Results of feeding inhibition rate (mean±SE) of D. magna, D. longispina C and D. longispina V when exposed to natural waters from Crestuma reservoir (A, C) and lake Vela (B, D) collected in summer and spring. C − control treatment with ASTM hard water medium; UW − Unfiltered water treatment; FW − Filtered water treatment. *stands for significant differences compared to control treatment (Dunnett's test).