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 ad jacent waters of the eastern Gulf of Finland (Baltic Sea) under late s ummer conditions. Special emphasis was given to the role of N and P as production limiting factors. The data originated from 5 cruises perfo rmed in 1990-1992 and include CTD measurements and spatial distributio ns of different nutrient fractions as well as nutrient depletion measu rements onboard. The results revealed quick losses of both total and i norganic nutrients from the euphotic layer of the inner estuary under varying physical conditions. On the basis of nutrient depletion experi ments, the very striking non-conservative behaviour of inorganic nutri ents was explained by effective biological fixation of nutrients into planktonic biomass, followed by sedimentation. For total nutrients, ph ysico-chemical loss processes might also be important in the salinity gradient prevailing in the inner estuary. The theoretical limiting nut rient for planktonic production varied according to the physical state of the water mass, especially to vertical mixing conditions. In gener al the inner estuary was P limited, while the open eastern Gulf was ma inly N limited. In the outer estuary low mixed layer depth (MLD) favou red N limitation, while thick MLD (i.e. windy weather) favoured P limi tation. This indicates that reduction of both N and P load into the ea stern Gulf would improve water quality in this area. However, effectiv e N removal would probably induce the most extensive improvement in th e trophic status of the entire Gulf of Finland.