Long-term temporal and spatial trends in eutrophication status of the Baltic Sea

Much of the BalticSea is currently classified as 'affected by eutrophication'. The causes for this are twofold. First, current levels of nutrient inputs (nitrogen and phosphorus) from human activities exceed the natural processing capacity with an accumulation of nutrients in the BalticSea over the last 50-100 years. Secondly, the BalticSea is naturally susceptible to nutrient enrichment due to a combination of long retention times and stratification restricting ventilation of deep waters. Here, based on a unique data set collated from research activities and long-term monitoring programs, we report on the temporal and spatial trends of eutrophicationstatus for the open BalticSea over a 112-year period using the HELCOM EutrophicationAssessment Tool (HEAT 3.0). Further, we analyse variation in the confidence of the eutrophicationstatus assessment based on a systematic quantitative approach using coefficients of variation in the observations. The classifications in our assessment indicate that the first signs of eutrophicationemerged in the mid-1950s and the central parts of the BalticSea changed from being unaffected by eutrophicationto being affected. We document improvements in eutrophicationstatus that are direct consequences of long-term efforts to reduce the inputs of nutrients. The reductions in both nitrogen and phosphorus loads have led to large-scale alleviation of eutrophicationand to a healthier BalticSea. Reduced confidence in our assessment is seen more recently due to reductions in the scope of monitoring programs. Our study sets a baseline for implementation of the ecosystem-based managementstrategies and policies currently in place including the EU Marine Strategy Framework Directivesand the HELCOM BalticSea Action Plan.