The phosphorus (P) concentration is increasing in parts of the Baltic Sea following the spring bloom. The fate of this excess P-pool is an open question, and here we investigate the role of microbial degradation processes in the excess P assimilation phase. During a 17-day-long mesocosm experiment in the southwest Finnish archipelago, we examined nitrogen, phosphorus, and carbon acquiring extracellular enzyme activities in three size fractions (<0.2, 0.2-3, and >3 µm), bacterial abundance, production, community composition, and its predicted metabolic functions. The mesocosms received carbon (C) and nitrogen (N) amendments individually and in combination (NC) to distinguish between heterotrophic and autotrophic processes. Alkaline phosphatase activity occurred mainly in the dissolved form and likely contributed to the excess phosphate conditions together with grazing. At the beginning of the experiment, peptidolytic and glycolytic enzymes were mostly produced by free-living bacteria. However, by the end of the experiment, the NC-treatment induced a shift in peptidolytic and glycolytic activities and degradation of phosphomonoesters toward the particle-associated fraction, likely as a consequence of higher substrate availability. This would potentially promote retention of nutrients in the surface as opposed to sedimentation, but direct sedimentation measurements are needed to verify this hypothesis.

Centre for Coastal Research University of Agder Universitetsveien 25 4604 Kristiansand Norway

Department of Biological Oceanography Leibniz Institute for Baltic Sea Research Warnemünde IOW Seestrasse 15 18119 Rostock Germany

Department of Fisheries Oceanography and Marine Ecology National Marine Fisheries Research Institute ul Kołłątaja 1 81 332 Gdynia Poland

Department of Plankton and Microbial Ecology Leibniz Institute for Freshwater Ecology and Inland Fisheries Alte Fischerhütte 2 OT Neuglobsow 16775 Stechlin Germany

Department of Plant Physiology Institute for Biosciences University of Rostock Albert Einstein Str 3 18059 Rostock Germany

Institute of Biology and Biochemistry Potsdam University Maulbeerallee 2 14469 Potsdam Germany

Laboratory of Anoxygenic Phototrophs Institute of Microbiology Czech Academy of Sciences Novohradská 237 Opatovický mlýn 379 01 Třeboň Czech Republic

Marine and Freshwater Solutions Finnish Environment Institute Latokartanonkaari 11 00790 Helsinki Finland

Tvärminne Zoological Station University of Helsinki J A Palménin tie 260 10900 Hanko Finland

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