This study investigated relationships of hydrological variability and potential food resources for metazooplankton groups: rotifers, cladocerans, copepods and nauplii. Samples were collected monthly during ice-free season from March until November in 2006. Two sampling stations were chosen, the one located in Lake Sakadaš, as a segment of Kopački Rit floodplain (Croatia), and the other in the River Danube. Due to hydrological conditions we divided our samples into two hydrological groups which corresponded to: 1/ increased water level i.e. disturbed phase and 2/ decreased water level representing a more stable phase. Abundance of metazooplankton was not significantly different between hydrological groups in the River Danube. However, it was significantly different between hydrological groups in Lake Sakadaš (one-way ANOSIM R = 0.688, P = 0.024), where during a decreased water level higher abundance of metazooplankton was recorded. Rotifers were the most abundant metazooplankton group during the whole investigated period at both stations and between both hydrological groups comprising almost 99% of total metazooplankton abundance. Rotifers were significantly positively correlated with the total number of bacteria, as well as with ammonium and total phosphorous concentrations. The metazooplankton community in Lake Sakadaš was negatively influenced by flooding, but not in the River Danube. Compared to the River Danube the investigated floodplain lake showed potential as a storage zone for metazooplankton development during more stable hydrological periods. During that time abundance of rotifers was related to the heterotrophic component of microbial food web. Hence, this investigation adds to the understanding of the metazooplankton dynamics in riverfloodplain systems as well as of their relations with trophic levels under variable hydrological conditions.
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