Seasonal Variation of Biomass and Secondary Production of Thermocyclops (Cyclopoida) and Brachionus (Rotifera) Spp. in a Shallow Tropical Lake Kuriftu, Ethiopia

Abstract

Monthly net samples were taken at inshore and midlake sites in Lake Kuriftu from August 2008 to May 2009 to estimate biomass of the dominant zooplankton, the cyclopoid copepod (Thermocyclops consimilis, 71% in abundance) and the rotifer Brachionus calyciflorus (12%). T. consimilis was cultured in the laboratory to obtain life history data on duration of embryonic and post-embryonic development at 20 and 25.9°C, fed mixed algal food enhanced with F/2 culture medium. Laboratory duration times were close to biomass turnover rates calculated from field data for both species.  Laboratory results and field data were used to estimate production of T. consimilis with the growth increment method. The mean biomass and production of T. consimilis were 23.8 mg dry weight (DW) m^-3 and 0.78 mg DW m^-3d^-1, respectively, and daily and annual P:B ratios were 0.033 and 13.10, respectively. Cyclopoid production was highest during the post-rainy month of November enhanced by a month lag of decomposed detrital food in October.  For B. calyciflorus, its dry mass, determined from biovolume, and recruitment rates of new individuals, were used to calculate secondary production. The mean biomass and production of this species were 629.66 µg DW m^-3 and 180.38 µg DW m^-3 d^-1, respectively, with daily and annual P:B ratios of 0.29 and 104.6, respectively. Rotifer production was highest during the dry months of December – March with high temperature and chl a. Overall, secondary production  rates in Lake Kuriftu were low compared to other tropical lakes and related cyclopoids and rotifers, and  some reasons for this observation are discussed.