Population responses of Daphnia magna, Chydorus sphaericus and Asellus aquaticus in pesticide contaminated ditches around bulb fields.

The goal of this study was to investigate the effects of ambient concentrations of pesticides combined with abiotic factors on the key aquatic species Daphnia magna, Chydorus sphaericus and Asellus aquaticus by means of 21 days field exposure experiments. In situ bioassays were deployed in ditches around flower bulb fields during spring and autumn 2011-2012. The results showed that phosphate was the most variable parameter followed by pesticides expressed as toxic units, as the main factors explaining differences between sites. Variation in reproduction and growth of cladoceran D. magna was largely explained by nutrients, whereas dissolved oxygen contributed mostly to variations in reproduction of C. sphaericus. Dissolved organic carbon contributed to variations in growth of the detrivore A. aquaticus. It is concluded that abiotic stressors rather than pesticides contributed significantly to the performance of aquatic invertebrates.