The purpose of this study was to evaluate the effects of four diets on
the results of seven-day Ceriodaphnia dubia toxicity tests. Survival
and reproduction were used as indices to detect the sensitivity of thi
s species to acute and chronic copper stress. All toxicity tests were
conducted using the moderately hard reconstituted water recommended in
1989 by the U.S. Environmental Protection Agency. Diet differentially
affected the acute and chronic toxicity of copper. Daphnids fed Selen
astrum capricornutum (alga) showed the greatest sensitivity, followed
by those fed the alga Chlamydomonas reinbardti, then by animals fed a
Yeast-Cerophyll(TM)-Trout Food (YCTF) mixture plus Selenasrum, and fin
ally by animals fed YCTF alone. These differences may result from the
poor nutritional adequacy of Selenastrum when fed alone, the different
caloric contents of the diets, the increased toxicant uptake by the o
rganisms through ingestion of copper-laden algal cells, and/or copper
ions sequestered by fats and insoluble substances in YCTF. We recogniz
e that diet is an important variable in seven-day toxicity tests, and
that the selection of a diet should not be based only on its effects o
n long-term culturing of C. dubia, but also on its possible effects on
test results.