Alternative assessments of large scale eutrophication using ecosystem simulations : hind-casting and scenario modelling

The Marine Strategy Framework Directive (MSFD) aims to achieve Good Environmental Status (GES) of the EU's marine waters by 2020, and to protect the marine resources upon which economic and social activities depend. The progress achieved in marine modelling gives the possibility of more realistic simulations of many aspects of the marine environment. Therefore, now the use of marine modelling can support the assessment process of the marine environment as foreseen in the MSFD by defining baselines, addressing data gaps and allowing for scenario simulations. We are here focusing on demonstrating the usefulness of ecosystem model data for assessing eutrophication aspects, as covered by MSFD descriptor D5. The assessments are based on calculating indicators, namely first the long established trophic indicator TRIX and for comparison the more recent HEAT indicator (as applied by HELCOM). We show that the use of ecosystem model data allows identifying sensitive areas and assessing long term trends in the development of eutrophication in 2 major European water bodies. Specifically strong spatial gradients from the open sea to the coast are detected in many variables and indicators. The available high resolution of the simulations allows the identification of such spatial gradients. The investigation of long term trends points to slightly increasing eutrophication problems in the Mediterranean Sea and the Baltic Sea. This increasing eutrophication trend seems to be caused by increasing nitrate concentrations in the Mediterranean Sea. In contrast, in the Baltic Sea the increase in TRIX and HEAT indicators seems to be due to increasing phosphate concentrations. We performed scenario simulations for investigating the impact from changing climate variability and from reducing nutrient inputs in the Mediterranean Sea. According to the model results, reduced climate variability (by using climatological atmospheric forcing) would lead to increasing eutrophication problems in many coastal regions and especially in the Aegean Sea. The proposed nutrient reduction scenario achieves surprisingly minor overall improvements, which are clearly identifiable only in the Adriatic Sea and the Aegean Sea, regions actually suffering from the most pronounced nutrient inputs. Finally possible methodological improvements and a way forward are discussed. We conclude that further nutrient reductions in the Mediterranean Sea and in the Baltic Sea will be necessary to reduce the eutrophication impact on marine and coastal ecosystems. However, it seems illusionary to aim at fully restoring past ecosystems, rather ecosystem management should develop iterative adaptation strategies to deal with shifting baselines and to maintain ecosystem services at a sustainable level.