Effects of water level variations on the dynamics of the reed belts of Lake Constance

Abstract

Following the extreme flood at Lake Constance in 1999 the reed belts of the shore of Baden-Württemberg lost approximately 30 ha (24%) of their lakeside reed beds. This loss is comparable with the situation in the late 1960s, when approximately 40 ha died back due to the extreme flood in 1965 and due to the high spring water levels in the subsequent years. In the time period between the extreme floods of 1965 and 1999, the reed areas expanded to nearly 85% of their original area before 1965. As a consequence of the extreme flood in 1999 a loss of 44% of the above ground biomass of 1998 occurred. One third of this loss was regenerated in the subsequent years 2001 and 2002. Similar results were obtained for the calculation of the biofilm area provided by the submerged culm parts of the reed stands. A loss of 47% of the biofilm surface area was estimated. By 2002 only one fifth of the lost area had been regained. This reflects the fact that the regeneration of the lakeside stands at lower elevation levels proceeds much more slowly than that of the inner stands on higher elevation levels, which con-tribute less to the submerged surface areas of the reed stands. Simulations of the extreme flood event of 1999 show a strong association with the results of the aerial photo interpretation. This confirms the concept of the model and supports the hypothesis that water level fluctuations play a major role in the reed dynamics of Lake Constance. The results demonstrate the close interaction of hydrological processes with dynamics of biota, thus the necessity of an ecohydrological approach for the understanding and for the sustainable management of littoral ecosystems