Original Articles

POPULATION DYNAMICS AND DEMOGRAPHY OF AN ESTUARINE COPEPOD (PSEUDODIAPTOMUS EESSEI) IN LAKE SIBAYA, A SUBTROPICAL FRESHWATER COASTAL LAKE


Abstract

Population size and stage structure of Pseudodiaptomus hessei (Mrázek) was studied over 27 months at a single site in subtropical Lake Sibaya. Population density varied seasonally from about 1 to 4 individuals per litre, but a fairly stable population structure was maintained by continuous breeding of this strongly multivoltine copepod. Embryonic durations were determined at several temperatures and used to estimate instantaneous birth rates, death rates and rates of population change. Overall mean values obtained for these population statistics were 0,345, 0,350 and, ignoring signs, 0,018 respectively, the former two values being particularly high. Some evidence linking death rates, and more particularly naupliar survival of P. hessei to cyclopoid copepods is presented; although it was not possible to discriminate between cyclopoid predation and cyclopoid competition, the former is favoured as the factor mostly influencing P. hessei mortality. Statistical analysis of interrelationships between various population attributes of P. hessei, and other biotic and abiotic factors in the plank-tonic subsystem of Lake Sibaya revealed the overriding influence of temperature. The implications of the present observations to the production ecology of the pelagic subsystem of this lake are discussed

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