Optical signatures of seawater and potential use for verification of mid-ocean ballast water exchange

摘要

ABSTRACT: Mandatory requirements for exchange of ships’ ballast water in the open ocean require an effective means of verifying that open ocean exchange has occurred. This study demonstrates that UV fluorescence of chromophoric organic matter in seawater has great potential to verify open ocean ballast water exchange. Ballast water samples, obtained for excitation-emission spectral analysis during 2 flow-through ballast water exchange events in the trans-Pacific voyage of a chemical carrier, establish a clear difference between the UV fluorescence characteristics of ballast water before and after mid-ocean exchange. The exchange of water in mid-ocean shifted the fluorescence signatures of ballast water from those typical of the coastal water at the donor ports to those of the exchanged mid-ocean water. Estimates based on the fluorescence data suggest that at least 90% of the coastal water was exchanged. The shifts in fluorescence intensity and structure were concomitant with a 99% reduction in the concentration of tracer dye added to the ballast tanks during ballasting in the ports of origin. In contrast, changes in phytoplankton and zooplankton species composition and diversity were not as great, which limit their potential for verification of mid-ocean exchange. The consistently low fluorescence intensity and lack of fluorescence structure of mid-Pacific ocean water relative to the coastal waters, and the stability of the fluorescence in the ballast tanks before and after exchange, suggest that fluorescence techniques can form the basis of verifying the presence of open ocean water in ballast tanks.