Computation of nutrient pools and fluxes at the entrance to the Baltic Sea, 1974–1999

Monthly time series of nutrient pools and transport in the Baltic Entrance were computed for the period 1974–1999 from a combination of hydrodynamic model results and observational data obtained in national monitoring programmes. Significant seasonal variations in both nutrient pools and transport were found. For example, inflowing dense nutrient-rich Skagerrak water causes net transport of dissolved inorganic phosphorus (DIP) and inorganic nitrogen (DIN) towards the Baltic during summer where surface concentrations are low. Surface-water concentrations of DIN and DIP increase during winter and the net transport may reverse. The computations indicated that the pools of dissolved inorganic phosphorus and total phosphorus (TP) were larger in the 1980s than in previous and following decades. Variations in pools of dissolved inorganic nitrogen (DIN) and total nitrogen (TN) were more irregular throughout the period. Computed nutrient transports likewise exhibited long-term variations. Net transport of DIN and DIP was southward, from the Skagerrak to the Kattegat and further into the Belt Sea, during the 1970s. In the same period, the transport of DIN and DIP from the Baltic Sea to the Baltic entrance was below average. In both the Kattegat and the Belt Sea transports reversed during the 1980s. In the 1990s the southward transport of DIP was restored, but the transport of DIN was not restored to the same extent. Between the Kattegat and the Skagerrak the transport diminished while the Belt Sea continued to export DIN to the Kattegat. Northward transports of TN and TP persisted throughout the period. A notable maximum TP transport was found to have taken place during the 1980s.