While many studies have projected climate change effects on species, little is known about the severity of these changes compared to historical alterations.Researchers led by IGB and the German Centre for Integrative Biodiversity Research (iDiv) used a unique 300-year time series of hydrological and climate data to explore the vulnerability of 48 native fish species in the upper Danube River Basin to past and potential future environmental changes. They show that fish have been particularly sensitive to changes in flow regimes in the past, while higher temperatures will pose the greatest threat in the future. The threat assessment will remain at least as high in the future. However, it could probably be mitigated by reconnecting former floodplains and improving river connectivity.
With almost a third of all freshwater species threatened with extinction – and a quarter of all known freshwater fish species – freshwaters belong to the most vulnerable ecosystems on earth. Fish species in river ecosystems are sensitive to alterations in discharge and temperature. Human interventions such as straightening and damming have historically altered river flows around the world, negatively affecting fish abundance, demography and diversity. When assessing the conservation needs of freshwater biodiversity under future climate change scenarios, it is important to understand the often dramatic historical environmental changes and their effects on species or populations.
"Fish communities in the upper Danube River Basin have already come under severe pressure over the last 200 years due to massive changes in the water regime. Our results show that the vulnerability of species in the past was mainly due to anthropogenic interventions such as river straightening. This has severely impacted the river. Future potential vulnerability is mainly due to temperature," said Dr Martin Friedrichs-Manthey, lead author of the study. He is scientist at the Friedrich Schiller University Jena, guest scientist at the Helmholtz-Centre for Environmental Research (UFZ) and works at the German Centre for Integrative Biodiversity Research (iDiv) in Halle-Jena-Leipzig.
Future changes for Danube fish: at least the same overall vulnerability as in the past
The research team investigated the vulnerability for the fish species to future climate scenarios. In the most likely scenario they found an increase in fish vulnerability. This is mainly due to a 2°C increase in the average annual temperature. Moreover, the study shows that future environmental conditions for riverine fish species driven by temperature would change at least at a similar magnitude as past hydrological changes, driven by anthropogenic river regulations. "This result surprised us, as we had assumed that future climate change would not lead to such a high threat assessment as the massive regulation of the Danube in the past," said IGB-scientist Dr Sami Domisch, who led the study.
Sins of the past concerning the upper Danube River: 15 per cent of its length lost, 90 per cent of its banks embanked
The regulation of the Danube for flood protection and shipping began at the end of the 16th century. By the end of the 19th century, the German and Austrian main stem had lost 15 per cent of its length due to channelisation, and today more than 90 per cent of the shoreline of the upper Danube mainstream is embanked. And more than 70 hydroelectric power stations in the main river alone fragment the upper Danube. This has led to the loss of almost all free-flowing river stretches. These structural changes have decimated the number and diversity of native fish species. Take the sturgeon, for example: six different species used to live in the Danube. Today, five are classified as critically endangered and one is already extinct.
Vulnerability of fish communities could be buffered by floodplain connectivity and improved river continuity
However, in their study, the researchers also point to possible solutions: "Other studies on invertebrates or marine fish have already shown that a reduction in environmental pollution - for example by improving water quality - can promote resilience to the expected climatic stresses. According to our assessment, this would also be possible in the Danube," said Dr Sami Domisch.
The authors identified the restoration of floodplains as an effective measure for the conservation of fish communities. Other studies have shown that in the upper Danube catchment area, around a quarter of the historically available and currently cut-off floodplain area has good potential for renaturalisation measures. In view of rising temperatures, the upper Danube catchment with its many headwaters could also offer temperature refuges for sensitive cold-water fish. To achieve this, however, river continuity must be improved so that species can also reach these areas.
