Properties of an aquatic micro-ecosystem

Abstract

A mlcro-ecosystem, composed of three trophic levels, was built in 1973. Up to the present time (1976) the biological and chemical state has remained essentially the same. The trophic levels are spatially separated, but interconnected by a continuous flow system. Especially steady state properties of the autotrophic subsystem are discussed. The herbivorous subsystem, mainly populated by Daphnia rnagna, has a population density of 400-700 individuals per liter, though strong fluctuations can be observed. The composition of the decomposer unit is unknown. A high rate of mineralization and a high buffering capacity, at least for phosphor, are present. This results in a constant output rate of phosphor over long periods. The autotrophic subsystem contains different species of algae, although at most times one or two are dominating. The algal biomass fluctuates around a mean concentration of 1.6 • 10 TM /~m 8 1-1. A "statistical steady state" is present with a mean relative rate of change in total particle volume equal to the dilution rate. The kind of frequency distribution of the relative rate of change seems to depend to a large extent on the conditions of the system. An autocorrelation technique was used to analyse the particle volume time series for periodic oscillations. For stationary periods a periodic function is involved which might be independent of the algal association present. The micro-ecosystem shows a strong tendency for selfmaintenance. It seems to be suitable for long-term observations and for experimentation with various ecosystem properties. It is especially suited for studying the behaviour of toxic substances in the foodweb, as well as the fate of these toxicants a~er mineralization and reentering of the foodchain.