Biodiversity Reversals in Alpine Rivers

A recent study on the Borgne d’Arolla, a glacier-fed stream in the Swiss Alps, shows that there is less biodiversity among macroinvertebrates than expected in the summer and higher biodiversity than expected in the winter. Chrystelle Gabbud, a geologist at the University of Lausanne in Switzerland, and her associates, found that the rates of streambed disturbance in the Borgne d’Arolla were also much more frequent than normal observations of disturbance in glacial rivers, even during times of peak discharge. The team’s results were published in September in Science of the Total Environment and attribute the above biodiversity inversion phenomenon to the increased frequency of flushing events.

The Borgne d’Arolla (Source: bulbocode909/Flickr).

Why is it that glacier-fed rivers in the Alps are experiencing even more flushing events? Evidence points toward the impacts of global climate change, as rising temperatures influence increased glacial melting and sediment production during the summer months, which in turn means that flushing must be facilitated more often.

Summertime runoff in glacier-fed Alpine rivers is exceptionally useful for supplying water for hydroelectric power production. The flow of water is abstracted at water intakes, which hold back both water and sediment, functioning similarly to dams but on a smaller scale. Intakes also have a relatively low threshold for how much sediment can accumulate before they must be flushed. This means that in basins with high erosion, namely glaciated basins, this flushing happens more frequently. In the summer months, when glacial melt is at its peak, flushing of water intakes can occur up to several times a day. Flushing disrupts the streambed, increases water turbidity, contributes to river aggradation, and negatively affects the macroinvertebrate community both in abundance and biodiversity.

Gabbud and fellow researchers collected samples of macroinvertebrates (animals that do not have a backbone but that are large enough to be seen with the naked eye, such as crustaceans, worms and aquatic insects) at several locations over the course of two years (2016 and 2017) to determine the impacts of flushing water intakes on species biodiversity and abundance. The surrounding tributaries served as controls for the Borgne. The researchers’ findings effectively contradicted the normal expectations for seasonal biodiversity changes.  

Normal biodiversity expectations anticipate that both species richness and abundance should be higher during the summer months, from June to September, which also correspond to the highest water temperatures. However, Gabbud and her team found that biodiversity of macroinvertebrate populations in the Borgne d’Arolla during winter months (and coldest water temperatures) was comparable to the expected levels for the surrounding tributaries during the spring and summer. The Borgne was found to be mostly devoid of life in the summer months, a result which the researchers primarily attribute to the high frequency of flushings.

Figure A depicts the geographical location of the study. Terms in bolded black are the locations of each water intake, and red circles indicate sampling stations. Figure B shows the Bertol Inférieur intake (Source: Gabbud et al., 2018).

The team also compared observations in 2016 to those in 2017. Variations in flushing frequency and duration between the two years led Gabbud and her associates to two determinations. One, that more flushing had a direct negative impact on the presence of macroinvertebrate biodiversity and abundance. Two, that flushings with shorter duration also correlated with higher rates of streambed disturbance.

In addition, they found that as the frequency of flushing decreased, macroinvertebrate populations started to return. Outside of the summer months, flushing happens much less frequently. In a four-day period between flushes, biodiversity was almost able to reach pre-disturbance levels.

A graphical abstract, magnifying both a water intake and a macroinvertebrate species downstream (Source: Gabbud et al., 2018).

The researchers’ observations led them to recommend that the frequency of flushing at the water intakes be decreased and the duration of flushing be increased. They stipulate that higher magnitude flushings, resulting from taking too much time between events, could also have negative impacts. Thus, this situation creates a tension between maintaining hydropower and maintaining biodiversity, a major policy issue.

Currently, Switzerland has a single set of regulations regarding mitigating impacts and restoring ecological areas being used for hydropower generation. There are provisions related to sediment management; however, guidance provided by the Swiss National Government does not mention water intakes by name, instead only addressing dams and maintaining sediment connection.

Seeing as water intakes govern over 50 percent “of hydropower impacted rivers by basin area” in the Swiss Alps, Gabbud and her team emphasize that future regulations must incorporate both sediment management and flow management.

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