Responses of coastal zoobenthos to stress induced by drifting algae in the Neva estuary

The present study focuses on destabilizing changes in macroinvertebrate communities in shallow stony-sandy littoral (depths of 0-1 m) of the Neva estuary, easternmost part of Baltic Sea caused by eutrophication phenomena (proliferation of fast-growing filamentous algae Cladophora glomerata, depletion in oxygen and enrichment in total phosphorus during the algae decomposition). Habitat characteristics (temperature, salinity, total phosphorus, and oxygen), biomass of attached and drifting C. glomerata, taxonomical composition, abundance and biomass of zoobenthos were studied since May to October 2003-2005. Also, horizontal distribution of macroinvertebrates in the shallow areas during period of maximum biomasses of drifting algae (in 2005) was studied in the shallow zone up to 60 m-distance from the shore. We conclude that coastal eutrophication phenomena (pollution of habitats by drift algae, temporal hypoxia) caused widespread mortality of macroinvertebrates resulting in shifts of structure and at least 10 times decrease in biomass of macroinvertebrates in shallow areas. Maximum effects were recorded at near-shore areas with high biomass (315-445 g dry weight m^-2) of drifting algae, while at the greater distances (30-60 m) the biomass of drift algae decreased, hypoxia was not recorded, and biomass of benthic macrofauna reached 24.3-30.8 g wet weight m^-2. Only species with opportunistic life strategy and high resistance to oxygenation are able to survive and dominate in coastal area of the Neva estuary with temporal oxygen depletion inducing by macroalgae decomposition.