WATER-LEVEL FLUCTUATIONS Ecological effects of water-level fluctuations in lakes: an urgent issue

Water-level fluctuations (WLF) affect the ecological processes and patterns of lakes in several ways. Aquatic habitats and feeding or breeding grounds are gained or lost, as light, climate and wave impacts change to mention only a few phenomena. While the phenomenon of WLF has been studied in many rivers (better known as flood pulse, Junk & Wantzen, 2004) and environmental flows (Bunn et al., 2006), so far it has not received due attention in lakes (Coops et al., 2003). A potential reason for why WLF in lakes have been overlooked for such a long time may be that central Europe and North America—the cradles of limnological research—are situated in landscapes which have been shaped by man for long time. Most central European lakes have been regulated since the middle ages; therefore, naturally fluctuating lakes such as Lake Constance (Fig. 1) are rare today. This lack of knowledge is most remarkable, as all climate change models consistently state that severe increases in climatic stochasticity can be anticipated and water availability will be reduced due to climate change (IPCC, 2001; Sophocleous, 2004). This means a decrease in snowfall and hence spring floods at midand high-elevations in temperate regions, which is countered by an increase of exceptionally strong rainfall events and severe droughts for all areas. Thus, there is an urgent need for comprehensive scientific approaches that address the impacts of WLF through socially and economically feasible mitigation schemes. However, it needs to be stressed that WLF are natural patterns which are necessary for the survival of many species, i.e. natural WLF in lakes guarantee both productivity and biodiversity (Gafny et al., 1992; Gafny & Gasith, 1999; Wantzen et al., 2002). Only extreme or untimely floods and droughts have deleterious effects for both biota and man (Sparks et al., 1998, Bond et al., 2008). The dramatic effects of climate change that can be witnessed today (extreme droughts and floods) remind us of the necessity to implement studies to Guest editors: K. M. Wantzen, K.-O. Rothhaupt, M. Mörtl, M. Cantonati, L. G.-Tóth & P. Fischer Ecological Effects of Water-Level Fluctuations in Lakes