Chronic Toxicity Tests with Daphnia magna for the Examination of River Water Quality

− 19 − ABSTRACT The toxicological effects of river water on the reproduction of the water f lea Daphnia magna were assessed with the standard 21-d chronic test(OECD TG211)at four river sites(St. 1–4)in rural districts of the Kanto Plain(Ibaraki Prefecture, Japan) at monthly intervals from April 2007 to March 2010. Over the 3 years, we recorded the following average numbers of live D. magna offspring after the 21-d tests: 17–233, 14–223, 29–210, and 84–209 in St. 1, 2, 3, and 4, respectively. The average numbers in August 2007, May and September 2008, and April 2009 at St. 1; August 2007 and May 2008 at St. 2, and June 2007 and April 2009 at St. 3 were significantly lower than those of the control(ANOVA, P < 0.01). The results of the above tests suggest that D. magna may be affected not only by the overall pesticide toxicity in the river water, but also by other factors that were not clarified in the present study. In addition, significant positive correlations were observed at each site between the body length of D. magna and the total number of offspring at 7 d(r = 0.783), 14 d(r = 0.772), and 21 d(r = 0.931)after the test began on 10 May 2008. These findings suggest that the body length of D. magna might be used to evaluate the toxicological effects of river water, even at 7 d, by using the standard chronic toxicity test.