Introduced marine macroalgae and habitat modifiers : their ecological role and significant attributes

Abstract

Invasive, non-indigenous species (NIS) have become an increasing problem worldwide, with impacts on the diversity and ecosystem functioning of native communities. Marine invasive NIS also have a negative economical impact through increased abundance of toxic species, fouling of man-made underwater structures, and reduced recreational values of beaches. Only a small proportion of the NIS becomes invasive (i.e., having a negative ecological and/or economical impact), but once a species has been established much effort and resources are needed to remove it.

In the present thesis I discuss possible factors determining the success of macroalgal introductions and their impacts. A species of special concern in this thesis is the non-indigenous marine red alga Gracilaria vermiculophylla (Ohmi) Papenfuss, seen for the first time in the archipelago of Göteborg, Sweden, in the summer of 2003. Firstly, I highlight some positive and negative impacts caused by NIS as habitat modifiers. Secondly, I describe, by quantitative ranking, whether there are any common patterns of species traits increasing the likelihood of macroalgal NIS, introduced into a new area, becoming established and spread. In general, introduced and invasive species were ranked more hazardous than the native and non-invasive species introduced in Europe. Applying the quantitative species traits ranking on G. vermiculophylla rendered it among the most invasive red algae in Europe. Thirdly, I show the ability of G. vermiculophylla to withstand an emerged situation of more than five months, e.g. simulating transportation in a dredger or among fishing nets. The results indicate that G. vermiculophylla can easily survive long transportation in darkness such as in a ballast tank, and without being submerged in water. It also survived salinities down to 2 in a laboratory experiment, indicating that this species can survive in the innermost parts of the Baltic Sea (the Bothnian Bay). With the help of an event tree I illustrate the potential impact an establishment of G. vermiculophylla could have in the Baltic Sea. Fourthly, I show the distribution pattern within 150 km of the Swedish west coast in two years time for G. vermiculophylla. Furthermore, I describe the community associated with this species collected from Sweden, Denmark and the United States. In total, nearly 100 different taxa in twelve phyla were found associated with G. vermiculophylla. Finally, the impact of G. vermiculophylla on the native eelgrass, Zostera marina, was assessed using a modelling approach. The model output showed a negative effect on Z. marina already at low densities of G. vermiculophylla. This thesis contributes to a wider understanding of macroalgal introductions in general and of the ecology and ecophysiology of the invasive red alga G. vermiculophylla in particular. Such knowledge is important for management and stresses the importance of monitoring the Swedish coastline for early detection of NIS.