Utvärdering av biotopvårdande åtgärder i Grimåsbäcken. En analys av fysikaliska egenskaper och sammansättning av makroevertebrater.

Abstract

Many streams have been exposed to extensive anthropogenic impact for the benefit of forestry and agriculture. The effects has mainly been channelization characterized by lowered and straightened channels. This homogeneous morphology leads to decreased ability to retain water as well as lost connection to the floodplain. Restoration measures aims to achieve similarities with the streams pristine characters and increase the heterogeneity of habitats. Grimåsbäcken in Säve has historically been heavily affected by channelization to gain more land for forestry which has led to recurrent draught. Biotope preservation measures was performed in the years 2021 and 2022 to increase the retention capacity of water and to recreate a more natural stream water biotope. The stream was treated with four different methods; 1: high pressure pump and hand held tools, 2: five ton excavator, 3: high pressure pump and adding log in the stream 4: one ton excavator. Each section was treated with one of the methods to raise the stream bed, widen the furrow and increase the retention of water. This study investigates the physical and biological response following the different restoration methods. In the survey, the four restored reaches were compared with each other and with nearby control reaches. For each reach, the distribution of bottom substrate, degree of shading, amount of woody debris, pH, alkalinity and temperature as well as stream width in comparison with width before the measures were noted. A visual assessment of the streams ability to retain water was made during the dry period. Macroinvertebrate composition was analyzed to provide a snapshot of the ecological status of the reaches. Stoneflies (Plecoptera) have a high sensitivity to disturbances and were therefore used as indicators. The physical analysis showed that stream width increased with 800% and the amount of water in the environment showed an increased potential to retain water. Regarding recolonization of the benthic fauna, 30% of the species found in the control reaches were observed in the restored reaches already one year after the habitat improvement was carried out. In two of the reaches, however, only 13% of the species were found, in these two the amount of dead wood from spruce was significantly higher than in the others. A higher number of individuals could be detected in the reaches with less dead wood from spruce. The controls showed a higher family diversity (Shannon diversity index) and the intact reach within the restored area had the highest index. Of the reaches treated, high pressure pump and hand tools were the method that resulted in the most varied biotope with its connection to the floodplain and alterations of habitats. This reach showed the highest numbers of taxa, individuals and Stoneflies among the restored reaches. The conclusion enlightens the differences in macroinvertebrate composition between the reaches with potential connection to attributes like amount and type of woody debris and substrate. The fact that the succession is at an early stage motivates a restudy in a couple of years to follow up the processes in the stream.