Brownification Affects Phytoplankton Community Composition But Not Primary Productivity in Eutrophic Coastal Waters : A Mesocosm Experiment in the Baltic Sea

Climate change is projected to cause brownification of some coastal seas due to increased runoff of terrestrially derived organic matter. We carried out a mesocosm experiment (15 d) to test the effect of this on the planktonic ecosystem. The experiment was set up in 2.2 m 3 bags using four treatments, each with three replicates: control (Contr) without any manipulation and organic carbon additive HuminFeed (Hum; 2 mg L -1 ), inorganic nutrients (Nutr; 5.7 µM NH 4 and 0.65µM PO 4 ), and combined Nutr and Hum (Nutr+Hum) additions. Measured variables included organic and inorganic nutrient pools, chlorophyll a (Chla), primary and bacterial production and particle counts by flow cytometry. The bags with added inorganic nutrients developed a phytoplankton bloom that depleted inorganic N at day 6, followed by a rapid decline in Chla. Brownification did not reduce primary production although the light penetration was clearly lower in the Hum and Hum+Nutr treatments. With increasing light attenuation, we expect that it will at some point, and hypothesize that brownification has a non-linear effect on primary productivity. Bacterial production was lowest in the Contr, but similar in the three treatments receiving additions likely due to increased carbon available for heterotrophic bacteria. Picoeukaryotes clearly benefited by brownification after inorganic N depletion, which could be due to more effective nutrient recycling, or alternatively lower grazing pressure. In conclusion, brownification shifted the phytoplankton community composition towards smaller species with potential effects on carbon fluxes, such as sinking rates and export to the sea floor