Resolving the Baltic Sea into seven subbasins: N and P budgets for 1991–1999

Total nitrogen and phosphorus budgets based on more than 32,000 oceanographic stations have been constructed for the first time for all seven major marine basins within the Baltic Sea, including the Kattegat. The budgeted time interval, 1991–1999, comprises both extreme and more typical states of the Baltic Sea. The budgets were compared with published independent estimates of water flows and biogeochemical fluxes to ensure consistency. The budgets show that the Baltic Sea annually exported to Skagerrak masses of nutrients equivalent to 15% of nitrogen and 45% of phosphorus inputs from land and atmosphere during the period studied. The high phosphorus export during this decade is explained by an internal phosphorus source in the Baltic Proper of 18,000 tons released annually from anoxic sediments. A network flow analysis, which demonstrates how changing the loads to each individual basin changes the total nutrient inputs into all basins, indicates that the Baltic Proper is more susceptible to such changes than any other subbasin of the Baltic.