Role of sedimentation and resuspension of particulate matter in fluctuations of trophic status of the Neva Estuary

Primary production of phytoplankton, concentrations of chlorophyll a, suspended particulate organic and mineral matters and total phosphorus were measured in the Neva Estuary during the field summer expeditions in 2003-2007. Obtained data were compared with the results of earlier investigations. Impacts of natural and anthropogenic factors on the trophic status of the Neva Estuary have been analyzed. Relationships between biotic and abiotic environmental factors of natural and anthropogenic origin, which influence the trophic status of the Neva Estuary, have been analyzed. Analysis of the data shows that fluctuation of trophic status of the inner part of the Neva Estuary closely related with resuspension and sedimentation processes. Resuspension of bottom sediments during engineering works in the Neva Estuary probably lead to return of phosphorus to water column and to involvement of phosphorus into small biological cycle of elements. Moreover, according to our data, maximum rate of photosynthesis and concentration of chlorophyll a in the Neva Estuary were observed in zone of geochemical barrier, where fresh river waters mix with saline marine waters. The main part of suspended matter in the Neva Estuary deposited in the zone of geochemical barrier, which is characterized by the highest rate of photosynthesis. The investigations have shown that primary production of the Neva Estuary affected by sedimentation and resuspension of particulate matter and its transformation in a water column. Combine influence of natural and anthropogenic factors (resuspension and sedimentation of particulate matter) results in considerable fluctuation of the trophic status of the Neva Estuary during the last thirty years.