Aquatic ecosystems are characterized by fluctuating conditions that have direct effects on aquatic communities but also indirect influences such as changing the toxicity of chemicals. Because the effect of food quality on pesticide toxicity has rarely been studied, in the present study Daphnia magna juveniles supplied with 4 different food quality levels were exposed to a range of imidacloprid concentrations for 21 d. Food quality was expressed as carbon:phosphorus ratios of algae Pseudokirchneriella subcapitata (C:P 35, C:P 240, C:P 400, and C:P 1300). Survival, growth rates, and reproduction of D. magna were monitored, and the combined effects of imidacloprid exposure and the phosphorus content of algae were analyzed. A stronger effect on survival was observed at the P-deficient diet (C:P 1300), confirmed by lower 10% effect concentration (EC10) values at days 7, 9, 15, and 21 compared with diets with higher phosphorus contents. Similarly, the growth rate was reduced when D. magna were supplied with algae of low phosphorus content at imidacloprid exposure conditions. The highest reproductive output was observed for D. magna fed the optimal phosphorus diet (C:P 240), both at control and exposed conditions. Poor food quality increased the sensitivity of nontarget species to pesticide exposure, potentially leading to an underestimation of adverse effects on aquatic communities in the field.

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Analyzing functional species’ characteristics (species traits) that represent physiological, life history and morphological characteristics of species help understanding the impacts of various stressors on aquatic communities at field conditions. This research aimed to study the combined effects of pesticides and other environmental factors (temperature, dissolved oxygen, dissolved organic carbon, floating macrophytes cover, phosphate, nitrite, and nitrate) on the trait modality distribution of aquatic macrofauna communities. To this purpose, a field inventory was performed in a flower bulb growing area of the Netherlands with significant variation in pesticides pressures. Macrofauna community composition, water chemistry parameters and pesticide concentrations in ditches next to flower bulb fields were determined. Trait modalities of nine traits (feeding mode, respiration mode, locomotion type, resistance form, reproduction mode, life stage, voltinism, saprobity, maximum body size) likely to indicate pesticides impacts were analyzed. According to a redundancy analysis, phosphate -and not pesticides- constituted the main factor structuring the trait modality distribution of aquatic macrofauna. The functional composition could be ascribed for 2–4 % to pesticides, and for 3–11 % to phosphate. The lack of trait responses to pesticides may indicate that species may have used alternative strategies to adapt to ambient pesticides stress. Biomass of animals exhibiting trait modalities related to feeding by predation and grazing, presence of diapause form or dormancy, reproduction by free clutches and ovoviviparity, life stage of larvae and pupa, was negatively correlated to the concentration of phosphate. Hence, despite the high pesticide pollution in the area, variation in nutrient-related stressors seems to be the dominant driver of the functional composition of aquatic macrofauna assembly in agricultural ditches.Electronic supplementary materialThe online version of this article (doi:10.1007/s10646-016-1671-5) contains supplementary material, which is available to authorized users.

Temperate aquatic communities are highly diverse and seasonally variable, due to internal biotic processes and environmental drivers, including human-induced stressors. The impact of drivers on species abundance is supposed to differ fundamentally depending on whether populations are experiencing limitations, which may shift over the season. However, an integrated understanding of how drivers structure communities seasonally is currently lacking. In order to partition the effect of drivers, we used random forests to quantify interactions between all taxa and environmental factors using macrofaunal data from 18 agricultural ditches sampled over two years. We found that, over the agricultural season, taxon abundance became increasingly better predicted by the abundances of co-occurring taxa and nutrients compared to other abiotic factors, including pesticides. Our approach provides fundamental insights in community dynamics and highlights the need to consider changes in species interactions to understand the effects of anthropogenic stressors.