Scientists document aquatic species decline at dams and weirs

Sep 14, 2011

Dams and weirs have a stronger impact on the ecosystem of watercourses than was previously realized. Species diversity in the dammed area upstream of weirs shows a significant decline: the diversity of fish species is one-quarter lower on average, and species diversity among invertebrates is up to 50 percent lower. The interruption of a river course thus has greater effects on the biodiversity than the geological origin of the river itself. Scientists from the Technische Universitaet Muenchen have demonstrated this on the basis of a survey of five rivers in the catchment areas of the Elbe, Rhine/Main, and Danube. Their analysis records, for the first time, both abiotic factors, for example chemical composition, current, and river bed substrate, and biotic factors, such as the number, size, and diversity of all important animal and plant groups on the two sides of weirs. The TUM researchers will present their findings, which have already been published online in the Journal of Applied Ecology, at the annual conference of the German Limnological Society, which takes place from September 12 to 16, 2011.

Whether it is done for the generation of electricity, for , or collection of drinking water, the damming of a river represents a drastic intervention in its ecosystem. Weirs and dams alter the chemical and physical characteristics of the water and the riverbed. Their construction is accompanied by a clear decline in upstream , and this is exacerbated by successive damming. This phenomenon, which is known as "serial discontinuity," has now been measured for all important animal and plant groups by system biologists from the TUM for the first time; the information available up to now concerned only individual taxonomic groups or species. The scientists carried out a systematic survey of the on both sides of the weirs in five rivers of different geological origins. Their survey revealed a significant reduction in the number, biomass and variety of periphyton, invertebrates, and fish species upstream of the weirs. Compared with downstream areas, the diversity of fish species measured upstream of weirs was 25 percent lower on average; a threefold decline in biomass was also observed.

Current-loving fish species, many of which are on the "Red List" of endangered species, are particularly severely affected. "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. According to the Bayerische Landesamt für Umwelt (Bavarian Environmental Agency) there are more than 10,000 weirs in Bavaria's rivers alone.

According to the researchers, the main reason for the species decline is not the impermeability of the barrier to migrating fish species. Instead, the deciding factor is the chemical and physical alteration of the river itself which leads to a reduction of biodiversity. If the current is decelerated or interrupted, the flow rate upstream of the weir declines, and this is accompanied by increased water depth. In all of the surveyed dammed areas, the TUM scientists also recorded major differences in the oxygen content and temperature of the water and the sediment in the river bed, which hinders reproduction in current-loving fish species. The situation is compounded by differences in the structure of the sediments. The particles in the sediment found downstream of weirs are twice as large on average as those upstream, providing more and better quality spawning grounds.

River sections in the direct proximity of weirs should, therefore, be given greater consideration in ecological quality assessments, notes Prof. Geist: "During the evaluation of new weirs or modernization of hydroelectric power plants, attention should no longer be focused exclusively on the migration of , but on the consequences of the structure and function for the river ecosystem as a whole." To ensure this, affected river sections must be covered by the relevant laws or regulations, such as the European Union Water Framework Directive. "A set of instruments for the quantification of the effects of weirs is now available, which records both the quality of the habitat and its biodiversity," says the TUM scientist.

Explore further: Wolf-like animal seen roaming in northern Arizona

More information: M. Mueller, J. Pander, J. Geist: The effects of weirs on structural stream habitat and biological communities, Journal of Applied Ecology (early online) onlinelibrary.wiley.com/doi/10… 011.02035.x/abstract

Provided by Technische Universitaet Muenchen

5 /5 (1 vote)
add to favorites email to friend print save as pdf

Related Stories

Maine salmon may be facing extinction

Jan 31, 2006

Maine salmon might be facing extinction despite a $20 million, five-year rescue effort and inclusion on the federal government's list of endangered species.

Shifting Baselines Confound River Restoration

Sep 01, 2009

Steep reductions in the abundance of fish, shellfish, and other aquatic fauna in recent centuries are not restricted to animals that live in the sea: historical records show that species in rivers and lakes worldwide also ...

Recommended for you

Is fleet diversity key to sustainable fisheries?

2 hours ago

Concern about fisheries is widespread around the world. Over the past several decades, a robust discussion has taken place concerning how to manage fisheries better to benefit ecosystems and humans. Much of the discussion ...

Strange, fanged deer persists in Afghanistan

3 hours ago

More than 60 years after its last confirmed sighting, a strange deer with vampire-like fangs still persists in the rugged forested slopes of northeast Afghanistan according to a research team led by the Wildlife ...

Captive rhinos exposed to urban rumbles

4 hours ago

The soundtrack to a wild rhinoceros's life is wind passing through the savannah grass, birds chirping, and distant animals moving across the plains. But a rhinoceros in a zoo listens to children screaming, cars passing, and ...

User comments : 0

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.