Dammed rivers lose up to 25% of fish biodiversity

fish biodiversity drops 25% upstream of dams (credit: TUM )

A newly published study has demonstrated that rivers upstream of dams and weirs could have as much as a quarter of it’s biodiversity lost. The biggest losers tend to be the fish that live in fast water currents and those are also the fish species which tend to be endangered.

Fish diversity drops 25% and invertebrate diversity falls 50%.

Just as worryingly the study showed that invertebrate biodiversity- the base of the food chain – saw as much as 50% of species reduction. The study which was published in the journal of Applied Ecology indicates that the impact of dams and weirs on rivers is much stronger than previously thought.

The researchers from the Technische Universitaet Muenchen studied 5 different rivers in the catchment of the rivers Elbe, Rhine/Main, and Danube. The researchers looked at a range of different influences on the rivers including chemical composition, current and river bed substrate. They then took at look at the diversity, size and health of the fauna of the river. Finally they compared the results on both sides of weirs and dams.

The ecological impoverishment of rivers is particularly dramatic when series of dams prevent the sufficient interlinking of different habitat types.

More dams and weirs leads to progressively more losses to biodiversity.

Weirs and dams were built on the rivers for a number of different reasons including flood defence, power generation and water collection. Whatever reason for the dam or weir the ecologists found that the river make up and characteristics were different on both sides of the dam. Because many of the rivers had a number of dams and weirs the differences built up as more obstructions were encountered. This phenomenon is known as ”serial discontinuity”.

While serial discontinuity has been known about the new study has started to measure the impacts. As well as losses of biodiversity upstream there was also a 3 fold decrease in biomass upstream of a dam compared to downstream.

Current loving fish particularly hard hit by dams.

The real danger is for the fish which prefer to live in river currents. Many of these fish are listed in the IUCN Redlist as being endangered.

“Brown trout, grayling and Danube salmon are demanding fish species that require oxygen-rich water and spawn in coarse gravel areas. As typical residents of the upper reaches of rivers, they are unable to find suitable habitats in dammed areas,” explains Juergen Geist, Professor of Aquatic Systems Biology at TUM.

“These river sections are often dominated instead by bream, chub, and even carp – generalist species that are actually adapted to stagnant waters. The ecological impoverishment of rivers is particularly dramatic when series of dams prevent the sufficient interlinking of different habitat types,” says Geist.

The impact of weirs can be substantial when looked at a national or regional level. The Bavarian Environmental Agency for instance states that there are as many as 10,000 weirs on rivers in Bavaria.

Changes to river composition and not physical barrier is biggest problem.

The researchers highlight that it is not the physical barrier of the weir that causes the problem. Rather the dam or weir will actually change the river composition on a physical and chemical level and this is what causes the decline in biomass and biodiversity.

An example of this is that in all the dammed regions the ecologists found the water and sediment temperatures were higher and this could reduce reproduction in various fish species. Sediment grain size upstream of dams and weirs were also much smaller than downstream and this provided less favourable spawning grounds.

“During the evaluation of new weirs or modernization of hydroelectric power plants, attention should no longer be focused exclusively on the migration of fish species, but on the consequences of the structure and function for the river ecosystem as a whole.” explained Prof. Geist.

External sites:

M. Mueller, J. Pander, J. Geist: The effects of weirs on structural stream habitat and biological communities, Journal of Applied Ecology