Effects of management and climate change — transitional and coastal waters (workpackage 5.3)
After the detrimental effects of eutrophication on the integrity of marine waters became apparent, many countries in Europe initiated plans for reducing nutrient inputs from land and atmosphere in the late 1980s and 1990s. Coastal ecosystems and transitional waters having experienced such changes in nutrient inputs are used to assess the effect of reduction efforts. This will be pursued through statistical analysis of time trajectories of ecosystem responses vs. pressures and through mechanistic modelling of two specific sites, the Nervion Estuary in Spain and Helsinki Bay in Finland.
The workpackage addresses the combined effects of nutrient reduction and climate change on the ecological status of transitional and coastal waters. Through development of quantitative tools the reduction required under projected temperature increases (e.g., due to global warming) to achieve good ecological status will be assessed, together with estimations of uncertainties associated with such calculations.
Our results show that coastal ecosystems are resilient and may exhibit regime shifts suggesting that reversing the effects of eutrophication may be more complicated than anticipated at the onset of the WFD. Particularly, the expanding extent of hypoxia observed in many coastal ecosystems imposes a feedback mechanism through enhanced releases of nutrients from the sediments and thus sustaining an undesired ecological state. Similarly, the loss of benthic vegetation enhances sediment resuspension and therefore contributes to maintaining a turbid ecosystem that will not allow for recolonisation at deeper depths. Additionally, the alteration of coastal food webs, by overfishing the top predators of the system, may have alleviated the grazing pressure on phytoplankton through trophic cascades.
For instance, our results have shown that despite significant declines in nutrient levels in Danish coastal waters, light conditions have not improved because the reductions were in the dissolved inorganic fractions only. Such results suggest that there could be thresholds in the ecosystem response such that no improvement is observed until nutrient levels go below a certain level and alternatively that the top-down control of the ecosystem must be managed as well through sustainable fishing quotas maintaining a well-function food web. These mechanisms described above, a combination of regime shifts and shifting baselines, illustrate the complexity of restoring coastal ecosystems to a good ecological status. Understanding this complexity and quantifying expected outcomes of nutrient reduction plans is essential to the successful implementation of WFD.