Studies on the community structure,population dynamics and production of the limnetic zooplankton in a tropical lake,lake naivasha, Kenya

Abstract

The first quantitative study of the limnetic zooplankton community of Lake Naivasha, Kenya, was undertaken from July 1978 to July 1980. Using a 105 um mesh net, vertical haul zooplankton samples were taken from 3 stations in the limnetic zone of the main lake, at weekly intervals during the first year and at fortnightly intervals during the second year of the study. The vertical net haBl numerical estimates of zooplankton abundance were adjusted by employing net correction factors determined by using a newly developed volume sampler, similar ln principle to the conventional Van Dorn sampler. The pelagic zooplankton community of Lake Naivasha was composed of 2 cyclopoid and 1 calanoid copepod species;11 cladoceran and 13 rotiferan species. During the two year study period the limnetic zooplankton was ,numerically dominated by the following species, in order of their numerical dominance: Thermocyclops oblongatl Sars; (Copepoda), 54-61%); Diaphanpsoma excisum_Sars (Cladocera)(18-29%)) Brachionus caudatus Barrois and Daday (Rotifera),(8-11%); ~. calyciflorus Pallas (Rotifera) (7-10%); arre-Simocephalus vetulus King (Cladocera) (1-3%); the values in parentheses are mean ranges recorded during the two.year study period. Other occasionally important liMnetic zooplankters were Mesocyclops eguatorialis Kiefer (Copepoda), Daphnia laevis Sars (Cladocera), and,Tropodiaptomus banforanus Kiefer (Copepoda). Throughout ..the study period the species cbmposition of the zooplankton community in Lake Naivasha was relatively stable In proportion and variety when compared with the zooplankton of temperate lakes. Significant temporal chan&es in abundance of the zooplankton populations occurred on a seasonal basis associated with the dry and wet periods in the lake basin. High densities of the zooplankton are positively correlated with h\ph 'concentrations of chlorophylla_ , as well as rlslng lake level. Annual mep.ntotal zooplan~ton biomass was 237 and 3 • 425 mg dw/m for the first and second years of study, I respectively. Zooplankton biomas peaks coincided with the rainy seasons in the lake basin. Thermocyclops oblongatus and Diaphdnosoma excisum (which have been studied in detail) dominated the zooplankton biomass contributing 50-60% and 29-35% respectiyely. Rotiferan contribution to biomasswas usually low (Less than 7%) and rnainly from the two Bramionus speci.es . The dissolved oxygen and thermal regimes, which are mainly seasonal In temperate lakes, were found to be confined to diet events in this tropical lake. There was a pronounced (though temporary) oxygen and thermal stratification in the lake during calm sunny weather, usually between 11.00 hrs and 14.00 hrs. The stratification was initially broken down by the afternoon winds and subsequently by further night time cooling. Diel vertical distrjbution of the major zooplankton species was found to be homogen.eous throughout the water column; but with an apparent tendency for most zooplankters to aggregate above .the 4 metre zone, usually at noon, and most probably ln association with feeding and breeding activities No evidence of diel vertical migration by any of the zooplankters was 'recorded in the present study. Light does not seem to play a major role as a synchroniser to the diel activities and vertical distribution of zooplankton ln Lake Naivasha. The durations of development of the embryonic (egg) and post-embryonic stages of Thermocyclops oblongatus and Diaphanosoma excisum were determined ln the laboratory. The embryonic development times - 0 of both species determined at 19, 22, 25 and 28 C were found to be inversely and linearly correlated with temperature. The durations of embryonic and post-embryonic development determined at the lake's mean annual temperature of 220C were used to provide the time element in the calculation of production by the two species. Instantaneous birth rates and rates of change of oblongatus and D. excisum population densities were estimated. The calculated birth rates gave mean turnover times of numbers of 5.3 days for !. oblongatus and 13.5 days for D. excisurn. Turnover times 0 f biomass were 11. 4and 13.7 days for !. oblongatus and Q. excisu~respectively. The daily losses of the population biomass of 9% for T. oblongatus and 7% for D. excisum In Lake Naivasha are extremely low when compared with zooplankton ,, populations In lakes where predation pressure on the zooplankton lS intense. Estimates of production,by the two dominant limnetic zooplankters Thermocyclops oblongatus and Diaphanosoma excisum were calcuated using Winberg's (1971) weight increment method. The annual mean :' 3 production rates were 11.0 m~ dw/m /d. for !. oblongatus and 6.0 mg dw/m3/d. for D. exclsum. Eggs, nauplii and copepodites of T. oblongatus contributed 5%, 48% and 47%, respectively to the total annual net production, in dry weight, by the copepod. Since there are virtually no zooplanktivorous predators in the open lake, predation of the limnetic zooplankton in Lake Naivasha is virtually zero; in the littoral area, however, fish fry of Sarotherodon leucostictus and the adults of Gambusiaaffinis exert substantial predation pressure on the zooplankton. The bulk of all the zooplankton productionpasses to the decomposer food chai n and is not utilised by higher trophic levels.