Nitrogen and the Baltic Sea: managing nitrogen in relation to phosphorus.
The Baltic is a large, brackish sea (4 x 10(5) km2) extending from 54N to approximately 66N, with a fourfold larger drainage area (population 8 x 10(7). Surface salinity (2 to 8 PSU) and hence biodiversity is low. In the last century, annual nutrient loads increased to 10(6) metric tons N and 5 x 10(4) ton P. Eutrophication is evident in the N-limited south, where cyanobacteria fix 2 to 4 x 10(5) 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.
Elmgren
R
R
Stockholm University, Department of Systems Ecology, Sweden. Ragnar.Elmgren@ecology.su.se
Larsson
U
U
D004777:Environment; D005068:Eutrophication; D005843:Geography; D008386:Marine Biology; D009584:Nitrogen / Q000032:analysis*; D009792:Oceans and Seas; D010758:Phosphorus / Q000032:analysis*; D012623:Seawater / Q000032:analysis*; D013548:Sweden; D014874:Water Pollutants, Chemical / Q000032:analysis / Q000592:standards
D014874:Water Pollutants, Chemical; D010758:Phosphorus; D009584:Nitrogen