Agriculture, wastewater, dams, the runoff of fine sediments from croplands and, last but not least, climate change with its rising temperatures are changing the quality and structure of freshwater ecosystems, especially rivers. However, until now there has been no clear picture of how strongly individual man-made stressors are associated to different species groups.

A team led by lead author Dr Willem Kaijser, from the working group Aquatic Ecology at the University of Duisburg-Essen (UDE) has now closed this gap. The researchers reviewed more than 22,000 scientific articles and analysed 1,332 data sets from 276 studies. Their findings resulted in the first global summary relating stress factors with five key biota: microorganisms, algae, aquatic plants, invertebrates and fish.

Across all groups, biodiversity is particularly associated with the following stressors: increased salinity, oxygen depletion and excessive sedimentation. These factors often occur together and worsen living conditions – for example, through metabolic stress or silted habitats.

The effects of other influences, such as nutrient enrichment and warming, vary with species group. For example, some algae benefit from moderate amounts of nutrients, which can promote growth and species diversity. Macrophytes, however, lose diversity when salinity or nutrient levels increase. Invertebrates and fish are particularly related to oxygen depletion and fine sediment deposits that cover their habitats.

The researchers used statistical models and probability theories to reveal these relationships and identify associations with stressors. Commonly, these associations differ between regions and organism groups. Nevertheless, patterns are emerging that are crucial for the protection of freshwater ecosystems: ‘Salinisation, sediments and oxygen deficiency are almost always harmful,’ summarises Prof. Dr. Daniel Hering. ‘Our analyses now allow us to quantify these relationships and use them for predictions.’

The study was conducted at the RESIST Collaborative Research Centre, which is coordinated by the German Research Foundation and based at the UDE.

Image: Salinizsation is a worldwide problem, particularly in arid regions, and affects both aquatic and terrestrial habitats.

Further Information:
Dr. Willem Kaijser, Aquatic Ecology, + 49 201/18 3-3113, willem.kaijser@uni-due.de
Prof. Dr. Daniel Hering, Aquatic Ecology, +49 201/18 3-3084, daniel.hering@uni-due.de

Editor: Birte Vierjahn, +49 203/37 9-2427, birte.vierjahn@uni-due.de