During the unusually long European drought between 2000 and 2003, the water level of the large and shallow Lake Balaton, Hungary (area 5 596 km 2 , mean depth 5 3.25 m), decreased by 28%. Although food availability for zooplankton remained unchanged, and the fish stock declined more than the water mass, the density of populations of several planktonic rotifers, cladocerans, calanoid copepodes, and veligers decreased by 60-90% simultaneously with the water-level decrease and regenerated only after the drought. The generally strong turbulence of the lake was intensified during the four consecutive years of low water, as verified by instrumental monitoring of the turbulence intensity and by the estimation of the turbulent kinetic-energy dissipation rate. In our tank experiments, turbulence conditions similar to those that existed in the lake during low water were simulated, but mineral suspended material was minimized and food was regularly resupplied. Under these experimental conditions, zooplankton taxa showing the highest mortality were the same as those that were most susceptible in situ. Increased turbulence coupled with the water-level decrease is especially unfavorable for rotifer Keratella, the cladoceran Daphnia, Bosmina, and copepodit and adult stages of the calanoid copepod Eudiaptomus gracilis in this lake.Moderate turbulence intensity is beneficial for zooplankton, because it increases the producer-consumer encounter rate, and improves consumers' capture efficiency, feeding, and growth rates (Saiz and Alcaraz 1991; Visser and Stips 2002;Visser et al. 2009). High turbulence, however, interferes with the feeding current of the suspension feeders, erodes the remote detection of prey by their predators, inhibits their food ingestion, and increases their metabolic energy expenditures Kiørboe and Saiz 1995;Peters and Marrasé 2000). And finally, very high turbulence could disorientate the movement, lead to uncontrolled vertical drifting, and could even injure the largest bodied organisms (Reynolds 1992;Visser et al. 2009).Earlier we proved experimentally and quantified the relationship between frequently resuspended fine mineral sediment (mineral turbidity) of Lake Balaton and inhibition of the effective food collection and development of cladocerans, especially Daphnia, even if food abundance exceeded threshold concentrations (G.-Tó th 1984; G. -Tó th et al. 1986). However, the direct physical effect of turbulence on the zooplankton was never established in Lake Balaton or any other shallow lake.Lake Balaton is the largest shallow lake in Central Europe. Because the lake is very shallow, the vertical space for the dissipation of the kinetic energy content of the water mass induced by the wind is very limited, and so dissipation per unit of volume is very high. Higher than 4 m s 21 wind velocity occurs, on average, every third day and causes a turbulence sufficient to resuspend sediments. Wind faster than 10 m s 21 occurs , 12-15 times annually, resulting in sediment resuspension up to 600 mg dry wei...

Eutrophication of shallow lakes often triggers a series of cascading ecological effects. Among these are shifts in the zooplankton community structure due to phytoplankton changes, or shifts in the fish community reducing size-selective feeding of planktivorous fish. In such conditions larger zooplankton (e.g. Daphnia) can have a selective advantage. Re-oligotrophication can reverse such food web interactions. Europe's largest wetland system (the Danube Delta) went through a period of eutrophication and is now slowly recovering again. However, changes in the Daphnia populations triggered by eutrophication in this system have remained unstudied. We used different sampling strategies to screen 24 lakes (which differ in their ecological state) for the presence of Daphnia as well as for biotic and abiotic parameters potentially explaining Daphnia abundance. Daphnia densities were surprisingly low. We found D. magna ephippia in the sediment, but no D. magna in the water column. Microsatellite analyses on pelagic Daphnia populations confirmed the presence of the Daphnia longispina complex and provided evidence for significant hybridisation events. FluoroProbe data showed that Daphnia was mainly present in lakes with a higher phytoplankton production. Our study provides insights into the spatial and temporal distribution of Daphnia in a very dynamic wetland system.

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