From flatfish to sticklebacks: assemblage structure of epibenthic fauna in relation to macroalgal blooms

On the Swedish west coast, blooms of green macroalgae have become a common feature over recent decades. Assemblage structure of epibenthic fauna was studied in randomly selected shallow soft substratum bays, with and without macroalgae, using quantitative sampling techniques. The study was performed biannually (June and August) over 4 yr in 4 regions along the coast. Mat-forming macroalgae on average covered 35% of the surface area in vegetated bays, with an overall mean biomass of 100 g DW m(-2). The assemblage structure of epibenthic fauna was different in the presence of macroalgae, a pattern which was consistent over years and regions. Within the subset of bays containing vegetation, there were no bay-scale relationships between the biomass of algae and the abundance, biomass or number of species of epibenthic fauna. However, a detailed analysis revealed that the species had different responses to an increase in algal biomass at the scale of individual samples (0.5 m(2)). The flatfish Pleuronectes platessa occurred predominantly in samples completely free of vegetation. The shrimp Crangon crangon and the gobies Pomatoschistus spp. were found most commonly in the open sand habitat, and rapidly decreased in abundance with increasing biomass of algae. Gobius niger, Mysidacea, Anguilla anguilla, Sygnathus typhle and Palaemon spp. were most prevalent at a low to moderate biomass of algae. Sticklebacks (Pungitius pungitius and Gasterosteus aculeatus) increased in abundance with increasing algal biomass, and remained dominant at the highest level of algal biomass recorded. Thus, macroalgal blooms have the potential to change the structure and function of shallow soft substratum habitats, lowering their value as nursery and feeding grounds for commercial fish species.