Uptake of excess phosphate at low inorganic N:P ratio in a coastal sea afflicted with eutrophication

Abstract

Following the spring bloom in the northern Baltic Sea, nitrogen limits phytoplankton growth and there is typically a residual phosphate concentration (>0.2 µmol l-1) remaining that is often assumed to induce the recurring blooms of nitrogen-fixing cyanobacteria. However, these cyanobacterial blooms typically occur 2-3 mo later during the summer, when the phosphate concentration has been depleted, and it is unclear what organisms take up the excess phosphate. We studied the removal potential of excess phosphate (0.55 µmol l-1) at different temperatures (10, 13, 16°C) and with or without nitrogen addition in an indoor 20 l tank experiment. In addition, we followed the element pools and plankton community composition. As expected, the phosphate uptake rate was up to 3-fold faster in nitrogen-amended than non-amended tanks, but complete drawdown of phosphate also occurred under severe nitrogen limitation. The uptake ratio of dissolved inorganic nitrogen to phosphorus was 4.6, which is substantially lower than the Redfield ratio (16) and indicates excessive phosphate removal potential relative to nitrogen. A large part of the excess phosphate ended up in the particulate pool, which has a higher potential to sink out from the surface. The nitrogen-fixing cyanobacteria Nodularia spumigena grew close to summer bloom concentrations only in the highest experimental temperature. However, the combined biovolume of all 3 major bloom-forming cyanobacteria accounted for only 5.3% of the total autotropic biovolume, and their potential phosphate uptake was calculated to be <3% of the excess phosphate available at the beginning of the study. Therefore, our results demonstrate that the contribution of filamentous cyanobacteria to the removal of excess phosphate is small.