Seasonal distribution of the zebra mussel larvae in the dam reservoirs Dubrava, Cakovec, and Varazdin, Croatia

摘要

Populations of the zebra mussel (Dreissena polymorpha) have become common in the 3 dam reservoirs of the hydroelectric power plants Dubrava, Cakovec, and Varazdin (in the north-western part of Croatia) since they first appeared in the late 1980s in the dam reservoir Dubrava and subsequently spread upstream (LUTHER 2005; Fig. D. The reservoirs are part of the Drava River system, and upstream movement was enabled by the navigation and recreational activities on the reservoirs. The zebra mussel has free-swimming veliger larvae (ACKERMAN et al. 1994), which attach themselves to the artificial substrates of the reservoirs and power plant facilities, develop into adults, and then disturb the functioning of the power plants.Due to these problems we have started a broad monitoring program that includes an investigation of the ecology of zebra mussel in these reservoirs. As part of monitoring we investi-gated the vertical and longitudinal distribution of veliger larvae in all 3 reservoirs. The changes in veliger density and seasonality can help in predicting the zebra mussel infestation in a particular reservoir. After their invasion, zebra mussels alter food webs (STRAYER et al. 1999) and can cause significant changes in water quality (CARACO et al. 2000). We measured algal biomass as chlorophyll a (chl-a), as well as some physical and chemical parameters of the water. Previous studies have shown that the population distribution of the zebra mussel is controlled by temperature, pH, calcium concentration, and physical stresses such as strong currents and turbulence (HOR- VATH & LAMBERTI 1999).