Impact of heat wave on brooks harbouring the white-clawed crayfish (Austropotamobius pallipes) in western France (Deux-Sèvres)

In the summers of 2003 and 2006, severe heat waves occurred in France, with temperatures exceeding 40°C for several consecutive days in some regions. The summers of 2004 and 2005 were also exceptionally hot and dry. The extreme temperatures over these four years may have had major consequences for brooks. The present study deals with seven headwater brooks in Western France (Deux-Sevres) which contain populations of the threatened white-clawed crayfish (Austropotamobius pallipes). Thus, the main aim of this study was to assess the impacts of disturbances caused by heat waves and droughts on crayfish brooks. A direct consequence of the heat waves was an increase in water temperatures, however, the brooks reacted differently according to their geographical location and geology. Evaporation reduced the flow such that some study sites which used to be still-water areas (i.e., pools) were more characteristic of flowing water habitats. This phenomenon resulted, for example, in a clear increase in the number of macroinvertebrates belonging to the family Ptychopteridae. Changes in the fish fauna were also observed. The results highlight that it is fundamentally important to understand the changes in headwater areas in order (1) to develop ecosystem management plans for conserving threatened species and (2) to anticipate resulting downstream disturbances.