BIOMASS AND PRODUCTION OF THE MAJOR DOMINANT CRUSTACEAN ZOOPLANKTON IN A TROPICAL RIFT-VALLEY LAKE, AWASA, ETHIOPIA

Seasonal variation of the biomass (B), production (P) and P/B ratio of the numerically dominant crustaceans in Lake Awasa (Mesocyclops aequatorialis similis, Thermocyclops consimilis and Diaphanosoma excisum) were studied during 1986 and 1987. Quantitative net samples (64-mu-m mesh) were taken at three stations on approximately 10 day intervals throughout 1986, and the dry weights and development times for each life stage were obtained from laboratory measurements and cultures. Total biomass of most of the dominant crustaceans, determined from 390 samples during 1986, was 44.85 mg m-3 (dry weight, DW) with adult females of Mesocyclops making > 43.5%. Alona diaphana, another common crustacean, is dealt with in a separate paper, as are the Rotifera. Production of the dominant crustaceans during 1986 was estimated by the growth increment summation (Winberg) and instantaneous growth (Ricker) methods. The annual integrated production of the two dominant cyclopoids is 535.2 mg (DW) m-3 (Winberg) while annual crustacean production totals 2.5 g (DW) m-3 (Ricker). The mean annual P/B ratio for individual species and stages varied from 221.0 for Diaphanosoma, to 121.7-143.0 for nauplii and 9.8-18.7 for copepodites of the cyclopoids. It was 55.8 for the dominant zooplankton species. Low or high zooplankton production and biomass turnover rates (P/B) cannot be used to characterize all tropical lakes consistently. However, production per unit biomass is likely to be higher in tropical lakes.