Micro-ecosystems and toxicants: A search for effects on the ecosystem level of organisation
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Micro-ecosystems might play an important role in the evaluation of the environmental hazards of toxic substances. Besides arguments that micro-ecosystems in principle can be expected to be more sensitive than classical mono-species tests are, it must be stressed that an evaluation of the effect on the functioning of complete ecosystems is necessary. For the analysis of effects on this level of biological organization an ecosystem approach is chosen. The experiments were performed in circulating aquatic micro-ecosystems consisting of three units with a total volume of 7.5 litres. The separation in subsystems was made to obtain a more stable system by introducing spatial complexity. Moreover three trophic levels were more or less restricted to the three subsystems and could be analysed separately. The trophic levels were primary producers (algae), primary consumers (Daphnia magna), and decomposers (bacteria). These micro-ecosystems can be maintained for several years in a dynamic steady state. Applying stress to a system gives rise to strain. There are no obvious measures to quantify the strain of ecosystems. Systems can be described by state variables that can attain certain values. Ecosystem strain can be defined as the distance in the state space to a reference state. Stable ecosystems do not have one reference state, but they occupy a small region in the state space. This area can be called the normal operating range. The centre of this normal operating range can be chosen as the reference state. States within the normal operating range have a finite strain, which will be called endogenous strain. By relating the ecosystem strain to the normal operating range a normalized ecosystem strain can be defined. An example of the time course of the normalized ecosystem strain in an experiment with a pesticide is given. This example is restricted to the analysis in the state plane of two state variables,i.e., number ofDaphnia magna and algal concentration. The normalized ecosystem strain attained values in the period that the pesticide was present which were clearly outside the normal operating range. This strain can be considered as a response of the system to the applied toxic stress. This effect had not been discerned in the analysis of theDaphnia population or the algal concentration.