engleski

Seasonal distribution of the zebra mussel (Dreissena polymorpha, Pallas) larvae in the dam reservoirs Dubrava, Varaždin and Čakovec, Croatia

Zebra mussel was recorded for the first time in the dam reservoir Dubrava (north-west Croatia) at the end of 1980-ties while nowadays it is already detected upstream Drava River in other two dam reservoirs, Varaždin and Čakovec. All three dam reservoirs on the Drava stretch between Ormož ant the Mura River are part of the Drava River Hydroelectric Power Plants. In order to detect beginning of the reproduction and describe vertical and horizontal distribution of zebra mussel larvae we sampled larvae every week or two from May until November 2006 at 9 sampling points in each reservoir. Samples were collected along vertical profiles every meter or two depending on the water level in the reservoir until the depth of 7 meter was reached. Physical, chemical and bacterial parameters were measured as well as chlorophyll a. Density of the zebra mussel larvae was at the highest level in all three reservoirs during July (up to 141.210/10L measured in Dubrava at the 26th July 2006). At the same time the highest values of chlorophyll a were measured in all three reservoirs and were as high as 13.9 μ g/m3 measured in Čakovec. Zebra mussel larvae were most abundant in zooplankton community from June until September in all three reservoirs.

Dreissena polymorpha larvae; dam reservoir; hydroelectric power plants; Drava River; Croatia

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