Understanding human impact on the Baltic ecosystem: Changing views in recent decades

Grave environmental problems, including contamination of biota by organochl orines and heavy metals, and increasing deep-water oxygen deficiency, were discovered in the Baltic Sea in the late 1960s. Toxic pollutants, including the newly discovered PCB, were initially seen as the main threat to the Ba ltic ecosystem, and the impaired reproduction found in Baltic seals and whi te-tailed eagles implied a threat also to human fish eaters. Countermeasure s gradually gave results, and today the struggle to limit toxic pollution o f the Baltic is an international environmental success story. Calculations showed that Baltic deep-water oxygen consumption must have increased, and t hat the Baltic nutrient load had grown about fourfold for nitrogen and 8 ti mes for phosphorus. Evidence of increased organic production at all trophic levels in the ecosystem gradually accumulated. Phosphorus was first though t to limit Baltic primary production, but measurements soon showed that nit rogen is generally limiting in the open Baltic proper, except for nitrogen- fixing cyanobacteria. Today, the debate is concerned with whether phosphoru s, by limiting nitrogen-fixers, can control open-sea ecosystem production, even where phytoplankton is clearly nitrogen limited. The Baltic lesson tea ches us that our views of newly discovered environmental problems undergo r epeated changes, and that it may take decades for scientists to agree on th eir causes. Once society decides on countermeasures, it may take decades fo r them to become effective, and for nature to recover. Thus, environmental management decisions can hardly wait for scientific certainty. We should th erefore view environmental management decisions as experiments, to be monit ored, learned from, and then modified as needed.