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Eutrophication mitigation - modelling the effects on vegetation and fish

Eutrophication in the coastal zone reduces water clarity and thus has important effects on the distribution of many species. It is a pressing problem in the Baltic, and the HELCOM member states have committed to take extensive measures to reduce eutrophication through the Baltic Sea Action Plan (BSAP).

In PREHAB, we have explored how predictive mapping can be used to assess the ecological effects of changing eutrophication levels in a number of scenarios relating to mitigation measures. Using models involving water transparency (Secchi depth), which is the main status indicator for eutrophication in the BSAP, we evaluated effects on the distribution of key species in the Finnish-Swedish archipelago in the Northern Baltic Proper.

Species respond differently
The results illustrates the utility of habitat modelling for exploring ecosystem responses to management actions. Apart from revealing patterns which are important both from a biological and socio-economic perspective, the highly species-specific responses demonstrate the absolute need to put management actions in a spatial context when evaluating alternative measures.

Three scenario situations

We specified three situations - subscenarios - relating to changes in water transparency, based on the BSAP target and reference levels as well as analyses of Secchi depth trends in the study area:

1. Business as usual
As trends in available data suggest that the situation is on average getting slowly worse in the study area, the business as usual scenario is set to a decrease in water transparancy by 10 percent.

2. BSAP reference level
The BSAP reference level is based on historical data, corresponding to a 48 percent increase in water transparency compared to the current situation in the the Baltic proper.

3. BSAP target level
The target level is set 25 percent "worse" than the BSAP reference level, corresponding to an 11 percent increase in water transparency compared to the current situation for the Baltic Proper.

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