrak samples. It may be noted in this connection that fish, except salmon and flatfish, from a small area off the mouth of the River Viskan have been declared unfit for human consumption due to their high dieldrin content. 12.5. Comments on Geographical Differences The concentration of a substance in sea water is very much a function of the discharge at the boundaries of (sedimentation may be regarded a negative discharge at the bot- 36 tom) and the exchange with other sea areas by diffusion and advection (dispersion processes, see also Ch. 5.5.1.)- The concentrations in fish tissue are still more compli­ cated due to enrichment processes, migration and other causes. Higher concentrations in fish sampled in the German Bight and in the Öresund (and off Göteborg depending upon the definition of the coastal area) may be caused by large inputs and/or less effective dispersion. It will be important to study this problem more carefully with, e.g., mathematical models. The Skagerrak seems to be less affected by pollution, if lead and cadmium are excluded, concentrations of which must be checked (Cf. Pb/C ratios in sediments, Ch. 10.2.). In the open Kattegat the conditions seem to be acceptable concerning mercury, other parameters must be investigated more before a statement can be made. 13. Acknowledgements The author whishes to express his sincere thanks to Anita Taglind (drawings), Hubert Straka (references) and Birgit Stahm (typewriting), to Jan Johansson for the data processing necessary for the preparation of Chapter 9, and to S. Fonselius and A. Lindquist for valuable discussions. 37 References Aarkrog, A., 1974: Radioaktiv forurening af havet. Fisk og Hav 74. Danmarks Fiskeri- og F[avunders0gelser. Ackefors, Hans, 1970: Sillen förr och nu i Västerhavet och Nordostatlanten. Göteborgs Na­ turhistoriska Museum, pp. 23—86. Andersson, K. A., 1960: On the Causes of the Great Fluctuations in the Herring Fishery on the West Coast of Sweden. — Inst. Mar. Res. Lysekil, Series Biol., Rep. No. 12. 53 pp. Andersson, K. A. mil., 1964: Fiskar och Fiske i Norden. Stockholm, Bd. 1—2. 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IV H. 8. 31 + 81 pp. Goedecke, E., Smed, J. and Tomczak, G., 1967: Monatskarten des Salzgehaltes der Nordsee dargestellt für verschiedene Tiefenhorizonte. Erg.-heft zur DHZ, Reihe B, Nr. 9. Granquist, G., 1938: Zur Kenntnis der Temperatur und des Salzgehaltes des Baltischen Meeres an den Küsten Finnlands. Havsforskn. inst. skr. No. 122: 1—166, Helsingfors. Gustafsson, T. och Otterstedt, B., 1931: Strömmätningar i Kattegatt 1930. Svenska Hydro- grafisk-Biologiska Kommissionens Skrift Hydr. X. Hasselrot, T., 1971: Kvicksilverundersökningar 1966—1970. Statens Naturvårdsverk, Under- sökningslaboratoriet. Helland-Hansen, B., 1907: Current measurements in Norwegian Fjords, the Norwegian Sea and the North Sea in 1906. Bergens Mus. Aarbok 15: 1—61. Hermann, Frede, 1967: Hydrografiske unders0gelser företaget af D.F.H. og “Skagerrak”. 0re- sunds-vand-komiteens unders0gelser 1959^64, K0benhavn, Danskt-svenskt komitee. IV. Hydrografi. Kemi. pp. 55—77, (4.O.). (Vandudvekslingen i 0resund. 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Allmän rapport från Statens Skeppsprovningsanstalt Nr. 35. English summary. Walin, G., 1968: Vinddrivna vattenomsättningar vid östersjökusten — kommentar till några observationer. N. Forskningsrådet-Aktuellt. — 1972: Some observations of temperature fluctuations in the coastal region of the Baltic. Tel- lus, 24 (3), pp. 187—198. Vaux, David, 1955: Current Measuring in shallow waters by towed electrodes. J. of Mar. Res. V. 14: 194. Wattenberg, Herman, 1949: Entwurf einer natürlichen Einteilung der Ostsee. Kieler Meeres­ forschungen VI, 10—17. Wessel, L., 1971: Havsströmmar. Marinstabens hydrografiska detalj. Westöö, Gunnel and Rydälv, Marianne, 1971: Metylkvicksilverhalten i fisk fångad i mars 1968—april 1971. (Metyl mercury levels in fish.) Vår Föda, ârg. 23 (7—8): 179—321. Wyrtki, Klaus, 1954: Schwankungen im Wasserhaushalt der Ostsee. Deutsche Flydr. Z. Bd. 7: 91—129. —■ 1954b: Der grosse Salzeinbruch in die Ostsee im November und Dezember 1951. Kieler Meeresforsch. X/l: 19. Witting, Rolf, 1918: Hafsytan, geoidytan och landhöjningen utmed Baltiska hafvet och vid Nordsjön. Fennia 39/5: 1—346. ZwiEBLER, G., 1964: Beobachtungen auf den deutschen Feuerschiffen der Nord- und Ostsee im Jahre 1963 (sowie Monatswerte von Temperatur und Salzgehalt). D. Hydrogr. Inst. Hamburg, Nr. 2142. Data of currents, temperatures and salinities measured at Danish lightvessels were published by Det Danske Meteorologiske Institut, Charlottenlund: 1880—1896 Me­ teorologisk Aarbog, part 3; 1897—1961 Nautical—Meteorological Annual; 1962— Oceanographical observations from Danish light-vessels and coastal stations. Data of parameters measured at Swedish lightvessels (and Bornö station) were published: monthly means of temperatures, 1880—1913, in Svenska hydrografisk-biologiska kommissionens skrifter, fyrskeppsundersökningen 1923, monthly means of temperatures, 1914—1918, and monthly means of salinities, 1880— 1918, in Meddelande från Havsfiskelab. nr 102, daily measurements of temperature, salinity and current, 1923—1947, in Svenska hydrografisk-biologiska kommissionens skrifter, fyrskeppsundersökningen, daily measurements of temperature, salinity and current, 1948—1969, in Fishery Board of Sweden, Report, Series Hydrography, daily measurements of temperature, salinity and current, 1970—1972, in Meddelande från Havsfiskelab. nr 148. Data of sea levels measured on Swedish stations (Vattenstånden vid Sveriges kus­ ter) were published 1887—1944 by Statens meteorologisk-hydrografiska anstalt, thereafter by Sveriges meteorologiska och hydrologiska institut. Data of sea level measured on Finnish stations were published in Havsforsknings- institutets skrifter (after 1913). 44 Table 1. Volumes, Areas and Mean Depths Sea areas Mean depth m Surface m2 Volume m3 The North Sea (inch the Skagerrak, Sverdrup et al. 1946) 94 575,000 • 108 54,000 • 10® The Skagerrak 210 32,300 • 10s 6,780 • 10® The Kattegat (Ehlinet al. 1974) 23 22,040 • 108 506 • 10® The Belt Sea (Ehlin et al. 1974) 14 20,350 • 10° 291 • 10® The Baltic (excl. the Belt Sea and the Kattegat, Ehlinet al. 1974) 56 372,700 ■ 108 20,920 ■ 10® Swedish 1931- Table 2. rivers discharging to the Kattegat —1960 (U. Ehlin pers. comm.) Catchment area, km2 Discharge, m3/s Viskan 2,201 32 Ätran 3,343 46 Nissan 2,682 40 Lagan 6,444 70 Göta älv 50,181 530 Others 5,289 68 70,140 786 Norwegian rivers discharging to the Skagerrak 1911—1950 (Seep. 9) Catchment area, km2 Discharge, m3/s Tista 1,550 23.7 Glomma 41,284 708.0 Mosseelv 690 10.7 Dramselv 16,020 313.0 Numedalslågen 5,513 115.7 Skienselv 9,975 298.0 Toke 1,168 33.0 Vegardselv 491 15.6 Nidel v 3,907 124.5 Tordalselv 1,700 63.1 Otra 3,539 149.0 Mandalselv 1,746 87.0 Others 11,717 249.0 99,300 2190.0 45 Swedish rivers discharging to the Skagerrak Catchment area, km2 Discharge, m3/s Örekilsälven 1,327 21 Others 1,543 24 2,870 45 Table 2 a. Summary of discharges m3/s km3/year Baltic (Mikulski 1970, 1972), 1951—1970 13,900 439 Belt Sea (Brogmus 1952) 225 7 Kattegat 1931—1960 885 28 Skagerrak 1911—1950 2,245 71 17,255 545 Table 3. Measurements with automatically recording current-meters (Positions, see Figures 1 and 2) Station no Lati­ tude Longi­ tudeO t Depth bottom m Obs. depth m Period of observation Publication or availability S 10 June 1911 Bull. Hydr. ICES S 40 „ SJ „ S 6 57 56 09 40 100 80 Aug 1913 57 15.7 11 46.5 44 15.5 21.7—2.8 1930 Gustafsson och 57 15.7 11 46.5 44 7.5 2.8—14.8 1930 Otterstedt 1931 57 15.7 11 46.5 44 15.5 2.8—14.8 1930 57 15.7 11 46.5 11.5 2.8—9.8 1930 57 17 11 38 60 11.5 25.10—6.11 1930 57 17 11 38 60 31.5 25.10—6.11 1930 S. Kattegat 10.8—17.8 1931 Defant und Schubert 193^ 4030 57 56.9 11 19.7 58 12 21.9—3.10 1960 ZWIEBLER 1963 4031 57 49.4 10 50.9 76 65 21.9—25.9 1960 4032 57 23.3 09 17 39 13 22.9—30.9 1960 4034 57 43.4 07 56.8 446 8 22.9—1.10 1960 4035 57 42.9 07.56 438 75 22.9—30.9 1960 99 SW of Smögen 58 14 11 03 100 50 21.4—10.6 1965 Nyberg 1966 58 14 11 03 100 50 1.7—28.7 1965 58 14 11 03 100 50 2.11.-65—21.1.-66 32 57 26.8 06 59.3 123 17 21.6—9.7 1966 Anon. 1969 33 57 47.7 07 04 405 400 21.6—9.7 1966 34 57 51.7 07 51 516 511 22.6—10.7 1966 99 35 57 52 07 47 518 37 22.6—24.6 1966 36 57 50.3 07 48.2 513 18 22.6—22.6 1966 37 57 56.2 07 48.2 323 38 22.6—24.6 1966 38 57 39.6 08 01.8 304 28 23.6—12.7 1966 39 57 39.9 08 03.8 290 285 23.6—25.6 1966 ... 40 57 29.2 08 11 125 120 23.6—10.7 1966 41 57 28.8 08 11.8 110 19 23.6—10.7 1966 .99 42 57 11.5 08 23.6 33 10 23.6—10.7 1966 43 57 51 10 51 92 16 24.6—11.7 1966 44 57 50.1 10 51.9 81 40 24.6—11.7 1966 45 57 50.6 11 17.3 99 12 24.6—11.7 1966 46 57 51.1 11 17 97 40 25.6—11.7 1966 SW of Smögen 58 14 11 03 100 50 16.5—22.11 1967 Svansson 1969b W of Göteborg 57 41.15 11 34.65 25 5 AVz months 1970 AB Hydroconsult Uppsala 10 1966—1971, 34 months 16 1966—1972, 46 months 22 1966—1973, 51 months » Table 4. Currents, cm/s Depth m 0 2.5 5 10 15 20 25 1910—30* Läsö Rende 5/9 1912—14/11 1913 22.0 26.0 24.7 9.7 —0.2 —2.2 Rossiter (1968) Anholt Knob 17/6—17/9 1910 —5.0 1.7 —2.3 —4.3 —5.1 —4.3 —3.9 Jacobsen (1913) Schultz’s Grund 1910—1916 10.0 2.4 0.4 —9.4 —18.2 —19.0 —15.0 Jacobsen (1925) Lappegrund 1/9—22/11 1909 22/6—17/8 1912 35.0 27.0 17.5 —10.3 —13.2 —11.3 —9.0 Jacobsen (1925) Halsskov Rev** July 1969—Jan 1970 April 1970—May 1970 13.0 15.0 13.0 12.0 9.0 Hermann (1975) * Dietrich 1951 ** Preliminary 47 IT) on & Ö Z « Z a a Z « Z »a z a a z £ Y ea r POOenxf 17 1. 9 17 2. 5 43 8. 1 ffi u 1 14 o <4-1 S o i-< a Z Sê xf Z pt Z Tf a z St m a Z St xf a Z St ON 9 NN W Z a à Z m St z £ o 00 Z ob IAN D ec . 28 .0 14 .8 22 .9 19 .9 — 6. 5 26 .4 0 fi cd O xf p p O kO r-4 p T—4 H £ Ö Ö Ö Ö Ö1 Ö © Ö Ö Ö 5— ON On p kd P »d xf d p CS P CS s—''/ p xf T—I en p oo T—1 4—4 CS 4—4 CS CS 1 CS 00 X Ö Ö Ö Ö Ö Ö Ö Ö Ö Ö Z 1"oC/3C/3 HHX O xf en p p p CS CS p VD 00 p p SD On +L Ö Ö Ö Ö Ö Ö Ö Ö Ö Ö 4 , t4 VD xf id On 43 ÛÜ O en CS CS 1 CS 3 v—t SD O en cS >/n en ko t"* oo X es WD CS Xi" O p p o o S43 Ö Ö Ö Ö Ö Ö Ö Ö Ö Ö ÖC/3 o p r-H en xf p sq1 1 xf r-* en xf H CS 00 xf o On s SGo enen dCS den xf »des0 t§ X O p CS en 4—< p 4--1 CS CS 4^ \ COÖ ö Ö Ö © © Ö Ö Ö Ö CO»H a -3 C/3 1 S C3Û en p p p S e o § HH ko O ooP xf P */np xfp p r-*p H On op #o G < xfen On 00 »den CS en en å> > Ö 1 Ö Ö Ö Ö ! Ö Ö Ö Ö Ö S PQ p O p p p p 2 a g spO £ o> -2 ! en 00 en 1 en ko ko ON ITN CS kd O CDbß T3 G 0) G xf On xf 12 .9 6. 2 56 .1 p en Xf oo* o 53 <5 'S > p Ö1 P Ö en Ö PÖ en Ö1 xf Ö o Ö Ö O Ö Ö (D 3 H-> 3 ° 1 1 cdH CQ >4 en OO P SD p OSr, 0> S 5 a 1 o xf 00 en oo O CS o OO ko GCD cd S enko en es’4—1 oo > o p p xf O oo p xf CS 4-“1 ■H»Ö Ö Ö Ö Ö 1 © Ö © Ö Ö cd kocd .5 ko esp en esxf CSo f-r- 00 */■)en xf o G, c xf d xf enwn en dwn /<«i^ —> HH Ö Ö Ö Ö Ö Ö Ö Ö Ö Ö «4H ’Id 1 O •C > C/D P xf TH p p p 2 g kO on r-- SD cS tH xf »o ON •H*» G cd en d kd kden en ONxfo -5 H P *T) P Xf o ON p en p 4—1 (D S T 1 -X-cd 42 a) On OS ON On On On ON ON ON ON CS >» O G *-< *-< 4-h *-• '1 H T—1 T-* ON 1 ffi 'So s Sk ag en s R ev CL) > G .2 /--NCSVT3 ON + N II <1 ^ Ü On Ößi-4 G cd O 43 W — A H ) *d JD s SCO G 0) P4 cd G #G H W HÛÛ G3 C/3 ’s >O M UCD G ^R C/3 & d H-l *-C3 c/3 cd D S sG o 'o O 0) pi d CQ G w o3 UO S s o CO ° Z o w s S CS 42 cd H s 00 3 :0 CO :cd ►G oH CO o :0CO:cd H-l O 43 G < C4| Gcd G 0X)OU Û s o co C/3 *cd K C/3 T3 O O ö g > S aè Gcd > w ô Pi « d G CO o 1> d d G CO *o ON * 48 Table 7. German Current Data from 1966. Positions in Fig. 2. Date Current Components, cm/s. Stn no 34 Stn no 38 Stn no 40 Obs depht 511 m 28 m 120 m NE NE NE Stn no 41 19m N E 66 06 24 —2.3 —2.7 —2.7 1.5 13.4 11.7 20.0 27.9 25 —2.1 —0.1 —2.1 —1.2 9.2 10.7 15.0 29.9 26 —0.1 —1.2 —2.4 —0.7 8.4 10.1 16.6 35.3 27 —0.6 —1.2 —0.1 0 10.8 13.3 18.6 34.3 28 —1.3 0 0 0 12.3 13.0 24.1 32.3 29 0.4 —1.3 —0.2 0.6 13.0 14.1 14.5 44.3 30 0.2 0.5 —0.4 1.7 11.4 13.6 12.4 28.1 66 07 01 0.4 —1.4 0 0.5 14.1 14.0 15.8 29.6 02 —1.5 —2.8 0 0.2 15.3 16.9 12.6 27.4 03 —0.4 —1.0 0 0 12.5 16.2 14.6 34.5 04 0.1 0.2 —0.2 0 12.7 16.2 9.4 27.2 05 2.2 4.6 0 0 14.5 15.9 10.1 23.0 06 2.3 5.4 —0.1 0 15.7 19.7 6.3 22.6 07 2.1 4.0 0.8 2.2 13.3 17.4 7.0 16.2 08 0.1 0.6 1.4 4.6 17.1 19.3 12.5 23.8 09 —1.6 —4.4 1.8 5.1 23.6 27.6 15.9 36.3 Table 8. ~N-Component cm/s 1897--1908 at L/V Horns Rev (Jacobsen 1913). January 22.0 July 11.8 February 21.6 August 19.1 March 22.0 September 14.7 April 17.1 October 20.6 May 15.1 November 19.8 June 11.8 December 21.6 Table 9. Monthly Means of Currents Measured SW of Smögen. N-Components at 50 m Depth April 1971 17 cm/s September 1967 14 June 1967 19 October 1967 29 July 1967 8 October 1971 37 August 1967 9 Table 10. North Sea Inflows and Outflows according to various authors km3/year Dooley 1974 Svansson and Kalle Carruthers Cartwright Lybeck, 1949 1935 1961 1962 Inflow a) Orkney — Shetland b) Western Norwegian Trench c) Strait of Dover d) Baltic 9,500 34,800 18,500 500 23,000 2,000 7,400 Outflow 63,300 16,000 4— Physical and chemical 49 Table 11. Frequency (%) of salinities S %0 measured once a day at L/V Fladen 1951—1960 S %o 12.50— 14.99 15.00— 17.49 17.50— 19.99 20.00— 22.49 22.50— 24.99 - 25.00- 27.49 - 27.50— 29.99 30.00— 32.49 32.50— 34.99 >35 0 m 0.8 8.0 21.6 28.8 24.6 11.1 3.3 1.7 0.1 5 m 5.1 15.6 27.1 26.3 15.4 7.2 3.2 0.3 10 m 0.2 4.4 13.3 23.3 20.2 17.8 16.2 4.5 15 m 0.1 1.5 8.2 13.1 17.7 37.5 21.9 20 m 0.04 1.3 3.8 10.6 38.6 45.6 30 m 0.1 0.7 16.5 82.6 0.1 40 m 0.1 4.4 95.2 0.3 50 Ta bl e 1 2 a . S % 0 A nh ol t L ig ht ve ss el t-« O q n q n- *n o'vr oh CT o-1 n* cn ni CO CO CO CO CO CO co CO co co co CO co CO CO CO CO CO CO CO CO CO co CO CO CO CO CO co cn On cn cn q q q CO q q q oo oo q q OO' q n- q q q q CO q OO' q q q q q X NO O *n cn NO oo’ On y-S, ,_4 ON y-!< CO ON cn* o- i-h cn* Ö CO* in Ö in* Ö d cn* n in d i—< nico co co CO CO CO co CO CO CO co CO CO CO CO CO n On no’ 1-HCO CO CO CO CO CO CO CO co CO CO CO CO CO CO CO CO CO X oo ON VN ON q q oo q q »n cn q On q q q q ON CO q in NO cn q NO q q q q q n- no ON On ON Ö Ö y-lf On o: y-^ co’ od Ö Ö Ö Ö Ö Ö d od d ON r-i ni ON On CO CO CO CO co co CO CO v* CO CO CO CO CO CO co CO CO co l-H NO n cn q q q q q q q 'sb oo q q q CO' q q q in q OO' q q q q NO q q K non- Ö NO Ö co CO Ö y-1 ON od od Ö ö od n Ö On od On cn T-i CO d od d d cn > CO T-H CO (N co CO CO CO CO CO CO 1-H CO CO CO CO CO CO CO CO NO O oo CO q q q q of q q °°. q q q q q q q q q q q q oo 00 q CO q q > h-ooin ON ni Ö o- CO oo oo' CO oo’ © ni n 1-î On On l-H NO od od cn oo CO d ON od CO CO CO CO co «O CO y-* CO CO CO CO CO cn On CO q 00 CO 00 q q q q o cn q in q q q q cn q q q q OO' q > 00 oo* n y-h Ö y-lf ON y-^ oo in cn od Ö On od no n NO od ni no’ ON od ■^f T-i »n‘ in to od CO in* CO CO CO CO CO (O CO n n »o CO cn q q CO 00 q q q q q q q q q q q cn q q q cn q in q q > 00 oo oo Ö On ON ON r-i ON cn’ ni od Ö OO NO Tb od od n NO »* n O' co’ oo* O' n ON d NO CO CO CO CO co CO > CO ON CO CO CO q »n q 00 q oo q q q q oo NO q q cn q q °°. q •■sb q n NO cn NO* Ö Ö co’ oo’ Ö n y-lt xb cn cn od r-4 od 1-H On ON ni On n od in ON CO CO CO CO CO CO •O CO *-< CO CO CO 00 © oo q q q q q q q q 00 in q q q q q q q q q q cn q CO in NO "sb co co cn od co ,_4 CO On co cn od NO* ni oo NO* ni cn od od odHH CO co CO CO co co CO CO CO CO co CO CO co CO CO CO q q CO q q q q q q q OO' q q q CO q q cn q cn m q q q ,_4 ■"sC »n CO Ö co cn O' O' in’ 'sb NO in CO q Ö d NO co ni On -vb cnCO CO CO CO CO CO CO co CO CO CO co CO (O CO CO CO CO co co CO co co CO CO CO q q oo q oo q q q q q q On q q OO' q cn q m q q q q CO' q q q m 1—( cn Vn 'si- O* Ö *n co’ co j—H cn in Ö co *-i cn O' in NO Ö in o- n in CO O' NO cn >n* CO CO co CO CO CO CO CO CO CO CO CO co co CO CO CO H in^ \û 0\ in N ^ m h oo q 't oo no »n /n cd io '—' cd ^ no" On i—< r4 t-h cd cd cd ö ■d i/d O On On virHnnNNMNMMMMN^NrinrHMfNKNMMMrHrHM oooNOoomr^mmr 'ON^N'th-O'-H^OO'-HVû'tOO mNfb^o^inOsChnfnfn'tO^^^OtnooOO^NO'tMMMNrlMM'-r^MNnNnMNMNfNMMMMMNM VO «an ON Os rn r- '-h oo Os »o t^- 'O vn '«t On t-- oo rn p 'st O rl ro On On ÖOcdcdNdcdv-icdcdtd-^odcdÖcdi-McdONcd-^cdcdÖrdcdi-H cn! fvl y—i rA ——< rvi rv) rs) rv] r— »—< rsi rv] rvi rsl rO i—i rNl rs] n-l rsi cnI rsl rxJ rsl t^t^o\oot^CJ\'töNNO\oornh.'toooNooNh;0^^vqin\qh; cdÖ-^tÖÖONfdTtrdÖcdtdfSONrdrdfdONf-HONONÖONONONON NM^MMrH(SMNMM^N-HNN-- '. 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De ca de M ea n V al ue s 1 96 5— 19 71 (/ig at /1 ev er yw he re ex ce pt Og % ). K ul le n (N 5 6° 14 ' E 12 °2 2. 2') ç^[> y-( © 00 VO OS Od —^■^rt^r^ONr^ONcdvq Vi vd Is* ON't GO Th xh d xh r-"’ oo’d dh-4 J 1—4 cc>. °H o ^ 7- N OCHnOOnV) cct> 0£ O oo >o >h p> in m \o fhOVOCMrDC^t^-CDCDr^ O 6 rH Pi’ 7 £ 1 -d -d ri ri ri v v v H4 > c4 m xh c-4 rD > NnOhVNrMOOVNfO OOOOnONONOOOOOOOOONOOh i t-4 H-4 A I oo O o m On »h vn «o ii in o o On OO VO (MHH OOOONONOOhMO O w t^r^o^Hvo vot^-voOsONVornoO O O O oo O >—j OOOOnOnOnOnOnOn ^ ^ v—( r—( —-( ►H O oo On V) vo O On oo oo oo H4 ^rtO00C077mM OOOONONONONONON b A I NOOO-H^r^ o >;-—i O On rn m nJ-in n On in 1 vo vc o r-H o _ I •nxhioiDiovot^t^oo A d d d 6 h m Jh d d d d d d d d d > 00 O Tj- *-< CDOn vo r*- on in > O’—'fSONC^t^-rj-rDO »ovqv^Ttioiovovor^ Ö Ö Ö Ö Ö r—4 i 1 1—4 d d d d d d d d d o Ph H-4 On t—( 00 OO On C"- in Oh m oo-h ri ri o mJn +-S M o ^ H On dodo ö g. Oö H ’trnrncnmVNt^hco d d d d d d d d © rJ oo o *o On S t3 iniooo^vodNfno cnrn^vqinr^ooONON1-4 (Nj CD VO t"- CD > mxj-VNxfxvorNTtrh7 rnrnminh 1 r^cNr4rnro-^i-»riVoi>I dddod i d d d d d d d d d ri'trixt-p- ONON-^-ONr4r--r4»ooo> irnovo >> dodo ö H-4 d d d d d d d d d ■** oa oo On i> m *—i hhV0OhO77f6T—i »—( H CD t"- OK pOOnnrninvoM O-i ►*-< ö ö ö d d d d d d d d d d d HH (M T7' VD CO tT) OiOnO’-hO’—iONhON H4 ^HHVOh HH OOr-;mTj-»ov^i>C^ d d d d d d d d d d d d d d ON 00 o 'O’tVNrHOrlMONri 1“f vq vo vo oo HH VNVN»OVOVOVOVOVOt> d d d d d d d d d d d d d d s OVNOVNO s 0*oOv>00©OV">' rH 04 iHTHpimi-VNh 55 Table 16. Dry Weather Flow m3/day BOD Population tonnes/year N tonnes/year P tonnes/year Skagerrak 756,000 1,687,000 40.060 8,150 1,710 Kattegat (Sweden only) 427,000 1,094,000 82,420 15,165 1,545 Table 17. Primary Production at L/V Anholt Nord 1964—1967, 1969 g C/m2 and month X II III IV V VI VII VIII IX X XI XII 2 g C/m2 and year 1494687 97 9 42 71 Table 18. Primary Production (g C/nr and year) determined by the C 14-Method (Steemann Nielsen 1971) Year Halsskov Rev Fladen Anholt Nord Aalborg Bugt Läsö Rende 1953 80 1954 82 110 1955 88 80 1956 80 70 1957 105 1958 90 1959 100 1960 120 1961 1962 1963 1211964 65 1965 57 1966 51 1967 94 77 85 70 1968 134 861969 101 95 1970 56 300 m 200 m NORWAY /" Drl Arendal "Drache* 1 SWEDEN •09 lirtshals DENMARK •S.G. From Svansson (1965) Fig. 1. Chart for the Norwegian Channel and surroundings showing hydrographic stations. 57 5 — Physical and chemical 54° - •0- Lightvessel O Coastal Station +Open Sea Station -^Automat. Rec. Current Meter • Water Level Gauge R River ‘fa \\\ i v\\ \ Fig. 3. Lines of the same time interval between upper culmination of the moon in Greenwich (solar hours) and high water. (Reproduced from Defant [1961].) Fig. 2. Map of the Skagerrak, the Kattegat and the Belt Sea. The original purpose of the sec­ tioning was for model work. N.B. The Danish lightvessels Laesö Trindel, Anholt Knob and Schultz’s Grund were 1945 replaced by Laesö Nord, Anholt Nord and Kattegat SW respectively. The Danish lightvessel Östre Flak was 1943 replaced by Aalborg Bugt. 59 — 2.5 m 0.25 m 2m _ Fig. 4. Lines connecting the same average spring tide range in the North Sea. (Reproduced from Defant [1961].) 60 /9 cm 10 cm \ \4cm Phase in Hours Amplitude in cm Wcml /6 Fig. 5. Phases and amplitudes of the tidal component M2 (12.42 hours). No isolines have been constructed in W. Baltic due to lack of observations. Sea level gauges are indicated by filled circles. 0 Current flow into the paper 0 Current flow out of the paper Fig. 6. The different effect on opposite sides of a canal of a progressive tidal (Kelvin) wave. mb Atmospheric Pressure Stockholm 06 GMT 000 - 1040 - Water Level Smögen Daily means Water Level Landsort Daily means -20 - Surface Salinity Kattegat SW 06 GMT Oct 64 WIND pycnoctine Stage 1 Stage 2 WIND CURRENTS L Sep 64 Fig. 7. Nov 64 Fig. 8. The development of windinduced circulation in a two layered closed channel. Bei Nord-Wind . Stärke 6 Sfil \ ' cTV n am v f!\ t'A\< Bei Ost-Wind . Stärke 6 Y \ \ i y < » î ’ \ 2 i \ V \ / {\ nv >1 t ! » \Y /?nA\ ! : <" u °, / /» \2 \ /' X 4 V s \ \% \\ v x y ijA) -\\S)\ \ \ w --- \ \ \ V, \ fk A/? ' ;Y J ♦--- ''V MÖ * s^C. "W'~ Bei West-Wind i Stärke 6 Bei Süd-Wind Stärke 6 Current velocity, cm/s o <10 10-25 25-50 50-75 75-100 >100 Frequency of direction, % ______ >75 -------- 50-75 ..— 25-50 Dashed arrows do not refer to measurements Starke 3 corresponds to 3.4-5.2 m/s Fig. 9—12. Charts of surface currents at various wind conditions. Current observa­ tions made aboard 18 lightvessels in 1937 and at further 22 stations in other years provide the basic material. (Reproduced from Dietrich [1951].) 6 9.9 - 12.4 m/s Bei Nord-Wind . Stärke 3 10* 12* ■j?J <>\ Zr Y)\ t °v ] ii/ t ' ' L V /' ! t « / ♦ i \ Bei Ost-Wind . Stärke 3 \ l 5T üö: (K'< ’ ) j^WiW ° /T? n/t *' ‘ CÇ °0^ / /" o \ 1\ \ V ' \ \ „ c « V. ° 4 ! o \\ l 1 ) ■ J\ /% \ \ VW -SD ^fiLMM., \ (\>Y UW.V^f \ o ^-v w ,, »__ i/o / ^ wi0" 12* Bei Süd-Wind Stärke 3 Bei West-Wind . Stärke 3 Fig. 13. Daily means of currents measured SW Smögen every 20th minute and at lightvessel Halsskov Rev 8 times a day. JU LY , AUG US T , SEPT EM BE R , OC T Currents E-comp. 41 (57“ 28.8' 08” 11.8') Depth 19 m N-comp. 27 1 5 9 July 1966 E-comp. Depth 40 m N-comp. knots 2.0 E-comp Skagens Reu L/V Surface knots I rx N-comp. J Halsskov Rev L/V Surface Fig. 14. Daily means of currents measured during the International Skagerrak Expedition 1966. At the top a record from the Jutland current in the outer Skagerrak, in the middle two records from the border between the Skagerrak and the Kattegat and at the bottom a record from the Belt Sea. 66 Fig. 15. A comparison between the variations during one year (1964) of the daily means of hourly readings of the sea levels at Landsort and of the surface salinities measured once a day at the L/’V Kattegat SW. 67 Fig. 16. A comparison between the variations during one year (1964) of the daily means of hourly readings of the sea levels at Landsort and of the surface salinities measured once a day at Bornö Hydrographical Station. 68 JA N , F EB MAR , AP R , M AY , JUN E , J UL Y , AU G , SE P , O CT , NOV , DE C 196 4 Fig. 17. Schematic figure of an enclosed sea with the fresh water discharge Qft and the salinity Sj %o connected through a strait with an ocean of the salinity So. The compensation transport is designated Qo. River Vuoksi z/z irhultz's Grund, Fig. 18. Comparison of the ratio between the annual mean value and a long-term mean value of the discharge of the river Vuoksi, which flows into the Gulf of Finland through Lake Ladoga (top), and the annual mean of surface salinities at the L/V Schultz’s Grund (From 1945, L/V Kattegat SW). 69 ß.7 Fig. 19. Theoretically computed steady state concentrations (in mg/m3) in various parts of the Baltic and adjacent seas, concentrations caused by the hypothetical discharge of 10,000 tons pr year of a conservative substance in the middle of the Baltic. 70 Strömstad I Skagen Göteborg The Baltic Current Fig. 21. A simplified map of the surface currents of the Kattegat and the Skagerrak. In the Kattegat the “countercurrent” is indicated, which originates from the large mixing between the Jutland Current and the Baltic Current. The main bulk of this fusion is, however, leaving the Skagerrak along the coasts of Sweden and Norway. 71 CURRENT PROFILES JULY 9, 1966\\ Norway Denmark Resultant N & E-Component • Magnitude cm/s 0 10 20 Sweden Depth scale 0 m • 10 20 -30 - U0 ------50 -100 .200 .300 Fig. 22. Daily means of currents measured on July 9, 1966, during the International Skagerrak Expedition. 72 SEA SURFACE AUGUSTSALINITY 7, Fig. 23. Chart displaying long-term means of surface salinity. (Reproduced from Anon. 1927.) 4° 6° 8” 10° 12° 34.5 ' —'/ 5-9 July 1966 \ \ \ \ \%x \ \ 'i 34.7— Fig. 24. Salinities at 50 m depth measured during the International Skagerrak Expedition 1966. (Reproduced from Anon. 1970.) 6 — Physical and chemical 73 D ep th m 5 D ep th LTr LR laG HR Salinities 7« 1903 - 1926 25. Longituditional section of long-term means of salinity constructed by means of light- vessel data, published in Anon. 1933. Positions of lightvessels in Fig. 2. Salinities °U Dec. 11, 1951 Fig. 26. Longitudinal section of salinities measured on one day in December 1951 during the large inflow to the Baltic of water of high salinity. Positions in Fig. 2. 74 Sea Level Landsort S%o Kattegat SW J I F I M I AM I A I M ! J I J N I D %o 21.3 20.9 20.5 20.1 19.7 19.3 18.9 18.5 18.1 17.7 17.3 16.9 16.5 16.1 Fig. 27. Long-term monthly means of sea levels of the Baltic and surface salinities of the Kattegat. Schulz's-Grund 1903-1926 Salinities %o Fig. 28. Long-term monthly means of salinities measured at the L/V Schultz’s Grund. Data from Anon. 1933. 75 Läsö Trindel 1903-1936 Salinities %0 Fig. 29. Long-term monthly means of salinities measured at the L/V Läsö Trindel. Data from Anon. 1933. m 5 10 15 20 25 30 35 JAN FEB MAR APR MAY JUNE JULY AUG SEP OCT NOV DEC BORNO S 7« 1931 - 1960 Fig. 30. Long-term monthly means of salinities measured at Bornö Hydrographical Stations. Data from Svansson 1974. 76 — CM Fig. 31. Salinity section in the Skagerrak (See Fig. 2.) constructed by means of data obtained during a cruise in the spring of 1964. 77 M ar ch 17 19 64 m Å18B 17 16 15 14 13 12 11 Dec. 14 1964 Fig. 32. Salinity section in the Skagerrak (Positions in Fig. 2.) constructed by means of data obtained during a cruise in December 1964. (Reproduced from Anon. 1970.) N 57°48' E 10°42' Risör 134 Station no. 136 154 155 156 157 171 174 m 10 20 30 40 50 100 150 200 250 300 350 400 450 500 550 600 120 100 80 60 40 20 km 0 Fig. 33. Temperature distribution measured during the International Skagerrak Expedition on a section running from Skagens Rev to Norway approximately through station Å 18 B (Fig. 2.). Sk ag er ra k D ee p Fig. 34. Temperature versus time in the Skagerrak in the area of station M 6 (Fig. 2.). Fig. 35. Long-term monthly means of temperatures measured at the L/V Fladen. Data from Koczy (1954). J JAN I FEB J MAR| APR] MAY J JUNE]JULY J AUg| SEP ] OCT | NOV | DECm 0 — 5 — 10 — 15 — 20 — 25 — 30 — 35 — 40 — 45 — t C Fig. 36. Long-term monthly means of temperatures measured at the L/V Öiandsrev (Southern Baltic at N 56°07' and E 16°34'). Data from Koczy (1954). K SW La G HR G R (1 7m ) to C" 00 a> L- D □ i_ <\> CL E a> E o CM Fig. 37. Long-term means of temperatures measured at lightvessels (Positions in Fig. 2.) at 20 m depth. Arrows indicate minima and maxima. Data from Anon. (1933). 81 K SW LaG Fig. 38. Long-term means of temperatures measured at lightvessels (Positions in Fig. 2.) at 30 m depth. Arrow indicates maximum values. Data from Anon. (1933). 82 Te m pe ra tu re 30 m S % o Q ua rte r I s °u Q ua rte r T TTT i.. r rr Fig. 39. Decade (1962—1971) mean quarterly values of salinities measured approximately five times a year at a section in the Skagerrak (Position in Fig. 2.). Logarithmic depth scale. Q ua rte r i irr lu indea LU indaa LU q;d0Q Fig. 40. Decade (1962—1971) mean quarterly values of temperatures. See further Fig. 39. Q ua rte r I 0, % Q ua rte r I I I I I I I I I I I I J I I I Fig. 41. Decade mean quarterly values of oxygen saturation degree. See further Fig. 39. jjg at /l Q ua rte r I PO ^- P jj ga t/l Q ua rte r Ï... I.I I I I I Ö ® J I I I Fig. 42. Decade mean quarterly values of phosphate-phosphorus. See further Fig. 39. D ep th m D ep th m D ep th Quarter I-T5LS °/oo Quarter I-EZ Quarter I-E2 4—H-+ Fig. 43. Decade mean annual values of salinity, density-