Toxicity assessment of cobalt and selenium on marine diatoms and copepods

Cobalt and selenium are emerging concerns for marine organisms due to their raising levels in ambient environment and toxicity. Bioassay tests were conducted on marine plankton for evaluation and assessment of cobalt and selenium toxicity. Considering the ecological importance, diatoms such as Odontella mobiliensis, Coscinodiscus centralis, Skeletonema costatum and copepod Longipedia weberi, mysid shrimp were selected for the toxicity evaluation. The 96-h IC₅₀ of 1.08 ± 0.04 mg/l for the diatom O. mobiliensis and 3.52 ± 0.02 mg/l for C. centralis, 4.0 ± 0.42 mg/l, and 3.56 ± 0.22 mg/l were derived as 96-h LC₅₀ for the copepod L. weberi and mysid shrimp respectively, for Cobalt. The selenium concentrations of 9.30 ± 1.01 mg/l and 20.34 ± 1.05 mg/l were derived as a 96-h IC₅₀ value on the diatoms O.mobiliensis and Skeletonema costatum respectively while 96-h LC₅₀ values were derived as 0.97 ± 0.08 mg/l for copepod L.weberi and 1.13 ± 0.15 mg/l for mysid shrimp. The chronic toxicity values of Co were found in the range of 0.22-0.57 mg/l for Co and 0.12-1.72 mg/l for Se on plankton. The cobalt was found to be more toxic to the diatoms than selenium whereas copepods were more sensitive to selenium than the cobalt. The level of toxicity is varied due to the difference in the mode of toxicity and sensitivity among the species. The safe limits were derived by species sensitivity distribution (SSD) based on the chronic values of Co and Se. The ambient concentration of 22 μg/l for Se and 18 μg/l for Co are the safe level for the protection of plankton in the coastal waters.