engleski

Seasonal changes of zooplankton in the dam reservoir Modrac (Bosnia and Herzegovina)

The Modrac reservoir was built in 1964 (altitude, 200 m ; length, 11 km ; average depth, 4, 5 m ; maximum depth near the dam, 14 m). Studies on plankton and physico-chemical conditions in the reservoir were initiated in 1988 when zebra mussel (Dreissena polymorpha) appeared and for water supply to the nearby town Tuzla, and have continued with short breaks until 2003. All physical and chemical parameters were measured using standard methods (APHA, 1985). The number of plankton species and their densities were very low (<20 ind./L), except during spring and summer periods. Most rotifers feed on nanophytoplankton and benthic rotifers which are microfiltrators (Keratella cochlearis, K. cochlearis var. tecta, K. quadrata and Kellicotia longispina), were constantly present. When present, Polyarthra and Synchatea species were always more abundant than other macrofiltrators. In the spring or summer, rotifer maximum sometimes reached 500 to 600 ind./L, mainly due to K. cochlearis and K. cochlearis var. tecta. Three cladocerans that were constantly present: Bosmina longirostris, Daphnia cucullata and D. longispina, and indicated mesotrophic conditions in the reservoir. Among copepods, Thermocyclops crassus, T. oithonoides and Cyclops strenuus landei dominated. It is difficult to assess trophic degree of the reservoir although all measured physical and chemical parameters indicated mesotrophic conditions. K. cochlearis var. tecta, which sometimes represented over 50% of rotifer densities, indicated that eutrophic conditions prevailed in the reservoir.

zooplankton; seasonal changes; Modrac reservoir; rotifers; cladocerans; copepods

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