Plankton community responses to environmentally-relevant agrochemical mixtures

Chemicals do not occur alone in the environment but most ecotoxicological assessments target the effect of single chemicals on aquatic communities and the establishment of legal limits is based on them. The present study assesses how plankton communities respond to single and mixture treatments of copper sulphate and ammonium nitrate where both agrochemical concentrations are below legal limits. Twenty-five microcosms were used to assess the effects of four treatments (n = 5): (1) low nitrate (L) of 25 mg L−1; (2) high nitrate (H) of 50 mg L−1; (3) copper treatment (CU) of 0.04 mg L−1 of copper; and (4) interaction treatment (I) of 50 mg L−1 of nitrate applied together with 0.04 mg L−1 of copper, and the controls (C). Plankton abundance, phytoplankton biovolume and zooplankton community structure (changes in the diversity and richness) were used as structural endpoints, and oxygen production and litter decomposition as functional indicators. Overall, results show no effect on the plankton community exposed to agrochemical under legal limits in single neither in mixture treatments. Only by the end of the experiment, total zooplankton abundance shows differences between interaction treatment (I) and the rest of the treatments and controls. Concretely, the interaction treatment suggests how a nutrient enhancement from ammonium nitrate addition may counterbalance the toxic effect of copper sulphate on zooplankton, most likely as a result of higher phytoplankton availability that positively influences zooplankter survival. Both drastic and subtle effects on communities are relevant for disentangling how chemicals interact under European current legal limits.