Long‐term changes in macroalgal communities along ferry routes in a northern Baltic archipelago

Abstract

Species composition, abundance and biomass of littoral macroalgae were studied along a ferry route in the and archipelago, northern Baltic Sea. Two areas, originally studied in the 1970s, were revisited in 1999 and 2000. In one area the traffic has strongly decreased since the 1970s, in the other the frequency of the traffic has remained the same, but the size of the ferries has increased. During the revisit the highest number of species was found close to the ferry route, in contrast to the results of the original study. In the area with decreased traffic only minor shifts in species composition were observed. In the heavily trafficked area, red algae in particular, had increased close to the route. New species recorded were Polysiphonia fibrillosa, P.fucoides and Aglaothamnion roseum. On sheltered sites in this area, red and brown algae had decreased as a result of increased sedimentation and occurrence of drift algae, and were replaced by rooted macrophytes (e.g. Zannichellia sp, Ranunculus peltatus ssp. baudotii, Myriophyllum spicatum). In the 1970s a Fucus vesiculosus‐belt was present at all the 14 studied sites. During the revisit F.vesiculosus was belt‐forming at only 5 sites, mainly close to the ferry route in the heavily trafficked area. The biomass of Cladophora glomerata had increased and the occurrence of F.vesiculosus had decreased since the 1970s. The main reason to the documented changes is the ongoing eutrophication of the Baltic Sea. Close to the ferry route regular strong water movements seems to have a positive local effect on mainly perennial macroalgae by keeping the bottoms free from sediments and drift algae.