INFLUENCE OF SALINITY ON ATRAZINE TOXICITY TO A CHESAPEAKE BAY COPEPOD (EURYTEMORA-AFFINIS) AND FISH (CYPRINODON-VARIEGATUS)

The objective of this study was to determine the influence of a range of salinities (5 parts per thousand, 15 parts per thousand, and 25 par ts per thousand) on the acute toxicity of atrazine to nauplii of the c opepod Eurytemora affinis and larvae of the sheepshead minnow, Cyprino don variegatus. Ninety-six-hour LC50 values for the copepod were 0.5 m g l-1, 2.6 mg l-1, and 13.2 mg l-1 at salinities of 5 parts per thousa nd, 15 parts per thousand, and 25 parts per thousand, respectively. A comparison of LC50 values between adjacent salinities showed a statist ical difference between 15 parts per thousand and 25 parts per thousan d, but not between 5 parts per thousand and 15 parts per thousand. Atr azine was more toxic to E. affinis at the lowest salinity. The 96-h LC 50s for the sheepshead minnow were 16.2 mg l-1, 2.3 mg l-1, and 2.0 mg l-1 at salinities of 5 parts per thousand, 15 parts per thousand, and 25 parts per thousand, respectively. There was a statistical differen ce between LC50 values at 5 parts per thousand and 15 parts per thousa nd but not between 15 parts per thousand and 25 parts per thousand. In contrast to E. affinis results, atrazine was more toxic to C. variega tus at the highest salinity. The toxicity data from these species sugg est that development of estuarine water quality criteria is warranted.