Ecological surprises: how do multiple stressors impact European rivers and lakes?

Guest blog by Sebastian Birk and Daniel Hering

When does 1+1=3 for freshwater conservation and restoration? What seems like a trick question is actually the basis of a major European research project, MARS, begun in 2014. The 1’s in the equation refer to single stressors on freshwater ecosystems – things like nutrient pollution, water abstraction and temperature increases.

Adding the effects of single stressors together should give us an indication of the overall stress placed on the ecosystem by human activities, shouldn’t it? Not always!

Ecological surprises in European rivers and lakes

In our newly published study, we found that in about a third of cases such multiple stressors interactions in European rivers and lakes yield ‘ecological surprises’. Such ‘surprises’ occur where different stressors interact to intensify, or even cancel out, their combined effects.

For example, nutrient pollution in a drought-affected river could mean that 1+1=3, known as a synergistic effect. In contrast, the same nutrient pollution in a flood-affected river could mean that 1+1=1, known as an antagonistic interaction. What a headache for freshwater conservation and restoration efforts!

Our new paper, published in Nature Ecology & Evolution (open access pdf link), is the result of years of collaborative work by MARS scientists across Europe. We carried out 33 mesocosm (i.e. tiny ‘experimental lake’ systems), 14 river basin and 22 regional and continental studies to produce 174 sets of paired stressors (e.g. ‘nutrient pollution and drought’). The effects of each stressor pair were observed on the response of a biological variable, such as number of invertebrate species or phytoplankton density.

Multiple stressor impacts differ between lakes and rivers

We found differing results between lakes and rivers. In most of the lakes we studied, nutrient pollution was the overriding stressor. In addition, the frequency of ‘ecological surprises’ was similar irrespective of whether single lake mesocosms or thousands of European lakes were studied.

However, in rivers, the effects of nutrient pollution depended on both the stressor combination, and the biological variable used to measure their impact. In contrast to lakes, the frequency of ‘ecological surprises’ in rivers increased with scale. In other words, large – and often diverse – river systems can generate unexpected interactions between the stressors placed on them by humans, causing a range of impacts on their ecosystems.

Nutrient pollution a key stressor in European freshwaters

Our study shows that nutrient pollution is a key stressor affecting the health and status of European lakes and rivers. Nutrients – particularly nitrogen and phosphorous – reach freshwaters from agricultural run-off, industrial discharges and urban waste, and can cause a number of harmful impacts, such as algal growth, eutrophication and overall poor water quality.

However, we suggest that lakes and rivers require different conservation and management approaches. In lakes, the traditional approach of reducing nutrient use and discharge across catchments remains key, although the magnitude of the required nutrient reduction may increase with climate change.

Managing for multiple stressors is crucial in rivers

On the other hand, our results suggest that rivers require more bespoke management approaches, which take into account the different stressors affecting the system, and how they interact. This is particularly important in a time of increasing climate change impacts and extreme weather events.

Conserving and restoring our rivers and lakes is no easy task. Through MARS, and in our new paper, we’ve shown that the multiple stressors affecting freshwaters often have complex interactions and impacts. However, we know more than ever about these interactions: so whilst 1+1 might sometimes equal 3 (or even 1), we can now better plan for such ‘ecological surprises’!

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Birk S., et al. (2020) Impacts of multiple stressors on freshwater biota across scales and ecosystems, Nature Ecology & Evolution (open access pdf link)

Funding: This work was supported by the MARS project (Managing Aquatic ecosystems and water Resources under multiple Stress) funded under the 7th EU Framework Programme, Theme 6 (Environment including Climate Change), Contract No: 603378.