How are river ecosystems affected by regulation?

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, transport, irrigation, 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. In winter, because more electricity is needed for heating, hydropower companies take more water from the reservoirs and release it into streams, generating electricity. Therefore, many regulated rivers have relatively high discharges in winter. This is in contrast to unregulated rivers in Norway which normally have low discharges in wintertime because precipitation falls as snow and does not run off immediately.

Consequences and interaction

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 (known as macroinvertebrates), as well as bottom-dwelling algae (known as 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.

The researchers asked: 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?

- Studying the effects of river flow on stream biota is not straightforward, Susanne Schneider, senior research scientist at NIVA, says.

- 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. Properly replicated and controlled experimental designs in streams are rarely possible in practice.

Flume experiments

A convenient way to standardize conditions, i.e. to achieve the same conditions across many streams, are flume experiments. Flumes are not “real streams”, but artificial constructions made of e.g. steel, with water running through.

- They have the advantage that we can study the influence of a certain factor, e.g. an increase in flow, while all other conditions are kept constant, Schneider explains.

Nutrients matter

In this project, the scientists manipulated flow and nutrient (nitrogen and phosphorus) supply in the flumes. They 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.

- We also found that a moderate increase in flow caused an increase in benthic algal biomass, Schneider says.

- This may seem surprising, because we all know that large floods can “clean” the bottom, i.e. 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. What we learn from this, is that a flood in a nutrient-rich river will have different consequences than a flood in a nutrient-poor river.

Fieldwork

Schneider explains that differences in macroinvertebrate and benthic algae were found between the flumes, and the nearby river from where they took the water for their flumes.

- This was because “controlled” flumes can never really be the same as “real” ecosystems, Schneider says.

- Therefore, we also analysed “real” rivers.

The scientists compared 32 regulated river sites in Norway and Germany with an equal number of unregulated sites. None of the indices of macroinvertebrates and benthic algae used for ecosystem status assessment was affected by the water flow regime. This suggests that these indices can be applied to regulated rivers as well as non-regulated ones.

- We did, however, see some effect of the long-term flow regime, calculated from five years of discharge data, on the species composition of macroinvertebrates, Schneider says.

- We found that a flow regime which is comparatively stable over several years, such as may be caused by river regulation, changes the species composition of macroinvertebrates towards more species that prefer slowly flowing water. We also saw that less variable flow conditions lead to a reduced proportion of grazers among the macroinvertebrates, i.e. those species that directly feed on benthic algae.

Thus, a more uniform flow regime may lead to a higher biomass of benthic algae, via a direct and an indirect effect: direct, because the occurrence of lower and fewer floods will “clean” less algae from the river bottom, and indirect because fewer grazers will eat less algae.

Similar effects on biota

Another finding that caught the scientists’ interest 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.

That climate and natural variation in river flow affected benthic algae was something the researchers have seen before. Data collected over more than 20 years at the lake outlet Atna, and the headwater stream Li, both located in the Norwegian mountain area Rondane, showed similar results. In this remote location, there is practically no human interference except climate change.

Recolonization

- In summary, we found that there are short-term effects of extreme events like floods and droughts on benthic algae and macroinvertebrates, Schneider says.

- 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.

A prominent long-term effect was however that a new flow regime affected species composition of macroinvertebrates. In addition, variability affected grazing macroinvertebrates - which in turn may lead to a higher biomass of benthic algae in rivers with stable flows.

The study was funded by the Research Council of Norway (ECOREG) and by the EU 7th Framework Programme, Theme 6 (Environment including Climate Change) (MARS).