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Mitigating eutrophication - modelled effects on biodiversity

The scenario analyses show that the effect of a decrease in eutrophication - i.e. increasing Secchi depth - differ among species. The predicted changes will have direct effects on the ecosystem goods and services provided by the species.

We analysed the consequences of changing eutrophication levels on habitat areas for bladderwrack (Fucus vesiculosus) and eelgrass (Zostera marina), and recruitment areas for perch (Perca fluviatilis) and pikeperch (Sander lucioperca). For bladderwrack and eelgrass, water transparency (Secchi depth) has a direct effect on the distribution through light limitation. For perch and pikeperch it is known to affect foraging success as well as predation rates and thereby ultimately the distribution of the species.

The scenario analyses show that recruitment areas of perch will increase strongly after a decrease in eutrophication, while recruitment areas of pikeperch are predicted to decrease (fig. 1). For vegetation, increased water transparency is predicted to cause an increase in the areas suitable for bladderwrack, while eelgrass is largely unaffected (fig. 2).

Scenario based effects on fish from mitigating eutrophicationFigure 1. PREHAB scenario analyses predict that reducing eutrophication – i.e. increasing water clarity – will increase recruitment areas for perch. For pikeperch, the analyses predict the opposite effect.

Scenariobased effects from mitigating eutrophication on vegetationFigure 2. PREHAB scenario analyses predict that reducing eutrophication will cause an increase in the areas suitable for bladderwrack, while eelgrass will be largely unaffected.

Effects on ecosystem goods and services

The predicted changes in species distributions will have direct effects on the ecosystem goods and services provided by the species. Bladderwrack and eelgrass form the basis of diverse biotopes with great importance for coastal productivity. Perch and pikeperch are predators with a key role in the coastal ecosystem, and at the same time they are two of the most valuable species for commercial and recreational fisheries.

The observed effects correspond well to what is known about how these species are affected by water transparency, and the responses also agree with historical data on the distribution of the species. The responses differed not only between species, but also between methods. Quantifying these methodological errors inherent in the modelling is crucial for measuring and communicating the precision of the predictions to stakeholders and policy makers.

Predicting biodiversity from Secchi depth

We chose to use Secchi depth as a predictor variable in our management scenarios on eutrophication, because:

  • It is the primary indicator for eutrophication in the BSAP.
  • It has a direct impact on many organisms, including both plants and fish.
  • There is plenty of field data on Secchi depth, including historical data.
  • It is possible to construct maps of Secchi depth at a fairly high resolution using either modelling or remote sensing methods.

Indicator for a healty Baltic Sea

The BSAP (Baltic Sea Action Plan) has defined indicators that “describe the characteristics of a Baltic Sea which is unaffected by eutrophication”. The primary indicator for eutrophication is summer time Secchi depth (June-September). This indicator reflects the ecological objective “Clear water”. The plan also sets specific reference and target levels for the indicators. The reference levels are based on historical data while the target levels are set 25 percent “worse” than the reference level.

More about the BSAP

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