The Freshwater Blog

Multiple stressor effects on fish assemblages in the Austrian Alps

The Drava River at Villach, Austria. Image: Andrey | Flickr Creative Commons

The rivers and stream ecosystems of the mountainous European Alps are under increasing stress as a result of human activities. So-called ’hydromorphological’ alterations – those which change the hydrological regime (or flow) and morphological character (or shape) – are particularly common on Alpine rivers and streams.

Such hydromorphological pressures are often caused by the construction of hydropower plants – which harness the power of fast-flowing streams to generate renewable energy – and flood protection measures along river and stream channels. In addition to alterations to water flows and aquatic habitat, hydromorphological pressures (or stressors) can cause alterations to habitat connectivity for migratory species and – indirectly – to water quality.

Scientists are increasingly documenting the numerous negative ecological impacts of these stressors in Alpine rivers and streams – particularly on fish populations. However, knowledge of stressor impacts remains incomplete, particularly when multiple stressors act in combination.

In this context, a new study investigates the ecological impacts of multiple stressor combinations on fish populations in two river basins in the Austrian Alps – the Drava and the Mura. The open-access study, published in Science of the Total Environment, was supported by the MARS and AQUACROSS projects.

“The river ecosystems of the European Alps are highly under stress through various human activities which are affecting the physico-chemical conditions of running waters and are strongly influencing and impacting their morphological character, hydrological regime and as a consequence, inhabiting aquatic biota and the overall ecological integrity,” explains lead author Rafaela Schinegger, a researcher at the Institute of Hydrobiology and Aquatic Ecosystem Management at the University of Natural Resources and Life Sciences, Vienna (BOKU).

“This is particularly true for many rivers and streams in the European Alps, which constitute the ‘water towers’ of Europe, providing numerous aquatic ecosystem services to their inhabitants and the areas beyond”.

“By analysing an extensive dataset from the 2nd River Basin Management Plan of the EU WFD implementation in Austria, we found a general trend of decreasing ecological integrity with increasing number of stressors and maximum stressor in two Alpine river basins of Southern Austria – the Drava and Mura rivers. Fish metrics based on age structure, fish region index and biological status responded best to single stressors and/or their combinations”, says Schinegger.

A small hydroelectric dam in the Austrian Alps. Such projects can cause multiple stressors on the river ecosystem. Image: Michael Thomas | Flickr Creative Commons

The research team identified seven different stressor categories of single stress or multiple stressor combinations, with up to four stressors acting together at a single site. Just under two-thirds (62%) of stressor-affected ecosystems were impacted by multiple stressor combinations.

Over two-thirds (69%) of the 2590 water bodies investigated across the Drava and Mura basins were impacted by significant human stressors. Connectivity disruptions were the most common stressor found in the basins (1213 water bodies: 47% total), followed by morphological alterations (578 water bodies: 22%), and water abstraction (413 water bodies: 16%).

This analysis provides invaluable information for water managers in the Austrian Alps seeking to prioritise their efforts to mitigate the effects of multiple stressors in rivers and streams. It shows that increases in stressor occurrence reduces overall ecosystem integrity, and that fish assemblages may be negatively affected by such multiple stressor combinations. In terms of biological response, fish metrics based on age structure, fish region index and biological status responded most to single stressors and/or their combinations.

“The knowledge gained in this work provides a basis for advanced investigations in Alpine river basins and beyond, supports WFD implementation and helps prioritizing further actions towards multi-stressor restoration and management.”

One significant finding made by the study is that 37% of headwaters in Austria’s river basins are still in a very good or good ecological state. The authors argue that there is a pressing need for environmental policy and management to protect these headwater reaches from human pressures, particularly from hydropower construction.

Schinegger, R et al (2017) “Configuration of multiple human stressors and their impacts on fish assemblages in Alpine river basins of Austria”, Science of the Total Environment, http://dx.doi.org/10.1016/j.scitotenv.2017.10.283