Effects of proposed physical ballast tank treatments on aquatic invertebrate resting eggs

Adaptations in aquatic invertebrate resting eggs that confer protection from natural catastrophic events also could confer protection from treatments applied to ballast water for biological invasion vector management. To evaluate the potential efficacy of physical ballast water treatment methods, the present study examined the acute toxicity of heat (flash and holding methods), ultraviolet (UV) radiation (254 nm), and deoxygenation (acute and chronic) on resting eggs of the freshwater cladoceran Daphnia mendotae and the marine brine shrimp Artemia sp. Both D. mendotae and Artemia sp. were similarly sensitive to flash exposures of heat (100% mortality at 70°C), but D. mendotae were much more sensitive to prolonged exposures. Exposure to 4,000 mJ/cm2 of UV radiation resulted in mortality rates of 59% in Artemia sp. and 91% in D. mendotae. Deoxygenation to an oxygen concentration of 1 mg/L was maximally toxic to both species. Deoxygenation suppressed hatching of D. mendotae resting eggs at oxygen concentrations of less than 5.5 mg/L and of Artemia sp. resting eggs at concentrations of less than 1 mg/L. Results suggest that UV radiation and deoxygenation are not viable treatment methods with respect to invertebrate resting eggs because of the impracticality of producing sufficient UV doses and the suppression of hatching at low oxygen concentrations. Results also suggest that the treatment temperatures required to kill resting eggs are much higher than those reported to be effective against other invertebrate life stages and species. The results, however, do not preclude the effectiveness of these treatments against other organisms or life stages. Nevertheless, if ballast tank treatment systems employing the tested methods are intended to include mitigation of viable resting eggs, then physical removal of large resting eggs and ephippia via filtration would be a necessary initial step.