Keeping the emergency exit clear for aquatic organisms

Just like humans, the inhabitants of rivers and streams need an emergency exit when they are faced with impending danger. But structures that would normally provide shelter during floods or droughts are now almost non-existent in straightened, channelised and built-up watercourses. How these refugia can be retained and restored is the subject of research by river ecologist Christine Weber.

In the absence of human intervention, streams and rivers comprise a rich mosaic of diverse habitats. These constantly evolving habitats are formed by natural variations in discharge and the associated transport of sand, gravel and stones – known as bedload. All of these habitats play an important role for aquatic organisms: some are used for hunting or feeding, some are suitable for reproduction, and others serve as places of refuge during disturbances such as droughts or floods. For example, refugia can be found in pore spaces of the riverbed, side channels, undercut banks, accumulations of driftwood and on floodplains. They are essential to the survival of aquatic organisms in an emergency and therefore play an important role in preserving biodiversity.

“Until now, the importance of refugia has been underestimated in river management, and they’re often forgotten about altogether in restoration projects,” says Christine Weber, leader of the River Restoration Group at Eawag. Apparently, one reason for this is that there is still very little research into refugia because the organisms are only there occasionally and for short periods of time – and also because sampling is difficult and dangerous during flooding. What kinds of structures serve as refugia during floods, and for which organisms? When do the organisms go there, and for how long? How are refugia affected by bedload dynamics? These questions have been explored by Christine Weber in three studies that were conducted in collaboration with colleagues from Eawag and the Laboratory of Hydraulics, Hydrology and Glaciology (VAW) at ETH Zurich. The studies form part of the “Hydraulic Engineering and Ecology” research programme, which is sponsored by the Swiss Federal Office for the Environment (FOEN) and four institutions within the ETH Domain, including Eawag.

How organisms seek out refugia during floods

“In a field study on the River Spöl in the Swiss National Park, we were able to observe which tiny invertebrates use which refugia in an artificially triggered flood,” explains Weber. The key finding was that it all depends on the variety and resilience of the refugia, as well as their connectedness within the river network. Different species have different requirements when it comes to these places of shelter. Furthermore, not all aquatic animals have equal levels of mobility – while fish can also look further afield for shelter, insect larvae are reliant on places of refuge in their immediate vicinity. “Refugia must be connected to the residential habitats so that they can be accessed in time in the event of an emergency. What’s more, they must remain connected – or be connected again – when the flooding subsides so that animals can return to their residential habitats following the disturbance.”

Less bedload, fewer refugia

How well refugia are connected and their resulting availability for use is largely influenced by the supply of bedload. “We studied several streams using sediment traps and found that the substrate diversity of the stream bed decreases with the retention of bedload, leading to the loss of refugia,” Weber explains. Less bedload also means an incision of the river bottom due to erosion, such that the river becomes increasingly decoupled from the surrounding land. In a laboratory experiment, Weber and VAW-PhD student Cristina Rachelly studied the variation in available refugia in a restored river reach known as a dynamic river widening. If sufficient bedload was present, the widening was inundated even in small – i.e. frequent – floods and was available for wildlife to use as a refuge. “In the case of a bedload deficit, however, the widening is only inundated in the case of heavy flooding, the likes of which only occurs every 30 to 100 years. In smaller, less-frequent flood events, the organisms therefore have little chance of survival because they have nowhere to escape to.” Restoring the bedload regime could mean more refugia are available for aquatic organisms during floods. “But floods aren’t the only challenge, of course,” Weber points out. “Climate change will lead to more periods of drought, presumably calling for other types of refugia. There’s still a lot of research to be done in this area.”

Generating – and disseminating – knowledge

Christine Weber believes that, as well as more research, there is above all a need for greater dissemination of knowledge. In the past, the “Hydraulic Engineering and Ecology” research programme was primarily aimed at professionals and experts from industry or other practice-based backgrounds. In her view, however, that is not enough. “We need to do more to address the decision-makers in the hydraulic engineering and water management field – in other words, those whose decisions directly affect whether refugia are maintained or restored. In many cantons, these are the municipal councillors, who often come from a different professional background.” Weber and her colleagues have therefore commissioned the production of an easy-to-understand flyer, an interactive website and a series of entertaining explanatory videos. “In this way, we hope to raise awareness of the importance of refugia and offer recommendations so that, in future, refugia are no longer neglected in hydraulic engineering and restoration projects.”

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Created by Isabel Plana for the Info Day Magazine 2023