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.”