Fredrik Wulff, Oleg P. Savchuk, Alexander Sokolov, Christoph Humborg and Carl-Magnus Mo ¨ rth Management Options and Effects on a Marine Ecosystem: Assessing the Future of the Baltic We ?1 are using the coupled models in a decision support system, NEST, to evaluate the response of the marine ecosystem to changes in external loads through various management options. The models address all the seven major marine basins and the entire drainage basin of the Baltic Sea. A series of future scenarios have been developed, in close collaboration with the Helsinki Commission, to see the possible effects of improved wastewater treatment and manure handling, phosphorus- free detergents, and less intensive land use and live stocks. Improved wastewater treatment and the use of phosphorus-free detergents in the entire region would drastically decrease phosphorus loads and improve the marine environment, particularly the occurrence of cya- nobacterial blooms. However, the Baltic Sea will remain eutrophic, and to reduce other effects, a substantial reduction of nitrogen emissions must be implemented. This can only be obtained in these scenarios by drastically changing land use. In a final scenario, we have turned 50% of all agricultural lands into grasslands, together with efficient wastewater treatments and a ban of phosphorus in detergents. This scenario will substan- tially reduce primary production and the extension of hypoxic bottoms, increase water transparency in the most eutrophied basins, and virtually eliminate extensive cyanobacterial blooms. INTRODUCTION Eutrophication of freshwaters as well as marine coastal areas and seas is now a world-wide phenomenon (1, 2) The same nutrient load reduction may have quite different effects on various marine systems depending on differences in physical, biogeochemical, and ecological properties. Likewise, a specific measure on lands may result in quite different results in terms of nutrient load to the sea. Since the 1970s, the Helsinki Commission (HELCOM), the intergovernmental organization responsible for overseeing the protection of the Baltic Sea environment, has adopted several recommendations to reduce pollution by nutrients. Since the late 1980s, HELCOM has been working to implement a 50% reduction target for nutrient emissions and discharges. Despite all efforts, there have been no appreciable effects on the overall nutrient loads, and no drastic improvements on the scale of the entire Baltic Sea ecosystem have occurred. (3, 4) The approach of setting nutrient reduction targets is now gradually being taken over by a general objective to reach good ecological status, following the Water Framework Directive of the European Union (5–7). One of the priority activities within HELCOM is now the use of models to assess the implications of different policy scenarios on nutrient inputs and the resulting eutrophication status to indicate the most cost-effective measures to be undertaken in the different subregions of the Baltic Sea. Most of the models considered in HELCOM work have so far been ecological models related to the assessment of effects to the sea. The basis of the effect models is the scenarios of activities on land. Therefore, it would be important to further consider the possibility to link management scenario models with ecological models to assess the focus of measures in the future work to reduce nutrient inputs. Integrated modeling has arisen as a valuable tool for environmental decision support (8–10), and models can be used to communicate to the public and policy-makers the linkages between parts of the systems and between natural and human- induced changes (11). Here, we will demonstrate how a decision support system for the Baltic region, NEST, can be used to provide scientific advice and management options for restoring the Baltic Sea from its current eutrophicated state (12, 13). ?2 MATERIALS AND METHODS We used a drainage basin model and a marine ecosystem model to develop the scenarios in this article. These models are integrated components of NEST, a system that is freely available via the Internet (http://www.mare.su.se/Nest).The drainage basin and the marine models are described in detail elsewhere in this issue of the journal (14, 15). The drainage basin model, CSIM, is a lumped hydrologic model based on a generalized watershed model originally developed to simulate water runoff, sediment, and nutrient fluxes from watersheds in the United States (16). ?3 CSIM and its predecessors have been used successfully in large watersheds of the US (17), as well as in other watersheds throughout the world (18). Our version of the model divides the Baltic watershed into 105 subbasins (Fig. 1) with a number of land-use categories and considers the loads from each category separately. Daily fluxes are aggregated to annual fluxes that are then the forcing data for the marine model within NEST. Nitrogen (N) and phosphorus (P) loads in stream flow are calculated as the product of water discharge and the specified nutrient concen- trations, with the addition of three types of sources, i.e., handling of manure as well as sewage in rural and urban areas, respectively. ?4 Sewage load is calculated from the distribution of urban and rural population, the degree of connection to wastewater treatment plants (WWTPs), and the effectiveness of their waste treatment. In the MARE’s NEST, the major mechanisms and symp- toms of the Baltic Sea eutrophication are quantitatively described by an aggregated biogeochemical marine model, SANBALTS, that on an annual scale simulates the coupled nitrogen and phosphorus cycles in the seven major basins (Fig. 1) presented as homogeneous boxes (15, 19, 20). ?5 In the model boxes, the dynamics of organic and inorganic nitrogen and phosphorus pools are driven by external inputs (from land, atmosphere, and Skagerrak), the major physical transports (advection and mixing), and the biogeochemical fluxes (primary production and nitrogen fixation, pelagic recycling, sedimenta- tion, outputs from the sediments, denitrification, and sediment burial). SANBALTS is validated with observed and estimated concentrations and fluxes for contemporary nutrient loads (15, 20) as well as for the conditions assumed to exist a century ago (20]). In this article, the SANBALTS is implemented in its steady-state mode, when the boundary conditions are given as Ambio Vol. 36, No. 2/3, Month 2007 1 Ó Royal Swedish Academy of Sciences 2007 http://www.ambio.kva.se