Nitrogen and the Baltic Sea: Managing Nitrogen in Relation to Phosphorus

Abstract

The Baltic is a large, brackish sea (4 x 105 km2) extending from 54^ÅN to ~66^ÅN, with a fourfold larger drainage area (population 8 x 107). Surface salinity (2 to 8 PSU) and hence biodiversity is low. In the last century, annual nutrient loads increased to 10⁶ metric tons N and 5 x10⁴ ton P. Eutrophication is evident in the N-limited south, where cyanobacteria fix 2 to 4 x 10⁵ ton N each summer, Secchi depths have been halved, and O₂-deficient bottom areas have spread. Production remains low in the P-limited north. In nutrient-enriched coastal areas, phytoplankton blooms, toxic at times, and filamentous macroalgae reduce amenity values. Loads need to be reduced of both N, to reduce production, and P, to limit N-fixing cyanobacterial blooms. When large N-load reductions have been achieved locally, algal biomass has declined. So far, P loads have been reduced more than N loads. If this continues, a P-limited Baltic proper may result, very different from previous N-limited conditions. Reaching the management goal of halved anthropogenic N and P loads at minimum cost will require better understanding of biogeochemical nutrient cycles, economic evaluation of proposed measures, and improved stakeholder participation.