The Baltic Sea is heavily affected by eutrophication, with ambitious nutrient load reduction schemes inplace to reduce algal growth and the secondary effects of increased productivity, such as increasedturbidity and the spread of anoxic bottoms. Forcing an Ecosim model of the Central Baltic food web withobserved and predicted productivity trends suggest that changes in primary production channel stronglyinto the pelagic food web, modifying the predation pressure on phytoplankton grazers.To include the feedback of higher trophic levels onto primary producers into simulations, we coupled theFortran version of Ecopath with Ecosim (EwE-F) to the BALTSEM biogeochemical model of the BalticSea. In their coupled code, both models share phytoplankton and detritus components: BALTSEMcalculates phytoplankton growth based on nutrient concentrations and available light; and as a loss term,EwE-F provides predation mortality of phytoplankton that is calculated applying the foraging arena theory.Similarly, EwE-F components produce inputs to BALTSEM sediment and suspended detritus variables,such as non-predation mortality or unassimilated food, and use the BALTSEM detritus variables as a foodresource.Since nutrient turnover within the Baltic pelagic food web is about ten times as large as the riverine inputof nitrogen and phosphorus, small imbalances in simulated nutrient fluxes can generate sources or sinksthat easily reach the magnitude of planned nutrient load reductions. To ensure closed nutrient cycles, wetrack the nutrient intake for each consumer based on fixed prey stoichiometries. We have introducedcompensatory nitrogen and phosphorus fluxes into EwE-F that describe the excretion of nutrients that arenot incorporated into predator growth, and included nutrient limitation functions for EwE-F consumers.Coupling the zero-dimensional EwE-F food web model to the one-dimensional, vertically resolvedBALTSEM biogeochemical model creates vertical fluxes between the BALTSEM depth layers. We willtherefore also discuss strategies to describe the transfer of carbon and nutrients between the food weband the biogeochemical model
