Effects of excess phosphate on a coastal plankton community

Abstract

Eutrophication in the Baltic Sea has caused an imbalance in the inorganic nitrogen (N) to phosphorus (P) ratio, leaving excess phosphate (PO4) remaining after the phytoplankton spring bloom that terminates after N depletion. Here we investigated the fate of such excess phosphate in a two-week mesocosm (1.2 m3) experiment. The starting concentration of PO4was 0.66 µM, and treatments included a non-treated control (control), nitrate addition (N-add; 3.6 µM), glucose addition (C-add; 25 µM) and combined nitrate and glucose addition (N+C-add); all treatments done in triplicates. Addition of N (N-add and N+C-add) stimulated nano- and microphytoplankton, with the picophytoplankton abundance increasing after N depletion. Also the Copepod biomass was positively affected by the N-addition. N-fixing cyanobacteria were present but in low abundance. Carbon addition did not enhance heterotrophic bacterial uptake of PO4, nor affecting the phyto-or zooplankton community composition. The PO4concentration was reduced to ∼0.4 µM in the control and C-add treatments and to 0.16 µM in the two N-amended treatments, with an inorganic NP uptake ratio of 7.2. The results underscore the role of picophytoplankton in reducing the excess phosphate pool after the spring bloom, a function traditionally ascribed to bloom-forming diazotrophic cyanobacteria in the Baltic Sea.