| Summary: | Climate change predictions indicate that increased future rainfall in the north of Europe will result in elevated land run off, thus allochthonous material transported to the Baltic Sea will increase. Consequently, compounds including humic substances, organic matter and inorganic nutrients (N and P), all with different biological availabilities, will be transferred to the sea. Such compounds will be incorporated differently into the food web, influencing primary (PP) and bacterial (BP) production, phytoplankton and zooplankton composition as well as food web efficiency (FWE), which are our 3 tested hypotheses. Riverine inflow was simulated by the addition of natural soil extracts (one from a southerly (Daugava) and one from a northerly (Öre) site on a daily basis to 2000 L indoor mesocosms (at Umeå Marine Sciences Centre). These mesocosms contained a natural food web collected from the Baltic Sea, including bacteria, phytoplankton and zooplankton; plus the addition of 7 juvenile fish (Perca fluviatilis). 12 mesocosms were divided into 4 different treatments where the difference between treatment and respective control was made by the addition of soil extract. Results showed that the input of soil matter from both sites created a lower PP:BP ratio, increased net heterotrophy in the systems, and in general resulted in changes to the phytoplankton and zooplankton community composition. Fish production and FWE was however only reduced in the Daugava treatment. Understanding changes in FWE and the structure of the food web will be vital for management of this system under future climatic conditions.
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