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The ecological risk assessment of pesticides is generally based on toxicity data obtained from single-species laboratory experiments and does not take into account ecological interactions such as competition or predation. Intraspecific and interspecific competition are expected to result in additional stress and might increase the sensitivity of aquatic populations to pesticide contamination. To test this hypothesis, the effects of the fungicide carbendazim were assessed on the population dynamics of the micro-crustacean Daphnia magna under different levels of intraspecific and interspecific competition for an algal food resource, using the rotifer Brachionus calyciflorus as competing species. The experiments were performed in glass jars with three different carbendazim concentrations (i.e., 50, 100 and 150 µg/L), and had a duration of 25 days, with a 4-day pre-treatment period in which competition was allowed to take place and a 21-day exposure period. The endpoints evaluated were D. magna total population abundance and population structure. Results of these experiments show that competition stress on its own had a significant influence on shaping D. magna population's structure, however, a different response was observed in the intraspecific and interspecific competition experiments. The use of a 4-day pre-treatment period in the intraspecific experiment already led to an absence of interactive effects due to the quick abundance confluence between the different intraspecific treatments, thus not allowing the observation of interactive effects between competition and carbendazim stress. Results of the interspecific competition experiment showed that rotifers were quickly outcompeted by D. magna and that D. magna even profited from the rotifer presence through exploitative competition, which alleviated the original stress caused by the algal resource limitation. These experiments suggest that competition interactions play an important role in defining population-level effects of pesticides in a more complex way than was hypothesized ("increasing competition leading to a sensitivity increase"), as the interspecific experiment showed. Therefore, these should be taken into account in the extrapolation of single-species toxicity data to protect higher levels of biological organization.

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Abstract. Intensive agricultural practices have been characterized by an overuse of agrochemicals. The inputs of chemicals in a watershed are likely to alter trophic interactions affecting its ecological integrity. This ecotoxicological study aims to identify warning signals of agrochemicals effects on a plankton community. Eighteen outdoor microcosms were used to establish an experiment with 2 concentrations of copper sulfate above and below the legal limit with six replicates lasting two weeks. Phytoplankton and Zooplankton structure changes were studied. Chlorophyll-a concentration was used as a functional indicator. A rapid change in phytoplankton structural features, abundance and size classes, was detected after both, below and above legal, concentrations. Similarly, Zooplankton structural changes showed an effect of copper exposures on abundance and composition. As Phytoplankton response was so rapid, it could be used as an early and direct warning signal, but also to warn of future indirect effects on zooplankton structural features owing to a change of food resources. In summary, the shifts across both trophic levels could be use as warning signals. Moreover, even legal limits do not protect the plankton community, which emphasises the need of a more ecotoxicological and realistic approach to achieve a balance between agriculture development and ecosystems conservation.

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.