The fluorescence and absorbance optical properties of seawater samples from the Deepwater Horizon (DWH) oil spill were analyzed to examine the transport and fate of oil both during and after the spill, as well as the effect of chemical dispersant on the optical properties. Seawater samples were collected during three cruises: in June 2010, when the well was still discharging oil, in September 2010, after the well had been capped, and in January 2016 in an area around the Taylor Energy site, which has been leaking oil since Hurricane Katrina. Two excitation/emission wavelength pairs were chosen to trace oil from the DWH spill based on fluorescence excitation/emission matrices (EEMs) of seawater samples: 275/324 nm and 240/354 nm. These pairs were chosen based on similarities to oil signatures from the literature, as well as in situ sensor responses at corresponding depths showing decreases in dissolved oxygen, increases in CDOM, decreased transmission and increased chlorophyll for June samples, and decreases in dissolved oxygen, increased CDOM, depletions in the ?^13C of DIC, and enrichments in DIC for September samples. Increases in fluorescence intensity values at these chosen wavelength pairs provided evidence of crude oil from the Macondo well between 900 m and 1200 m depth at several stations in June, and between 1000 m and 1200 m depth at several stations in September. Fluorescence EEMs from the Acadiana station with known fresh surface oil were compared to EEMs from June and September stations to identify the presence of oil, although massive inputs from Mississippi River floodwaters complicated interpretation. Fluorescence intensity values of the 275/324 nm and 240/354 nm wavelength pairs at the surface and around 1100 m depth were used to generate maps showing the spatial distribution of oil from June to September. Samples exhibiting signatures similar to oil were generally found at a distance of up to 10 miles both southwest and northeast of the well in June and extended to the southwest almost 300 miles from the well in the months following the spill.
