Harming local species or preventing the transfer of exotics? Possible negative and positive effects of using zinc anodes for corrosion protection of ballast water tanks

Ballast water tanks in ships are today often corrosion-protected by sacrificial zinc anodes in addition to protective coating. Dissolved zinc has been found to be toxic to early life stages of many freshwater and marine organisms. We have therefore examined if the zinc released into ballast water could reach concentrations previously known to affect aquatic biota. When assuming a simplistic model for the release of zinc from the anodes, the zinc concentrations in ballast water could reach from 1.16 mg/l at pH 8.2 to 29 mg/l at pH 7.5. The former corresponds roughly to one, and the latter corresponds to two orders of magnitude times the levels found to kill or injure early life stages of various aquatic organisms. The use of zinc anodes in corrosion protection may thus have possible negative impacts on the biota in enclosed harbour waters, but can reduce the numbers of non-indigenous aquatic organisms surviving in ballast water.