2025/03/18 by Kristian Spilling, Mari Vanharanta, Mariano Santoro +4 · 1 voice
Earth and Planetary Sciences · Environmental Science · #Aquatic Ecosystems and Phytoplankton Dynamics #Isotope Analysis in Ecology #Marine and coastal ecosystems
paper · pdf · doi:10.1002/lno.70027
openalex publication_date 2025/03/18 · openalex created_date 2025/10/10 · openalex updated_date 2026/08/01
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.