Particulate N and P characterizing the fate of nutrients along the estuarine gradient of the River Neva (Baltic Sea)

The estuary of the River Neva is eutrophied due to the high nutrient load from St. Petersburg. Previous studies have suggested that a large part of the N and P load is efficiently retained in the estuary. However, the fate of particulate N and P in the estuarial sea area has remained uncertain. Variations in the C, N and P sedimentation rate and in sediment organic matter (loss on ignition, LOI), total nitrogen (TN) and phosphorus (TP) were therefore studied in the recent sediment deposits along the estuarine gradient of the River Neva. The high gross sedimentation rate of N and P indicated efficient retention of both N and P in the inner estuary. An increase in the concentrations of settling particulate C and N along the estuarine gradient was consistent with the increase in the concentrations and burial of organic matter and TN in sediments. However, although the concentration of P in settling matter also increased along the estuarine gradient, a decrease was noted in the sediment TP concentration. This decrease correlated significantly with an increase in the concentration of sediment organic matter, but not with water depth along the estuarine gradient. The result indicates that an increase in organic matter and changes in hydrodynamic conditions along the estuarine gradient can reduce the capacity of the top 0e10 cm sediment layer to retain P. The average concentration of N in settling particulate matter was 23.3 mg g 1 DW and in the sediment surface 6.5 mg g 1 DW. Corresponding values for P were much closer to each other, that is, 3.3 and 2.8 mg g 1 DW, respectively. This suggests that, in the surface sediments (0e1 cm), settled P was more efficiently retained and/or accumulated than N. In contrast, in the 0e10 cm sediment layer N seemed to be more efficiently retained than P.