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News article21 May 20215 min read

New modelling tool to assess the impact of reducing nutrients in European rivers and seas

Nutrient runoff from land management has a significant impact on water ecosystems.
Nutrient runoff from land management has a significant impact on water ecosystems.
© Norwegian Institute for Water Research, OSPAR, 2010

Scientists from the JRC and research institutes all over Europe have developed a new model ensemble to assess the pan-European impacts of improved land-based management to reduce the amount of nutrients being dumped into freshwater and coastal ecosystems and the marine environment.

The results of this collaboration have been published in three scientific papers describing the impacts of measures on freshwater systems and nutrient exports to the seas (Grizzetti et al., 2021), the nutrient dynamics in the marine environment (Friedland et al., 2021) and the consequences for marine food webs and biodiversity (Piroddi et al., 2021).

The issue of eutrophication in EU waters

Eutrophication, caused by excessive nutrients in water systems, has long been recognised as a high-risk anthropogenic pressure on many lakes, rivers, coastal and marine waters. Although rivers naturally discharge nutrients into the sea, this process has accelerated worldwide beyond sustainable limits due to human activities.

In the EU, the excess nutrients delivered to coastal areas via waterways and further transported throughout the open seas is the main cause of marine eutrophication, which has resulted in harmful algal blooms and the degradation of pelagic and benthic ecosystems. There are many examples of such human-induced eutrophication in almost all marine regions in Europe.

Legislative measures still to achieve their goals

Several pieces of legislation are implemented in Europe to mitigate the negative impacts of nutrient discharge in European waters and to achieve Good Environmental Status, such as the Nitrates Directive (91/676/EEC), Urban Waste Water Directive (91/271/EEC), Water Framework Directive (2000/60/EC) and Marine Strategy Framework Directive (2008/56/EC). Despite their well-defined targets, the measures adopted so far have not been effective enough to achieve the ultimate goals of the regulations.

One of the main problems is that the complexities of marine and coastal ecosystems make it difficult to observe simple pressure-impact relationships between nutrient discharges and the state of the marine ecosystem.

The ensemble modelling approach

In these recent studies, the researchers used hydrological, hydrodynamic-biogeochemical and food web models to carry out the first pan-European assessment of the impact of nutrient management scenarios in freshwaters and marine ecosystems, and to evaluate the predictions of different types of models and applications.

Fifteen marine models of the lower trophic food web and 14 marine models of the higher trophic food web were applied. These ensembles allowed for far more reliable and robust results than would be obtained from a single model system.

Measures could significantly reduce excess nutrients

The results of these studies suggest that policy measures that promote the upgrading of wastewater treatment and optimisation of fertilisers in agriculture could reduce the amount of nutrients exported to seas by up to 14% for nitrogen and 20% for phosphorus. This would improve the ecological status of rivers, lakes and most seas, although in some cases the nutrient imbalance in marine ecosystems could widen, thereby potentially increasing the eutrophication risk.

However, unless the current agricultural production and consumption system is addressed by the measures, the reduction in nutrient load is not sufficient to achieve the Good Environmental Status goals of EU water policy.

All water systems benefit from better management on land

A key finding of the studies is that improved nutrient management in the river basins has a strong impact on freshwater systems, resulting in fewer nutrients being delivered to regional seas and improved water quality as a result.

There is a clear indication that all waters benefit from land-based measures to reduce nutrient loads. This was particularly noticeable in freshwater systems and coastal seas, but less apparent in marine phytoplankton concentrations and primary production. While pelagic oxygen concentrations changed even less, the authors acknowledge that the study period of 8 years may have been too short to understand the full impacts of improved nutrient management.

Although improvements in water quality were evident in all regions, regional seas showed strong differences in the strength and speed of their reactions to the changed nutrient loads. These peculiarities were found throughout the model ensemble, which indicates that there is a strong coherence and reliability of the model outcomes.

No significant impact on marine ecosystems

From a food-web modelling perspective, results suggest that nutrient reduction measures may not have a significant impact on the structure and function of European marine ecosystems. Among the indicators assessed, the biomass of commercially important fish (e.g. sardines and anchovies) would be the most impacted, albeit by less than 2.5% reduction, while the impact on species diversity would be even lower.

The Black Sea and the North-East Atlantic would be most negatively impacted in terms of reduction of commercial fish, while the Baltic Sea was the only region showing signs of improvement. Coastal and shelf areas were more sensitive to environmental changes than large regional and sub-regional ecosystems, including open seas.

New models help inform better EU policy

These studies suggest that improved nutrient reduction management, in line within European directives, the European Green Deal and the Zero Pollution Ambition, will improve the quality of EU freshwater and marine environments with few negative impacts on their food webs.

The modelling tools used in this study can help assess and provide useful information on the impacts of cumulative anthropogenic pressures on every trophic level of a marine ecosystem, and evaluate short- and long-term forecasts of selected policy measures.

This first pan-European ensemble modelling study represents an extremely valuable first step towards large-scale policy evaluation within the EU, linking land activities and measures to their impacts on freshwater and marine environments.

Suggestions for further improvement of modelling tools are also given, such as standardisation of model evaluation, their sensitivity and creation of model ensembles that will provide reasonable confidence intervals for policymaking decisions.

Further information

Related Content

How EU policies could reduce nutrient pollution in European inland and coastal waters

Effects of Nutrient Management Scenarios on Marine Eutrophication Indicators: A Pan-European, Multi-Model Assessment in Support of the Marine Strategy Framework Directive

Effects of nutrient management scenarios on marine food webs: a Pan-European assessment in support of the Marine Strategy Framework Directive

Marine Strategy Framework Directive

Water Framework Directive


Publication date
21 May 2021