Trophic ecology of meiofauna : Response to sedimentation of phytoplankton blooms in the Baltic Sea

Abstract: Marine soft sediments are the second largest habitat on Earth. How animal communities in this habitat are structured is a central issue in marine ecology. Food is an important limiting factor for many benthic populations, and settling organic matter from phytoplankton blooms is of vital importance to them. This thesis discusses the effects of settling phytoplankton blooms on benthic meiofaunal populations in the Baltic Sea and how species interactions affect the fate of settled organic matter. Eutrophication in the Baltic Sea has altered phytoplankton community dynamics, with indications that toxin-producing cyanobacterial blooms may reach the benthos in greater quantity than previously. Paper I found that meiofauna feed on settled cyanobacteria, yet suffer no increase in mortality. However, growth of meiofauna is significantly slower on a diet of cyanobacteria than when fed spring bloom diatoms, indicating that the studied cyanobacteria are nutritionally poor (Paper II). In Paper III we found that the presence of macrofauna reduces the access of meiofauna to settled organic matter, presumably through interference competition that increases when several macrofauna species are present. We also found that meiofaunal populations influence the provision of ecosystem services by benthic microbes. Paper IV shows that when meiofauna is abundant, mineralization of organic matter is positively affected, presumably through facilitation mechanisms. In contrast, paper V reports that degradation of the contaminant naphtalene decreases significantly at high meiofauna abundance.In conclusion, this thesis shows that type and quality of organic matter available, as well as competition from macrofauna, affect how meiofauna grow and incorporate nutrients. Furthermore we found meiofauna to be an important functional component of the benthic ecosystem, with marked effects on ecosystem processes such as nutrient regeneration and contaminant degradation.