The effects of food limitation on the population dynamics, production and biological interactions of three Daphnia species, co-existing in a London reservoir

Abstract

This thesis is concerned with the population dynamics and production ecology of Daphnia magna Strauss, D.pulicaria Forbes and D.hyalina Leydig from a riverine impoundment, Wraysbury Reservoir, in the lower Thames Valley, U.K.The three species co-exist throughout the year and are the main contributors to the zooplanktonic biomass of the reservoir.Weekly samples taken from several stations in the reservoir and over seventeen months during 1985 and 1986 provide data on the numerical densities, size structure and fecundities of the three populations and permit the analysis of their population dynamics. These data, together with regularly determined length-carbon weight relationships within the field populations, enable estimates to be made of population biomasses. Periods of sexual reproduction were identified from the appearance of males and ephippial females and some attempt was made to quantify the benthic stock of viable ephippia in the reservoir. The appearance of 'bites' on the carapaces of some individuals indicated the presence of a predator whose identity has not yet been discovered. Some attempts were made to assess the quantity of food available to the daphnid populations and to identify the periods of food limitation. Food level was estimated by the direct determination of the less than 50 micron fraction of the chlorophyll a biomass, converted to units of Carbon from chlorophyll a : carbon ratio derived from field data. Periods of food limitation were indicated by the size of the primiparous female, the Carbon weight of a standard female and the relationship between fecundity and size of female for each daphnid species. It was possible to compare these field data with the results of Rocha (1983) from her long-term laboratory life cycle studies on the same species from Wraysbury Reservoir which were conducted under defined food and temperature conditions. This comparison permitted the prediction of field food levels, which could be compared with those measured directly.The identification of periods in the year when food was limiting to the daphnid populations provided the possibility of assigning both instar stages and duration of developmenmt to different-sized individuals, both based upon the experimental study of Rocha (1983). Knowing these population parameters, it is possible to make better estimates of the levels of population mortality rates at different times of year and of the secondary production of the three species in Wraysbury Reservoir.The seasonal time course of the population dynamics and production of the three species is considered in relation to food limitations, inter-specific competition and possible predation.