Response of Zooplankton to Restoration and Climate Warming in Alte Donau

Abstract

Lake restoration is commonly assessed by changes in water transparency, nutrients and biomass of phytoplankton, while information about changes in zooplankton that is triggered by lake management is often missing. In our 19-year study we used rotifers and crustaceans to document the effect of restoration measures on zooplankton in the oxbow lake Alte Donau, a former side-arm of the Danube River which is most popular for recreation and angler (cyprinid-dominated shallow water). The record covers four management periods: the period before restoration, the restoration (including years of chemical phosphate precipitation by Riplox treatment), the re-establishment of macrophytes and the sustained ‘stable conditions’. We found the highest abundance of all zooplankton in the first Riplox-year, with decreasing zooplankton abundance in following periods associated with the decline of phytoplankton. In the long term, the main compositional change related to a shift from a cladoceran-rotifer-rich to a copepod-rotifer-rich zooplankton assemblage. Thus, the large-bodied zooplankton shifted from a community composed of mainly filter-feeding herbivorous cladocerans under eutrophic algal-turbid conditions to mainly selective-feeding omnivorous and herbivorous copepods under mesotrophic transparent-water conditions. While the carbon ratio between zoo- and phytoplankton increased significantly during the first three periods and remained high under ‘stable conditions’, the mean body size of zooplankton did not exhibit a long-term trend. Short-term increases of large-bodied zooplankton coincided with an intermittent increase of calanoid copepod abundance (Eudiaptomus gracilis) during the chemical treatment concomitant with a drastic phytoplankton biomass reduction and the occurrence of large-bodied cladocerans (Simocephalus vetulus and Sida crystallina) in some years with re-established underwater vegetation. Besides the main response of zooplankton to the ‘bottom up’ control that was triggered by the reduction of phytoplankton food supply by one order of magnitude, we studied the zooplankton response to climate change. The impact of climate warming was evident from intra-annual coincidence of the climate signal (NAODJFM) and water temperature (WT) in winter and early spring, the increase of surface water temperature (SWT) by 1.52 °C per decade in April and the prolongation of the warm period (SWT > 22 °C) by 10.5 days per decade in summer. This prolongation of the warm season seemed to support the summer development of the medusa stage of freshwater jellyfish (Craspedacusta sowerbii). During the transition from spring to summer, the progressively earlier clear-water phase followed two trends. The first period with a 33-day earlier clear-water phase per decade coincided with pronounced ecosystem changes from a high to a low eutrophic state created by chemical restoration measures. The second period with a moderate earlier progression of 7 days per decade was accompanied by a further, slight TP decrease associated with the re-establishment of macrophytes. When comparing rotifers, cladocerans, calanoid and cyclopoid copepods, the latter group benefits most from seasonal temperature increases and climate warming.