Spatial distribution of zooplankton in Lake Ziway: A shallow tropical rift valley, Ethiopia

الملخص

Species composition and spatial distribution of zooplankton were studied in Lake Ziway based on biweekly sampling from April to July 2004. Microscopic analysis revealed that zooplankton species of Lake Ziway is composed of 21 planktonic rotifers, 5 cladocerans and 3 cyclopoid copepods. 28 more rotifer species were also identified from the littoral area covered with macrophyte. Calanoid copepods and harpacticoids were absent in the samples analyzed. The numeric contribution of crustaceans to the total zooplankton density was less than 15% (Cladocera 2% and cyclopoid copepods 12% (90% of which were naupliar stages) whereas rotifers contributed 86%. Four species of rotifers: Anuraeopsis fissa, Brachionus angularis, Filinia novaezealandiae and Trichocerca ruttneri, were dominant and contributed 24, 23, 10 and 10% to the rotifer community, respectively. Body size of crustaceans derived from inshore and offshore areas varied significantly. From five crustacean species analysed, only Ceriodaphnia cornuta did not differ in length distribution. The mean lengths of Daphnia barbata (p<0,0004), Diaphanosoma excisum (p<0,0001), Moina micrura (p<0,0001) and Thermocyclops decipiens (p<0,0001)  sampled from inshore were significantly smaller than those in the offshore areas. This difference could be attributed to selective predation mainly by invertebrates and juvenile fishes spending much of their  time  in  the  littoral  area  covered  with  dense  macrophyte.  The absence of size variation in spatial distribution of Ceriodaphnia cornuta would be due to size (mean length 295 µm, which is 3.4 times less than that of Daphnia barbata) that could reduce predation pressure.