Studies of the assimilation capacity and effects of nutrient load reductions in the eastern Gulf of Finland with a biogeochemical model

A model of the nitrogen and phosphorus biogeochemical cycles was applied to the Gulf of Finland for summer and autumn 1991 and validated with concurrent field observations. Estimates derived from modeled biogeochemical fluxes indicated that in August a relatively small easternmost area can assimilate up to 70%–100% of the nitrogen load from the St. Petersburg region. Phosphorus assimilation is equivalent to 160%–260% of the land load. In November, the nitrogen retention decreases to about 4%, while the phosphorus export exceeds the entire land input by 12% due to internal (sediment) loading. The nutrient load reduction scenarios imply that reduction of nitrogen load would result in locally reduced eutrophication and decreased nitrogen export into the open Gulf, while phosphorus reduction would increase assimilation of phosphorus imported into the easternmost area from the west.