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Abstract
The diversity, abundance and biomass of microcrustaceans were studied monthly in Lakes Ehoma and Iyieke from March 2005 to August 2006. Twenty-six (26) species were identified and Daphnia Obtuse was recorded for the first time in Nigerian freshwater. Contrary to our expectations, diversity, total biomass and cladoceran biomass were significantly higher during the rainy season (p < 0.05), and inter-lake variation in these was not significant, although the abundance of copepods was significantly higher at Ehoma. Diaphanosoma excisum accounted for over 50% of the total zooplankton biomass in the two lakes, low total biomass during the dry season was mainly due to the low population of this species. A well-defined succession pattern in which the cladocerans Moina micrura and D. excisum were predominant during pre-rainy and rainy seasons and the copepods Thermocyclops neglectus and T. decipiens dominant during dry and post-dry seasons was discerned. Canonical correspondence analysis showed that zooplankton succession and biomass were influenced by seasonal variations in the major environmental variables (temperature, dissolved oxygen, lake morphometry and pH) of the lakes. These variables were in turn influenced by the seasonal flooding of the lakes. Since the attainment of maximum zooplankton biomass coincided with the breeding season of the major fishes of the Cross River, we therefore propose that factors such as climatic changes and the damming of the Cross River that could alter the flooding pattern of the lakes may inadvertently affect the zooplankton community structure of the lakes and consequently the fish productivity of the entire Cross River ecosystem.