Trace elements in ships' ballast water as tracers of mid-ocean exchange

Recent regulation mandates that ships conduct mid-ocean ballast water exchange (BWE) prior to discharging foreign ballast in U.S. territorial waters. We investigated the utility of dissolved concentration measurements for 6 elements (Ba, P, Mn, U, V and Mo) in the ballast tanks of ships operating in the North Pacific and Atlantic oceans as tracers of mid-ocean BWE. Relatively conservative elements Mo, U and V provided little additional information beyond that obtained from salinity, whereas nonconservative Ba, P and Mn offered greater resolution. The utility of Ba, P and Mn was further examined in the context of three criteria: (1) stability, or whether tracers maintain stable concentrations in ballast tanks over time; (2) fidelity, or the degree to which tracer concentrations in ballast tanks faithfully reflect concentrations at their ocean source; and (3) predictability, or the degree to which ballast tanks have a predictable and restricted range of tracer concentrations following BWE. We found that in water held in ballast tanks over time, average stability increased for Mn < P < Ba, as reflected by decreasing coefficients of variation (30% > 21% > 3%) and fidelity increased in the same direction. While Ba and P usually increased discrimination at high salinities, Mn was typically the most sensitive indicator of BWE and the presence of residual port water in partially exchanged tanks. Ba, P and Mn in tanks exchanged in the Atlantic exhibited different concentration ranges compared to tanks exchanged in the Pacific, suggesting that if trace elements are to be used to verify BWE, criteria for discriminating between exchanged and unexchanged ballast tanks may need to be basin-specific.