WATER: Ecosystem responses to different regulation regimes (ECOREG)

Most rivers and streams across the globe experience natural variations in flow throughout a year. Whereas climate change increases the risk of extreme floods and extreme droughts, "normal" floods and droughts are part of the natural rhythm in river ecosystems. Natural variations in flow are important for these ecosystems because they "clean" the river bottom. Although this obviously is devastating for the organisms which are ripped off by a flood, or killed by a drought, it also creates open spaces which later can be colonized by other organisms. Therefore, disturbance by floods and droughts is - in the long run - important for maintaining biodiversity in rivers. However, across the globe, more and more rivers are regulated to serve human needs such as flood protection, hydropower generation or drinking water supply. In Norway, more than two-thirds of river basins are affected by regulation. This has altered natural flow dynamics. Scientists at The Norwegian Institute for Water Research (NIVA), in collaboration with The Norwegian Institute for Nature Research (NINA) and colleagues from Germany, wanted to find out more about how altered flow dynamics affect organisms that inhabit the bottom of rivers, such as insects, snails and other small spineless creatures (= macroinvertebrates), as well as bottom-dwelling algae (= benthic algae). Benthic algae are the very basis of many food webs in rivers. Macroinvertebrates feed on algae and detritus, while they themselves are an important food for fish. Will there be consequences of an altered flow regime for macroinvertebrates and benthic algae? Furthermore, macroinvertebrate and benthic algal indices are commonly used to assess the health (ecological status) of rivers with respect to acidification and the over-enrichment with nutrients. Can we use these indices also in regulated rivers? And how does river flow interact with other ecosystem stressors like acidification or nutrient enrichment? Each stream is special in its own way, has a slightly different flow regime, sediment, water chemistry, shading, etc., than its neighbour. The interaction of so many factors, which all affect macroinvertebrates and benthic algae, makes it difficult to detect the effects of flow among all other factors which also are important. A convenient way to standardize conditions, i.e. to achieve the same conditions across many streams, are flume experiments. Flumes are artificial constructions made of e.g. steel, with water running through. We manipulated flow and nutrient (nitrogen and phosphorus) supply in the flumes, and found that an increased nutrient supply - not surprisingly - after a few days lead to an increased biomass of benthic algae. This is simply because algae "feed" on nutrients, and more food leads to better growth. But a moderate increase in flow caused an increase in benthic algal biomass. This may seem surprising, because we all know that large floods can rip off benthic algae from their substrate and wash them away. But a moderate increase in flow can have a positive effect on algal growth, because it is not strong enough to rip off the algae, but transports more water, and with it nutrients, into the algal patches. This means the nutrients are more easily accessible to benthic algae, which leads to increased algal growth. Interestingly, when nutrients and flow both were increased at the same time, the effect on algal biomass was smaller than the sum of both individual effects. This was because a larger patch of benthic algae - caused by more nutrients - can more easily be ripped off already by a moderate increase in flow. However, "controlled" flumes can never be the same as "real" ecosystems. Therefore, we also compared 32 regulated river sites in Norway and Germany with an equal number of unregulated sites. We found that indices of macroinvertebrates and benthic algae used for ecosystem status assessment can be applied to regulated rivers as well as non-regulated ones. We did, however, see an effect of the long-term flow regime on the species composition of macroinvertebrates. We found that a flow regime which is comparatively stable over several years changes the species composition of macroinvertebrates towards more species that prefer slowly flowing water. Another finding was that natural differences in flow regime had similar effects on the biota as artificially modified flow regimes. Macroinvertebrates and benthic algae simply did not mind if a difference in flow regime was due to natural reasons, or if regulation caused it. We also found that there are short-term effects of extreme events like floods and droughts on benthic algae and macroinvertebrates. Benthic algae generally were more affected by floods, while macroinvertebrates were more affected by droughts. Within few weeks or months after extreme events, however, benthic algae and macroinvertebrates usually recolonize the rivers, such that few long-term effects were apparent.

Hydropower production, despite being presented as a clean source of energy, fundamentally transforms rivers, with concomitant consequences for river ecosystems. ECOREG aims at detecting the responses of primary producers (benthic flora) and primary consum ers (benthic macroinvertebrates) to different hydrologic regimes in rivers in Norway. We will focus on benthic invertebrates and benthic flora, because both are mandatory organism groups with respect to the WFD. In addition, differences in hydrologic regi me are expected to highly impact these organisms, while at the same time the ecological consequences are poorly understood. We will i) study the effects of detailed and continuously recorded hydrologic regime on species assemblies and traits at 40 regulat ed and unregulated river sites, ii) study basic ecosystem functions, i.e. food web structure and energy and nutrient flow in experimental flumes, and their reaction to changes in hydrologic regime, and iii) study the effects of multiple stressors (mainly eutrophication and regulation) on species assemblies and traits based on existing data on biology and water chemistry, which we will supply with information on hydrologic regime. We aim at i) contributing to the definition of good ecological potential in regulated rivers, ii) developing an easy-to-use method for defining minimum flow targets, and flow restrictions (e.g. hydropeaking), iii) setting the foundations for predicting ecosystem responses to the expected hydrologic changes related to climate chan ge, and iv) detecting whether or not the effects of river regulation are different from natural differences in hydrologic regime (testing the hypothesis that regulated rivers are a model for what will happen to unregulated rivers in response to climate ch ange). In addition, ECOREG will strengthen our knowledge on what controls species assemblies and ecosystem functions, thus helping us to understand what controls the delivery of important ecosystem services.