PRECISION OF THE NITOCRA-SPINIPES ACUTE TOXICITY TEST AND THE EFFECT OF SALINITY ON TOXICITY OF THE REFERENCE TOXICANT POTASSIUM BICHROMATE

The precision of the 96-h LC50 test with the eurohaline copepod Nitocr a spinipes was evaluated from a data set containing 85 tests with the reference toxicant potassium bichromate conducted at 12 laboratories d uring a 12-year period. The overall mean was 30 mg/L as K2Cr2O7 with a coefficient of variation (CV) of 47% (N = 85). Since salinity differe d between tests (7-33 parts per thousand), it explained part of the va riation (ca. 25%; Pearson two-tailed test r = 0.49; p = 0.0000; N = 85 ). The effect of salinity on Cr(VI) toxicity was confirmed in an exper iment with 4 salinities (3.4, 6.8, 14.6, and 27 parts per thousand) an d was explained to 99% by the equation 96-h LC50 (mg/L) = 2.58 + 1.79 x salinity (parts per thousand). Intralaboratory CVs for corresponding salinities ranged from 8 to 35% (repeatability). Interlaboratory CVs at two salinities were 42% (N = 11) at 7-9 parts per thousand and 47% (N = 4) at 15-16 parts per thousand. (C) 1993 John Wiley & Sons, Inc.