Effects of nutrient emissions and their reduction on phytoplankton in a Baltic bay, himmerfjärden, a 25-year study

Ragnar Elmgren and Ulf Larsson
Department of Systems Ecology
Stockholm University
SE-10691 Stockholm
Sweden

The ecological effects of the nutrient discharge of the Himmerfjärden STP (now serving 250 000 people) was followed in 1976-2000. Particular emphasis is given the introduction of 90% nitrogenc from mid-1997, studied in the MISTRA research programme Sustainable Coastal Zone Management (SUCOZOMA). Pre-reduction measurements from 1977-1996 showed that the large nitrogen load generally made phytoplankton in inner Himmerfjärden Bay phosphorus-limited, except in late summer, when sediment release of phosphorus caused nitrogen limitation. Following N-reduction in the STP, nitrogen concentrations decreased markedly in the Bay, giving lower phytoplankton biomass (as chlorophyll a) both during the spring bloom and the summer, and as annual averages, but there were only a minor reduction in Secchi depth (primarily during spring). No clear effect on oxygen consumption in the deep water can yet be discerned. Phytoplankton limitation by N, or N and P co-limitation, has now become dominant in the bay, with increasing nitrogen limitation towards the open sea, which is N-limited, except during summer blooms of nitrogen-fixing cyanobacteria. As predicted, nitrogen-fixing cyanobacteria blooms appeared in the bay system after reduction of the N load. These blooms are probably phosphorus-limited, but other phytoplankton seem to be co-limited by N and P during the blooms. Their stoichiometry shows that filamentous nitrogen-fixing cyanobacteria are less severely phosphorus limited in Himmerfjärden bay than in the open Baltic, and their stable nitrogen isotope ratios show that they are as dependent on fixed nitrogen in the Bay as in the open sea. Ongoing studies include experiments where more nitrogen is released for a period in late spring-early summer, to keep the availability of N and P balanced near the Redfield ratio of 16 (by atoms), and hence prevent or reduce the development of cyanobacterial blooms in the bay system.