Integrated ecosystem impacts of climate change and eutrophication on main Baltic fishery resources

Abstract The Baltic Sea is a heavily impacted ecosystem with multiple pressures acting simultaneously. In order to quantify ecosystem impacts of integrated climate change and eutrophication pressures under constant high fishing pressure, and to support decision-making and policies in generating environmental and economic sustainable systems, the Baltic Atlantis holistic and mechanistic ecosystem model was applied. The overall aim was to run scenarios of separate and integrated impacts of climate and riverine nutrient load changes, taking into account the interactions of the full food web in the entire Baltic Sea. This was done to identify which of those two pressures will likely dominate the future of the Baltic Sea ecosystem, and to test effects of different riverine nutrient forcing sources as well as the Baltic Atlantis functions in relation to hydrographic spawning thresholds. By integrating the hydrography, the biology covering all trophic levels of the food web, and multiple pressures, i.e. eutrophication, climate change and fishery, we were able to evaluate relative impacts of 3 climate scenarios and 3 nutrient load scenarios, using two sources of nutrient forcing and predict likely trends in ecosystem effects. With focus on major fish stocks, our model, with its assumptions, indicated that nutrient loads are the main driver of the changes in the ecosystem as long as the hydrographic thresholds for spawning are not reached. If the thresholds are reached for the Baltic cod, climate change impact will become most important. Furthermore, higher nutrient loads resulted in cod decrease, and increase in sprat and herring. This effect is amplified by stronger climate change. Overall, it is of crucial importance for the future of the Baltic Sea fisheries and stocks that potential impacts are considered both separate and integrated in a dynamic ecosystem-based management approach.