Early Detection of Contamination with Microplastics by Changing the Phototaxis of Freshwater Mesozooplankton to Paired Photostimulation

Our previous studies showed that the change in the plankton response to light could be an indicator of environmental pollution. This study experimentally reveals that the response of Daphnia magna Straus and Daphnia pulex plankton ensembles to photostimulation depends on the intensity of the attracting light. This makes it difficult to identify the occurrence and change of pollutant concentration. The large variability in the magnitude of the behavioral response is caused by the nonlinear response of plankton ensembles to the intensity of the attractor stimulus. As the intensity of the photostimulation increases, the variability of the phototropic response passes through increase, decrease, and relative stabilization phases. This paper proposes a modification of the photostimulation method: paired photostimulation involving the successive exposure to two photostimuli of increasing intensity. The first stimulus stabilizes the behavioral response, while the increase in response to the second stimulus makes it possible to more accurately assess the responsiveness of the plankton ensemble. The study demonstrates good reliability and increased sensitivity of this method of detecting changes in environmental toxicity when compared with single photostimulation or traditional bioindication through the survival rate of test organisms.