The Effects of Water Hardness on Daphnia Response to Chloride Exposure.

Freshwater salinization is a major issue in Canada with nearly 6 million tones of road salt (typically NaCl) used annually for road de-icing. Many aquatic organisms show sensitivity to salt, leading to the creation of the Canadian Water Quality Guidelines (WQG). The WQG were created to determine chloride (Cl) levels that would protect most aquatic species from salinization. Two concentrations were created for long-term and short-term exposure, 120mg/L and 640mg/L, respectively. However, research indicates aquatic organisms in soft water (low calcium, or Ca) can experience negative effects of Cl below the WQG. The mechanisms by which Cl and Ca interact are unknown, but one proposed hypothesis suggests that Ca tightens cellular junctions reducing the passive flow of ions into aquatic organisms. Collaborating with Dr. A. Donini (York U), we measured hemolymph Ca and sodium (Na) ion concentrations in Daphnia that were cultured in FLAMES media treated with 1 to 70 Ca mg/L (CaSO4 · 2H2O) crossed with 0.39 to 400 Cl mg/L (NaCl) to identify if they showed varying concentrations of hemolymph ions. Daphnia were cultured in untreated FLAMES media and fed Scenedesmus obliquus at 2mgC/L/day for 2 generations. Neonates <24 hours old were transferred to treatment media and fed the same regimen for 6 days, then hemolymph ion concentrations were measured using ion-selective microelectrodes. The experiment is still ongoing. We predict lower hemolymph Na concentrations for a given level of Cl as Ca treatment increases. Understanding the varying effects of Cl at different Ca concentrations will be helpful in adequately reforming the WQG.