1. Baltic Sea Research Institute Warnemünde iow thomas.neumann@io-warnemuende.de iow Ecological consequences of different nutrient abatement strategies for the Baltic Sea – a model simulation study Thomas Neumann and Gerald Schernewski Baltic Sea Research Institute Warnemünde Eutrophication of the Baltic Sea is a serious problem Different abatement strategies are under discussion Ecosystem models are appropriate tools for testing different measures What are the ecological consequences of different abatement strategies?

2. Thomas Neumann iow Baltic Sea Semi-enclosed, restricted water exchange with the ocean Positive water balance due to river discharge Basin-wide gradient in salinity Strong halocline Anoxic areas in the deeper waters No tides Ice coverage Area: 412 560 km² Volume: 21 631 km³ Water residence time:25-30 years South-north-spread: ca. 1300 km West-east-spread: ca. 1000 km Average depth: 52 m Maximum depth: 460 m Catchment:1 734 000 km² Population (catchment)85 millions

3. Thomas Neumann iow Nitrate Phosphate The Problem In the last century the phosphorus input has increased eightfold and the nitrogen input fourfold Baltic Sea ecosystem has changed from an oligotrophic state into a mesotrophic state Sensitivity of the public to environmental issues of the Baltic Sea has increased

4. Thomas Neumann iow Ministerial declaration (riparian states) of 1988 set reduction of the nitrogen and phosphorus loads by 50% as a goal. (proportional approach) Because of different anthropogenic loading of the Baltic Sea rivers from an economic point of view this is not an optimal solution. => An alternative approach suggested by Gren (2000), is to achieve the 50% nutrient load reduction goal at minimum total costs. This implies that nutrient load reduction takes place in countries and drainage basins where it is most cost-efficient. (cost-effective approach) Solution?

5. Thomas Neumann iow RegionNitrogen % reduction Phosphorus % reduction Denmark4660 Estonia5410 Finland4132 Germany1555 Latvia6655 Lithuania5852 Poland5958 Russia5765 Sweden4219 After Gren in Gren, Turner & Wulff (eds.) (2000): Managing a Sea Total annual costs (million €) Proportional reduction:7500 Cost-effective reduction:1800 The cost-effective approach

6. Thomas Neumann iow Model simulations to explore different nutrient abatement strategies.

7. Thomas Neumann iow Circulation model based on MOM3 MOM3 application to the Baltic Sea 3 nm … 9 nm horizontal resolution 77 vertical layers 3 m … 6 m Open boundary condition towards the North Sea ERA-15 / ERA-40 meteorological forcing 15 Model rivers

8. Thomas Neumann iow NO 3 O2O2 P NH 4 Zoopl. Detrit. Fixation Uptake Mortality Grazing Recycling Settling Resuspen- sion Respira- tion Denitrifi- cation Atmosph. Input Nitrifica- tion N 2 Solar Radiation O 2 P. only N 2 Flagellates Blue-Greens Diatoms Sediment The Baltic Sea Ecosystem Model ERGOM

9. Thomas Neumann iow Model Experiments Simulation period 1979 – 1990 Control Run Proportional 50% river load reduction for N and P Cost-effective 50% river load reduction for N and P Example for a 3D simulation

10. Thomas Neumann iow Reduction after 12 years of simulation time based on an annual mean. Phytoplankton biomass is reduced especially near the river outlets Strong reduction for Diatoms Almost no reduction for Cyanobacteria Quicker response of nitrogen compared to phosphorus concentrations

11. Thomas Neumann iow Reduction along an offshore section Reduced phytoplankton biomass in the Gdansk Bay due to Diatoms and Flagellates reduction Enhanced phytoplankton biomass in the central part during summer due to cyanobacteria

12. Thomas Neumann iow Relative decrease of the inventories for the two load reduction scenarios black line: proportional reduction green line: cost-effective reduction Quick response of N compared to P Reduced N to P ratio and in turn enhanced Cyanobacteria blooms

13. Thomas Neumann iow Differences between the two abatement strategies In general the cost-effective approach produces lower concentrations in the southern part and higher concentrations in the northern part of the Baltic Sea. This is as expected because of the stronger load reduction in the southeastern drainage area. However, the differences are comparable small. Cyanobacteria are most sensitive. Cost-effective approach minus proportional approach compared to the control run

14. Thomas Neumann iow Differences at single stations Gdansk Deep Enhanced nitrogen around the redoxcline Further nutrient reduction with the cost-effective approach

15. Thomas Neumann iow Differences at single stations off Stockholm With the cost-effective approach especially the phosphate reduction is less compared to the proportional approach.

16. Thomas Neumann iow Differences at single stations off Stockholm The cost-effective approach favors cyanobacteria summer blooms in the northwester part of the Baltic proper compared to the proportional approach.

17. Thomas Neumann iow Summary and conclusions Riverborne nutrient load reduction yields lower nutrient concentrations and phytoplankton biomass. However, for at least a transition period enhanced cyanobacteria blooms can be expected. The offshore regions are more affected than the central regions. A cost-effective abatement strategy does not restrict the overall response of the Baltic Sea ecosystem. The uneven load reduction to different areas produce stronger effects in the southeastern part of the Baltic Proper compared to the proportional reduction. Spatial differences in the achieved reduction effect are less then 10% related to the control simulation.