From local measures to regional impacts : Modelling changes in nutrient loads to the Baltic Sea

Study Region: Our study region is the Baltic Sea Drainage Basin (BSDB), which covers an area of 1.8 Mio km2 distributed over 14 countries in northern Europe. Study Focus: We use a large-scale hydrological and nutrient transport model (E-HYPE) to model basin-wide impacts of measure scenarios on the Baltic Sea, where eutrophication is a critical issue for the marine ecosystem. We constructed measure scenarios based on stakeholder acceptance, established in workshops in different regions around the Baltic. These measures include local stream reach to catchment scale measures aiming to reduce nutrient transport into the stream network (buffer strips, stormwater ponds) and measures aiming to reduce regional nutrient source releases (fertiliser leaching rates, rural household emissions). New Hydrological Insights for the Region: Nutrient load reductions are often needed to reduce eutrophication and improve overall surface water quality in fresh-water and enclosed bays and seas, where dilution is limited and load emissions have long residence times. To reduce riverine nutrient loads, remediation measures are necessary, e.g. establishment of buffer strips or improved wastewater treatment. Such measures are, however, typically not designed to target nutrient load reductions at sea outlets, but rather focus on local improvements. Here, we show that measures, notwithstanding other meliorating ecosystem benefits, must include reductions in load emissions across large basin areas to impact integrated net loads into coastal sea basins, because reduction measures that only target nutrient delivery to the stream network typically cannot be implemented in a significant enough proportion of the total drainage area of large coastal river basins. Our impact scenarios show BSDB-wide nutrient reductions of up to 9 % for nitrogen and phosphorus compared to a reference scenario, if load emissions are reduced in the scenario assumptions.Funding Agencies|BONUS, the joint Baltic Sea research and development programme (Art 185) from the European Unions Seventh Framework Programme for research, technological development and demonstration; BONUS, the joint Baltic Sea research and development programme (Art 185) from Innovation Fund Denmark; Swedish Environmental Protection Agency (Naturvardsverket); Polish National Centre for Research and Development; German Federal Ministry of Education and Research (BMBF)Federal Ministry of Education & Research (BMBF); Russian Foundation for Basic Research (RFBR)Russian Foundation for Basic Research (RFBR); Latvian Ministry of Education and ScienceLatvian Ministry of Education and Science; Projekttradger JulichHelmholtz Association; Swedish Research Council for Environment, Agricultural Sciences and Spatial Planning (FORMAS)Swedish Research Council Formas; Academy of FinlandAcademy of FinlandEuropean Commission; BONUS project Mediating integrated actions for sustainable ecosystems services in a changing climate (MIRACLE) ; BONUS project Reducing nutrientloadings from agricultural soils to the Baltic Sea via groundwater and streams (SOILS2SEA)