Interactive effect of salinity decrease, salinity adaptation, and chlorpyrifos exposure on an estuarine harpacticoid copepod, Mesochra parva, in South Africa

This study tested the hypothesis: "does adaptation to fluctuating salinities lead to enhanced survival of the harpacticoid copepod M. parva when exposed to a combination of particle associated chlorpyrifos (CPF) exposure and hypoosmotic stress during a 96h sediment toxicity test?" The CPF exposure concentrations of 5.89-5.38 mu g/kg and the salinity decrease from 15 to 3 ppt were based on conditions observed in the temporarily open Lourens River estuary, South Africa, in order to simulate changes during a runoff event. Results of the three-factorial ANOVA showed that pre-adaptation to varying salinities (p = 0.02; p = 0.001), salinity decrease (p = 0.035; p<0.001), and CPF exposure (p<0.001; p<0.001), all had a significant negative impact on the survival rate of female and male M. parva, with a higher sensitivity of males specimens. The significant two-way interaction of salinity x adaptation for females and males (p = 0.021; p <0.001), indicate that adaptation to fluctuating salinities was beneficial for male and female copepods, but the hypothesis of a three-way interaction was not supported. However, a trend indicated a lower survival rate of non-adapted females and males exposed to CPF and hypoosomotic stress (38 +/- 17%; 0 +/- 0%), compared to pre-adapted organisms (59 +/- 6.6%; 8.9 +/- 10%), which requires further elucidation. This study has important implications for the management of temporarily open estuaries in South Africa regarding regulation of freshwater abstraction from rivers. (c) 2008 Elsevier Inc. All rights reserved.