Picophytoplankton act as the primary consumers of excess phosphorus after the spring bloom in the eutrophic Baltic Sea

Abstract Eutrophication in the Baltic Sea has caused an imbalance in the inorganic nitrogen (N) to phosphorus (P) ratio, leaving excess phosphate (PO 4 ) after the phytoplankton spring bloom that terminates after N depletion. Using monitoring data, we demonstrated that the PO 4 concentration has continued to increase in the outermost Gulf of Finland during past decades. We further investigated the fate of such excess PO 4 in a two‐week mesocosm (1.2 m 3 ) experiment. The starting concentration of PO 4 was 0.66 μ M, and treatments included a non‐treated control (control), nitrate addition (N‐add; 3.6 μ M), glucose addition (C‐add; 36 μ M) and combined nitrate and glucose addition (N + C‐add). The addition of N, both in N‐add and N + C‐add treatments, stimulated nano‐ and microphytoplankton, while the picophytoplankton abundance increased after N depletion. Also, the copepod biomass was positively affected by the N addition. N 2 ‐fixing cyanobacteria were present but in low abundance. Carbon addition did not enhance heterotrophic bacterial uptake of PO 4 contrary to our expectations, nor did it affect the phyto‐ or zooplankton community composition. The PO 4 concentration was reduced to ~ 0.4 μ M in the control and C‐add treatments and to 0.16 μ M in the two N‐amended treatments, with an inorganic N : P uptake ratio of 6.7. These results underscore the role of picophytoplankton in reducing the excess PO 4 pool after the spring bloom, a function traditionally ascribed to bloom‐forming filamentous cyanobacteria in the Baltic Sea.