Variation in Fucus vesiculosus associated fauna along a eutrophication gradient

Fucus vesiculosus (here Fucus) is a key species on rocky shores of the Baltic Sea, providing habitat for a diverse invertebrate community. While the invertebrate species within Fucus belts and their general distribution in the Baltic Sea are well known, much less is known about variation in invertebrate abundances and in community composition on different spatial scales. With requirements to monitor the status of marine habitats and com-munities, knowledge on species variation across environmental gradients is increasingly important. Knowledge on the abundances of different grazer species is also needed to evaluate the effects of fauna on Fucus on different spatial scales.In this study, a large-scale field campaign in seven geographically distinct areas along the Baltic Sea coast was conducted to assess variation in Fucus-associated invertebrate communities along eutrophication gradients (areas classified as either good, moderate or poor status). In addition, local scale exposure effects on invertebrate communities were studied. Eutrophication-related variation in invertebrate communities was found, with generally lower species abundances in the good status areas compared to areas in poor or moderate status. Especially in poor status areas, Fucus hosted high abundances of gammarids and chironomids. No specific taxa clearly functioned as an indicator for the eutrophication status and Pielou's evenness was similar across studied areas. However, lower Shannon diversity and number of taxa were found in areas where the status of Fucus is poor. Furthermore, abundances of some taxa, e.g. Idotea spp. isopods and Theodoxus fluviatilis snails were lower in areas where the non-indigenous crab Rhithropanopeus harrisii has been observed. On a local scale, exposed sides of small islands had less T. fluviatilis. This study took the first step to unravel spatial patterns in Fucus- associated invertebrate fauna, and the results illustrate how different environmental and biotic drivers shape these communities both on large and local scales.