The filamentous green alga Cladophora glolmerata as a habitat for littoral macro-fauna in the northern Baltic Sea

Opportunistic filamentous algae are known to increase due to elevated nutrient levels (eutrophication) and to outcompete slow-growing perennial macroalgae in shallow coastal areas causing severe shifts in algal communities and habitat changes for the associated fauna. When the algae detach from the primary substrate and form sublittoral drifting mats, oxygen deficiency occurs frequently. We investigated the temporal succession of macrofauna in attached and loose-lying Cladophora glomerata to establish the role of this transient substrate. We also conducted a series of aquarium experiments using clean sea-water and water from fresh and decaying C. glomerata to study preferences among juvenile and adult crustaceans (Gammarus spp. and Idotea baltica). Both attached and loose-lying littoral Cladophora glomerata hosted rich macrofauna assemblages with significantly higher faunal abundance and biomass among the attached algae and clear temporal trends. The dominant taxa were the gastropods Hydrobia spp., the blue mussel Mytilus edulis, the crustaceans Gammarus spp. and Idotea baltica, as well as chironomids and oligochaetes. Aquarium experiments demonstrated that juvenile and adult I. baltica and juvenile Gammarus spp. preferred clean water and water from fresh algae to water from decomposing algae, whereas adult Gammarus spp. were indifferent. Our results demonstrate that both attached and drifting C. glomerata serve as useful habitats for littoral macrofauna.