Deutschen Gesellschaft für Limnologie e.V. Annual Meeting 2004 - Potsdam, September 20th-24th, 2004.
Oral presentation : Ecological effects of different shore protection types on the Elbe, East Germany. Garcia XF, Brauns M, Pusch M.
Long-term management of the sandy river Elbe for commercial navigation purposes has modified the river banks by building stony groynes (Buhnen) on the major part of the river channel. The groynes are mostly built on both banks perpendicularly to the main channel axis and spaced of 100 to 300 m from each to an other. These groynes, which aim to concentrate the river flow in the central channel strongly adverse the benthic macroinvertebrate fauna inhabiting the banks. Mainly, the groynes enhance the accumulation of sand and mud in the groyne fields, so that microhabitats depending on erosion processes disappear. In addition, invertebrate populations are affected by the action of the waves generated by the ships passing by, which flush the bank regularly. Consequently, the diversity of the fauna is strongly reduced due to the loss of habitat diversity and mechanical disturbance. Even the tip of the groynes, exposed to high current velocities, and thus in theory suitable to hard substrate inhabiting and rheophilic communities, are few colonised. It can be hypothesized that the susceptibility of the ecosystem Elbe to invasive species, such as Dikerogammarus villosus and Gammarus tigrinus, which have appeared in recent years, was also largely enhanced by the presence of the groynes.
This situation pushed, the German government to think, some years ago, about a sustainable management of the river Elbe, trying to find solutions able to conciliate commercial navigation and preservation of the biodiversity. Several alternatives have been developed concerning the shape of the groynes, like the "Absenkungsbuhne", the "Knickbuhne" (Anlauf 2002) and more recently the "Parallelwerk". The latter consists of a stony dike of 800 m long, built parallel to the bank in such a way that the water can flow between the bank and the dike. One of the benefits of such a structure is to avoid the direct flushing of the bank by ship passing by. However, the question of a disturbing high sand sedimentation rate remains.
Our project aims to assess the influence of the parallel groyne on biodiversity recovering. Five different site-types were prospected with three replicates each. Namely, the parallel groyne built in Gallin (Elbe-km 204) and some adjacent perpendicular groynes. Sites exhibiting perpendicular, but destroyed groynes were also prospected because of the higher hydrodynamic observed in such sites. Two former side channels (oxbow lakes) with different degrees of connection to the main channel were also prospected for comparison, as due to its length and its two openings upstream and downstream, the parallel groyne fields presents functional similarities with side channels. Five sampling campaigns of the benthic macroinvertebrates associated to the groyne fields were conducted in May, June, July, August and October 2003.
First results show a higher global biodiversity in parallel groyne fields than in perpendicular groyne fields, highlighting the positive effect of the protection against the waves. However a smaller global biodiversity is observed in parallel groyne fields than in the side channels partially, because attractive mesohabitats as woody debris are not present in the parallel groyne fields. An interesting aspect of the parallel groyne fields is to offer two contrasting hydrodynamic conditions along the year. During the spring the water passed through the parallel groyne field allowing rheophilic species to colonise the different microhabitats present. When the water level decrease, pools are isolated in which environmental conditions are changing fast. Such a dynamics supports the global biodiversity by a high turn-over of the limnephilic species colonising the pools. This mechanism was also observed for the broken perpendicular groynes where similar pools are progressively isolated by the summer decrease of the water level and was previously described on the French sandy river Loire too. In summary, alternative shaping of groynes promises to be an effective instrument to enhance the diversity of aquatic invertebrates in the Elbe while commercial navigation would be still possible.