Exploring the limits of dissolved organic matter fluorescence for determining seawater sources and ballast water exchange on the US Pacific coast

To minimize the risk of biological invasions associated with commercial shipping, vessels are required to conduct ballast water exchange (BWE) ≥ 200 nautical miles offshore when arriving in the US from foreign ports, and some states require coastal BWE ≥ 50 miles offshore along domestic routes. Previous research suggests that the intensity of fluorescent dissolved organic matter (fDOM) can be used to verify whether BWE was implemented. This study examined seasonal and spatial variability of fDOM in Pacific rim ports and the adjacent seas, using the North American coast as a model system to test whether regional fluorescence intensity thresholds consistently distinguish port sites from coastal and oceanic sites at increasing distances from shore. Over 2000 samples from major port systems on the US Pacific coast and along offshore (perpendicular) and alongshore (parallel) transects were analyzed. Overall, humic fDOM fluorescence intensity (C3* = 370/494 nm) effectively discriminated port versus oceanic sites located further than 100 miles from shore, but discriminated only a subset of coastal versus oceanic sources within the northeastern Pacific. Data from additional global ports are needed to predict the frequency of false positive or false negative ballast source determinations using fDOM for foreign vessel traffic.

► Ships are required to exchange ballast water to avoid facilitating biological invasions.
► Organic matter fluorescence (fDOM) can discriminate ballast water origins.
► fDOM thresholds for discriminating ballast sources in the North Pacific were tested.
► Port sites could be effectively discriminated from oceanic sites.
► Additional variables are needed to distinguish between coastal and oceanic sites.