Nitrogen and phosphorus as production limiting factors in the estuarine waters of the eastern Gulf of Finland

The fate and effects of high nutrient loading from the River Neva - St. Petersburg (NW Russia) region was studied in the Neva Estuary and adjacent waters of the eastern Gulf of Finland (Baltic Sea) under late summer conditions. Special emphasis was given to the role of N and P as production limiting factors. The data originated from 5 cruises performed in 1990-1992 and include CTD measurements and spatial distributions of different nutrient fractions as well as nutrient depletion measurements onboard. The results revealed quick losses of both total and inorganic nutrients from the euphotic layer of the inner estuary under varying physical conditions. On the basis of nutrient depletion experiments, the very striking non-conservative behaviour of inorganic nutrients was explained by effective biological fixation of nutrients into planktonic biomass, followed by sedimentation. For total nutrients, physico-chemical loss processes might also be important in the salinity gradient prevailing in the inner estuary. The theoretical limiting nutrient for planktonic production varied according to the physical state of the water mass, especially to vertical mixing conditions. In general the inner estuary was P limited, while the open eastern Gulf was mainly N limited. In the outer estuary low mixed layer depth (MLD) favoured N limitation, while thick MLD (i.e. windy weather) favoured P limitation. This indicates that reduction of both N and P load into the eastern Gulf would improve water quality in this area. However, effective N removal would probably induce the most extensive improvement in the trophic status of the entire Gulf of Finland.