, Volume 408-409, Issue 0, pp 251-262

On the occurrence of clear-water phases in relation to shallowness and trophic state: a comparative study

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Abstract

We present a comparative study on the occurrence of spring clear-water phases in six eutrophic lakes (two deep, four very shallow lakes dominated by plankton, TP: 0.06–0.13 mg l-1) in the Scharmützelsee region (Germany). Our aim was to analyse the occurrence and intensity of clear-water phases in relation to shallowness and trophic state. We defined a clear-water phase by a continuous increase of Secchi depth, a corresponding decrease in algal biomass and a shift in phytoplankton species composition during the growth phase of cladoceran zooplankton. For shallow lakes, we used the increase of the euphotic depth up to maximum depth as an additional criterion. Only in two of six lakes a clear-water phase occurred. In two lakes, no spring peak of cladocerans developed. In two other lakes, a high biomass of grazing resistant cyanobacteria was not affected by cladoceran maxima. Daphnia galeatawas an important component of the grazer community in lakes with a clear-water phase, whereas lakes without a clear-water phase were dominated by Bosmina longirostris. Top-down control of grazing by fish predation is discussed as the main factor determining biomass, composition and timing of the cladoceran spring peak. The assumed relative strength of fish predation between lakes corresponds to the response in algal biomass, dominance of cyanobacteria and is inversely related to the potential grazing pressure (ratio of cladoceran and algal biomass). Bottom-up effects (food quality) and additional factors (fungal infection, flushing rate) may also influence growth and timing of cladocerans. Our results support the view that a high biomass of grazing zooplankton is essential for the occurrence of a clear-water phase, but not sufficient to explain variability between lakes. High cladoceran maxima in hypertrophic lakes indicate that food quality seems not to limit grazer biomass. Decoupling of zooplankton and phytoplankton may be the result of early and fast growth of grazing resistant algae. We conclude that shallowness and trophy may indirectly reduce the extent and predictibility of a clear-water phase by enhanced growth of cyanobacteria and increasing predation pressure at least for lakes on this TP level.