The Differential Effects of Salinity Level on Chlorpyrifos and Imidacloprid Toxicity to an Estuarine Amphipod

Agricultural activity within coastal watersheds results in estuaries becoming the receiving environment for pesticide inputs. In estuaries, salinity can alter insecticide responses of exposed crustaceans. The acute toxicity of environmentally relevant doses of chlorpyrifos and imidacloprid were examined using the euryhaline amphipod Gammarus lawrencianus at 20 and 30 Practical Salinity Units (PSU). Responses were recorded every 24 h until an incipient (threshold) L(E)C₅₀ was reached. For chlorpyrifos, LC₅₀ ranged from 0.1 to 0.5 µg/L and was two-fold higher at 30 vs. 20 PSU at all time-points over the 96 h exposure. Imidacloprid immobility EC₅₀ ranged from 4 to 40 µg/L over the 144 h exposure. An effect of salinity was only observed at 48 h and the EC₅₀ values showed 1.4 times more potency at 20 PSU compared to 30 PSU. Measured concentrations of both compounds did not differ between salinities. Acetylcholinesterase activity in chlorpyrifos exposed amphipods showed no salinity effect at 96 h. We conclude that salinity level alters G. lawrencianus susceptibility to chlorpyrifos exposure, but not imidacloprid.