Factors affecting the role of macrobenthos in pelagic-benthic coupling in the Neva Bay (eastern Gulf of Finland)

Long-term fluctuations in composition, biomass and functional patterns of benthic animal communities in Neva Bay, which became freshwater artificial lagoon in the middle 1980s due to construction of flood protection barrier (Dam), are depended on both natural and anthropogenic factors. The influence of natural climatic induced factors is evinced by periodic high increase in zoobenthic biomass with a dominance of small mollusks, Pisidiidae, in the eastern part of the bay near the delta of Neva River (the area with highest sedimentation rate of particulate organic matter) for the years of high run-of of Neva River. It is periodically succeeded by dominance of large mollusks, Unionidae, in the other parts of Neva Bay in the years of low run-of of Neva River. Progressive decrease in abundance of glacial relicts, Pallasea quadrispinosa, and increase of Oligochaeta worms was observed since the beginning of 20
^th
Century probably due to an increase of eutrophication and organic pollution in the bay. Functional role of zoobenthic communities in decomposition of organic matter and regeneration of phosphorus in the bay was very high at the beginning of 1980psilas in the period of high river run-of. In that time their food consumption was much higher than primary production in Neva Bay and they decomposed the main portion of the particulate organic matter brought by the Neva River waters. The role of zoobenthos in decomposition of organic matter has decreased during the last two decades in a period of low run-of due to the shift of the dominance of small Pisidiidae to large Unionidae and increase of plankton primary production in the bay. Large scale digging and dumping of bottom sediments dealing with constructions of new lands and ports in the eastern part of Neva Bay, which began in 2006 and resulted in distribution of great amount of suspended particulate matters over the whole bay, negatively affects bottom animal communities decreasing their role in ldquomarginal filterrdquo of the Neva Estuary.