Spatial and temporal patterns in abundance and distribution of zooplankton in the Tanzanian waters of Lake Victoria

Zooplankton composition and abundance, and its spatial and temporal distribution patterns were investigated from September 2005 to October 2007 at 51 sampling stations at various depths in the near shore, intermediate, and offshore waters of Lake Victoria. Relative to near shore (54.86 +/- 32.33 ind. l-1) and far offshore (38.63 +/- 11.14 ind. l-1), the intermediate waters (109.56 +/- 64.70 ind. l-1) had highest abundance of zooplankton. A total of 43 zooplankton species were encountered: of those 26 species were rotifers, with Brachionus angularis, Brachionus calyciflorus, and Asplanchna sp. as the most abundant species. There were 9 species of cladocerans, with Diaphanosoma excisum and Bosmina longirostris as the most abundant species. There were 7 cyclopoid species with Thermocyclops emini, Tropocyclops tenellus, and Thermocyclops neglectus as the most abundant species. Only one calanoid species, Thermodiaptomus galeboides was encountered. Cyclopoid copepods were the most abundant (76.5%) followed by calanoids (11.3%), rotifers (8%) and cladocerans (4.3%). The present findings contrast with earlier studies from the 1930s and 50s, but agree with many thereafter. There is change of zooplankton composition and abundance with a trend towards small sized species instead of the large. Increased eutrophications, pollution, and predation pressure from an increasing abundance of dagaa, juvenile perches, and reappearing haplochromines, among others, are responsible for the observed spatial and temporal variability in distribution and abundance patterns, and generally, change.