Effects of copper on the energy budget of a stream detrivore : validation and ecological relevance
It is undisputed that single species toxicity tests are essential for obtaining information on concentrations and durations of exposures to chemicals that result in changes in survival, reproduction, physiology, biochemistry and the behaviour of individuals within a particular species (Caims, 1983). However, the extent to which single species toxicity tests can be used to predict effects in the natural environment and changes at higher levels of biological organization are largely unknown (Kooijman, 1985). The ultimate aim of laboratory tests is to predict the potential effects of toxicants in natural systems in order to protect the structure and functioning of the ecosystem. Hence, the aim of this study was to investigate the validity (i. e. the extent to which effects observed in the laboratory are mirrored in a more natural environment) and ecological relevance (i. e. the extent to which the tests are indicative of effects at higher levels of organisation which are important for the structure and / or functioning of the ecosystem) of a particular single species laboratory test. In order to increase its potential ecological relevance, both test species and response criteria for the laboratory test were carefully selected. The effects of copper, a reference toxicant, on energy budget parameters of Gammarus pulex, an important stream detritivore, were investigated. The validity and ecological relevance of these energy budget tests, under more natural conditions, were investigated with the use of outdoor artificial streams. In the laboratory tests, consumption rates were identified as the most sensitive energy budget parameter to copper-induced stress in G. pule-x. This decrease in consumption rates was due to an increase in the body copper concentration of G. pulex via copper uptake from the media, rather than to a rejection of copper contaminated food. As well as being the most sensitive parameter to copper stress in laboratory tests, consumption rates were identified as the most sensitive energy budget parameter measured in animals deployed in the artificial streams. Further, there was no statistically significant difference between the effects of copper on consumption rates of animals in laboratory tests and in the artificial streams. Therefore tests based on consumption rates as a response criterion were valid in more natural systems. Potential additional effects on the consumption rates of G. pulex due to copper-induced reductions in food quality were also indicated. The ecological relevance of these tests was investigated by trying to predict higher level effects in terms of growth and reproduction of whole organisms in the laboratory and in terms of the structure and functioning of populations and communities in artificial streams. Predictions regarding effects of copper on growth and reproduction of whole organisms were fairly accurate in indicating trends but were unsuccessful in predicting actual growth rates and reproductive output. Population density and leaf processing were selected as response criteria indicative of effects of copper on the structure and function of the freshwater ecosystem respectively. Effects on these two parameters were observed at the same concentration as adverse effects on consumption rates, and no adverse effects were detected at concentrations below those causing a decrease in consumption rates. Potential consequences, of the observed effects of copper on G. pulax, for the structure and functioning of the freshwater community are discussed. iii It is concluded that by careful selection of ecologically meaningful response criteria and test species, laboratory tests based on individuals can be representative of potential impact of a toxicant at higher levels of organisation in the natural environment. Consumption rates in G. pulex represent a sensitive, valid and ecologically relevant response criterion for the determination of the effects of a pollutant in natural ecosystems.