engleski

Metazooplankton abundance in Danubian floodplain lake during extreme flooding in 2006 (Croatia)

The main objective of the present study was to determine the influence of extremely high water level on abundance of metazooplankton groups inside the floodplain segment Lake Sakadaš. We hypothesized there is a difference in abundance of metazooplankton between periods of increased and decreased water levels. In parallel we examined relation of metazooplankton with environmental parameters and food resources. Finally, the results from the floodplain lake were compared with the River Danube, to enable determination of the lake storage potential. Metazooplankton samples were collected monthly during ice-free season from March until November 2006. Two sampling stations were chosen, the one located in Lake Sakadaš, as a segment of Kopački Rit floodplain, and the other in the River Danube. Due to extreme hydrological conditions inside the floodplain, which lasted for almost 4 months (111 days) we divided our samples into two hydrological groups: 1. increased water level or disturbed phase and 2. decreased water level representing a more stable phase. The classification resulted with distribution of samples from March, April, May, June and November in the first group and the samples from July, August, September and October were classified into the second group. Measured environmental variables were water temperature (°C), dissolved oxygen (mg l-1), pH, conductivity (uS cm-1), water transparency (cm), ammonium (mg l-1), nitrites + nitrates (mg I-1), Kjeldahl nitrogen (mg I- 1), total phosphorous (mg l-1) and chlorophyll a. Abundance of metazooplankton 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. Danube metazooplankton abundance showed no significant difference between hydrological groups (p = 0.53). 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. Cladocerans and copepods were completely redundant during the whole investigated period while only abundance of nauplii exceeded 6 ind. I-1 in few samples. Rotifers were significantly positively correlated with the total number of bacteria, as well as with ammonia and total phosphorous concentrations, whereas there was no significant correlation with chlorophyll a concentration. Results suggest importance of heterotrophic component of microbial food web for rotifer development. Hydrological groups were significantly different regarding physical and chemical parameters, chlorophyll a concentration and total number of bacteria (one-way ANOSIM R = 0.315, p = 0.018). Lake Sakadaš showed potential as a storage zone for metazooplankton development during more stable hydrological periods that followed extreme conditions.

metazooplankton, floodplain, Lake Sakadaš, storage zone

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