Impact of cadmium on copepod Oithona similis at threshold concentrations: Determining safe exposure period

The level of protection by the seawater quality criteria (SWQC) of cadmium (Cd) on planktonic population remains unknown. Therefore, this study assessed the level of protection and safe exposure period of SWQC of cadmium for protection of marine life. The copepod Oithona similis exposed to the Cd at SWQC such as predicted no-effect concentration (PNEC, 0.6 μg/l), criterion continuous concentration (CCC, 1.2 μg/l), and criterion maximum concentration (CMC, 5 μg/l). The in-vivo assays with fluorescein diacetate (FDA) and 2,7-dichlorofluorescein diacetate (DC-FDA) revealed that the SWQCs are safe except for CMC for prolonged exposure. The population growth was higher at 5.4 folds in the control culture, with 4291 ± 134 ind/l, than the culture with CMC of Cd with 2403 ± 149 ind/l which is 2.7 fold growth. Protein content in copepod biomass increased between 3.27 mg/g in the control and 5.73 mg/g in CMC. Biomarker enzyme activities varied among the treatments, and higher expressions were found in CMC. Inhibition of in-vitro FDA and induced DC-FDA activities indicate the stress due to Cd toxicity. Overall, results reveal the SWQC of Cd is not safe for long-term exposure. The PNEC may ensure the protection for up to four days, CCC may be prescribed for one day, and CMC may be prescribed for one-hour exposure.