Reed and roots provide protection

Up to 10.000 organisms can be found on a square meter of water bottom, of which a lot are also terrestrial insect larvae. Scientists call the whole group macrozoobenthos – these are all invertebrates living on the bottom and still visible with naked eyes. Researchers at the Leibniz Institute of Freshwater Ecology and Inland Fisheries (IGB) now study the impact that ship-induced waves can have on these small animals.

The larva of Calopteryx splendens, a dragonfly, crawls on a stone in shallow water. Then operates Friederike Gabel the wave machine. A wave, comparable to that of a sport boat, runs along the three-metre-long canal. The larva is washed out – “detached” say the researchers – and paddle around several minutes helplessly in the water until it found again the “solid ground” under its feet. “If they stay suspended in the water, the larvae take the risk to be eaten” explains F. Gabel, a specialist of the effect of waves on invertebrates. In addition, the larva spent energy to fix them back, which has negative effects on their growth and reproduction. The researchers fear that ship-induced waves increase larval mortality and subsequently biodiversity, which would have a long-term effect on the ecological quality of rivers and lakes.

Using an experimental set up, they have defined in laboratory for which threshold of wave strength the animals will be washed out from their standing place. The results are now published in the journal Freshwater Biology (2008, 53, 1567-1578). They found that the more complex was the structure of the habitat, less massive was the detachment. “The impact of waves is the lowest, when the shore is covert with tree roots,” explains F. Gabel. Even dense reed belts would provide a sufficient protection against the power of the waves to the animals. On the contrary, the detachment is maximal on sand and stones. Complex habitats reduce the impact of waves since they offer better hiding place and fixing possibilities for the animals, explains F. Gabel.

The researchers have now begun to collect samples in natural habitat. They want to determine the long-term impact of ship-induced waves on the invertebrates inhabiting the shore. They survey different locations of the River Havel and compare shore sections differently exposed to ship-induced wave intensity.

We are not against the ship traffic, stresses F. Gabel, however there is a need to find simple measures to protect the shores. “As a result of our investigations we could make recommendations for water management, such as the design of the shores or, that ships in certain areas should pass by far away from the shore or should lower their speed” said the scientist.