Effects of salinity and temperature on survival and reproduction of Boeckella hamata (Copepoda : Calanoida) from a periodically brackish lake

The freshwater calanoid copepod Boeckella hamata Brehm, occurs periodically in Lake Waihola, New Zealand, a shallow, tidally-influenced lake that experiences daily and seasonal fluctuations in salinity, and seasonal changes in temperature, To determine whether these environmental changes might contribute to the copepod's disappearance from the lake in summer, we tested the survival and egg production of B. hamata at a range of salinities (20-3000 mg l(-1) Cl) and two temperatures (10 degreesC, 20 degreesC). Adults survived for 20 days or more only at salinities less than or equal to 1000 mg l(-1) Cl; survival was higher at 10 degreesC than at 20 degreesC, and higher for females (86% at 10 degreesC, 14% at 20 degreesC) than males (43% at 10 degreesC, 0% at 20 degreesC). The mean total eggs produced per female in three successive clutches decreased with temperature from 20 eggs at 10 degreesC to 15 at 20 degreesC; egg production rates increased however, as clutches were produced more frequently. Staged increases in salinity did not increase copepod survival, indicating that B. hamata adults cannot tolerate the magnitude, or rate, of seasonal fluctuations in salinity to which they are exposed in the lake. We conclude that the ability of B. hamata to tolerate warm, saline conditions influences its presence and role in brackish ecosystems.