Deducing ballast water sources in ships arriving in New Zealand from southeastern Australia

The transfer of organisms in ballast water of commercial ships is a leading cause of bio- logical invasions in coastal ecosystems. Ships arriving in New Zealand are now required to treat their ballast water to reduce the risk of transferring invasive aquatic organisms between ports. Most of these ships conduct mid-ocean ballast water exchange (BWE), replacing coastal water with open ocean water, but methods to verify BWE have been lacking. Samples were collected from ballast tanks and the ambient ocean on ships trading between southeastern Australia and New Zealand, to test the use of chemical (chromophoric dissolved organic matter or CDOM, Ba, Mn and P) concentra- tions to discriminate ballast water sources. Australian ballast water provides a difficult and valuable test case for BWE verification due to its high salinity and low chemical tracer concentrations resulting from Australia's low rainfall and nutrient-poor soils. Our results indicate that elevated CDOM, Ba and Mn were robust tracers of port waters, whereas elevated P was not a diagnostic tracer except of bal- last water originating from Port Phillip Bay. Exchanged ballast tanks were diagnosed by CDOM flu- orescence below 2.1 (for wavelength pair C2*, Ex/Em = 320/414 nm) and 1.2 (for wavelength pair C3*, Ex/Em = 370/494 nm) (quinine sulfate equivalents, QSE), and Ba and Mn concentrations below 5.7 and 3.5 μg l -1 respectively. These results are consistent with recent studies in the northern hemi- sphere, indicating that elevated concentrations of these tracers are robust indicators of unexchanged ballast water. Whereas clear differences existed between port and oceanic signatures, coastal and oceanic samples could not always be distinguished due to precipitously declining tracer concentra- tions within short distances from land.