Mineral fertilizer. Foto: Lianem /Mostphotos.

Reducing eutrophication in the Baltic Sea is a slow process. This fact is further confirmed by a new large pan-European study where multiple marine models were used to evaluate the effects of comprehensive measures aiming at reducing the nutrient pollution in the European seas.

The objective of the study, where researchers from the Baltic Sea Centre took part, was to provide more consistent assessments of the impacts of riverine nutrient reductions. Modeling the outcome on indicators used in the EU Marine Strategy Framework Directive could give an overview of the effects of different measures on a European level, and support political decisions.

The modelers assessed a reduction scenario called MTFR, High Technically Feasible Reduction, by comparing it to a business-as-usual scenario, where no reductions to the nutrients loads to the seas were made. The MTFR scenario comprised the upgrade of all wastewater treatments in the regions to the highest level of nutrient removal, as well as some improvement of agricultural practices.

A suite of biogeochemical models, one of them being the Baltic Sea Centre’s BALTSEM model, then simulated the response to the two scenarios on eutrophication indicators used in the EU Marine Strategy Framework Directive.

Further measures and more time needed

Bärbel Müller-Karulis, Stockholm University Baltic Sea Centre. Photo: Niklas Björling.

The results show substantial differences between the individual seas in the response to the reduced nutrient loads. In the Baltic Sea, the reduction scenario changed nutrient concentrations, bottom oxygen, and phytoplankton biomass only slightly. The effects were most tangible near the outlets of the Odra and Vistula rivers, in the southern shore of the Baltic Sea.

– The low progress is mainly due to the fact thar the reduction scenario covered only eight years, which is too short to show the full response of the Baltic Sea to load changes, explains Bärbel Muller-Karulis, participating researcher from the Baltic Sea Centre.

Several characteristics of the Baltic Sea, such as the limited water exchange, the specific bathymetry and the presence of a permanent halocline separating the surface and bottom waters, result in a particularly high residence times for nutrients, especially phosphorus.

– To receive a new steady state in the Baltic Sea after making reductions to the nutrient loads, we need several decades, says Bärbel Müller-Karulis.

However, the measures included in the MTFR scenario were also not sufficient to reduce nutrient losses from agriculture to the extend required by the EU Farm to Fork Strategy, and for the Baltic Sea the nutrient loads to the sea remained higher than the set goals in the Baltic Sea Action Plan.

– Further measures are needed to reach these goals, especially for phosphorus, says Bärbel Müller-Karulis.

Small but varying effects on ecosystems

Maciej Tomczak, Stockholm University Baltic Sea Centre. Photo: Niklas Björling.

The effects of the nutrient reduction measures to the marine food webs were, in a connected study, shown to be small in all European seas, but, perhaps surprisingly, slightly larger in the Baltic Sea.

This time, a modeling ensemble of fourteen higher trophic level (HTL) models, the majority of them of the type Ecopath with Ecosim (EwE), simulated the effects on spawning stock biomass of fish, species diversity and trophic level indicators.

In almost all seas, the spawning stock biomass of commercial small pelagic fish showed a slight decrease after the nutrient reductions, even so in the studied part of the Baltic Sea; the Baltic Proper, where the effect was simulated for sprat. The Baltic Proper did, however, as an only region show a small increase in species diversity.

– This could be explained by the small, but nevertheless, improvement in oxygen conditions that leads to better spawning conditions for the Baltic cod stock, says Maciej Tomczak, who has performed the simulations in the Baltic Proper using the EwE Baltic model.

The overall small effects that nutrient reduction shown to have on the higher trophic level of marine ecosystems in the study, should not be interpreted as improved nutrient management is irrelevant, he adds.

– Just as it takes time for the reductions of riverine nutrient to show up as decreased concentrations of dissolved nutrients in the sea, it takes even longer for the changes in primary production and oxygen levels to have full impact on the higher trophic levels. Also, the riverine nutrient pollution is only one of the stressors impacting our seas. But the robust tools and methods as presented here need to be ready to assess the effect of measures at whole ecosystem and European scale.