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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. Nitrate-copper mix within legal limits resulted on higher zooplankton abundance by the end of the experiment. It is explained by nutrient enhancement what may counterbalance the copper toxicity on zooplankton, most likely increasing phytoplankton availability. Consequently, studies on assumed safe toxicant mixtures are needed as subtle effects can have relevant consequences on communities. [ABSTRACT FROM AUTHOR]

More field relevant ecological assessments, apart from single species tests using standard species, are needed to better predict agrochemical effects at higher ecosystems levels. Therefore, an experiment using a non-target aquatic community was used to test the hypothesis of the negative effect of a single pulse of copper sulfate on plankton abundance, structure, richness and diversity endpoints. Microcosms (20 l volume) were established during 21 days of experimentation, using six replicates for controls and with two concentrations of copper sulfate (High treatment, H: 20 µg Cu l−1; and Low treatment, L: 2 µg Cu l−1), both within the copper legal threshold following the Water Framework Directive (2000). The general lineal model found significant differences at the phytoplankton abundance endpoint at the end of the experiment, with an increase of phytoplankton abundance in L treatments dominated by the smaller cell size class. The principal response curve on zooplankton data, despite being insignificant, pointed out some dissimilarities in abundance and structure between treatments and controls: treatments showed lower abundances and were richer in cladocerans and copepods than the control microcosm where rotifers and nauplii were dominant. This indicated that trends change in community structure due to the effects of copper sulfate, and that even if the copper concentrations under study were within legal limits, they showed potential to induce changes in planktonic communities. [ABSTRACT FROM AUTHOR]