Modeling of eutrophication processes in the Gulf of Finland during 1991–2010

The hydrodynamic model GETM coupled with the ecosystem model ERGOM was applied to the Baltic Sea area for the period 1991-2010. The first part of the modeled period is characterized by considerable socioeconomic changes in the area that are generally considered favorable for the status of coastal marine ecosystems due to a decrease in external nutrient loads. Model results were used to describe and analyze changes in salinity, nutrient (nitrate and phosphate) and oxygen dynamics in the central part of the Gulf of Finland, the most eutrophied sub-basin of the Baltic Sea, over the last two decades. Model validation showed very good agreement between modeled and measured parameters in the Gulf. In the surface layer nitrates were slightly underestimated, while phosphates were overestimated during the second half of the modeled period. Near-bottom parameters were generally accurately reproduced. Model results show that in fact the eutrophication status of the Gulf of Finland has worsened during the last two decades, especially during the latter part of the modeled period. This is reflected in an increased frequency of anoxic events and consecutive high phosphate loading and decrease of near-bottom nitrate concentrations. Stronger stratification favors anoxic conditions in the near-bottom, while a decrease in salinity induced by mixing processes allows for oxygenation of deep areas. However, the effect of these mixing events is short-term and deep water quickly returns to the anoxic state.