Did the northeastern Gulf of Mexico become greener after the Deepwater Horizon oil spill?

Assessment of direct and indirect impacts of oil and dispersants on the marine ecosystem in the northeastern Gulf of Mexico (NEGOM) from the Deepwater Horizon oil spill (April – July 2010) requires sustained observations over multiple years. Here, using satellite measurements, numerical circulation models, and other environmental data, we present some initial results on observed biological changes at the base of the food web. MODIS fluorescence line height (FLH, a proxy for phytoplankton biomass) shows two interesting anomalies. The first is statistically significant (>1 mg m −3 of chlorophyll‐a anomaly), in an area exceeding 11,000 km 2 in the NEGOM during August 2010, about 3 weeks after the oil well was capped. FLH values in this area are higher (i.e., water is greener) than in any August since 2002, and higher than ever since 2002 in an area of ∼3,000 km 2 . Analyses of ocean circulation and other environmental data suggest that this anomaly may be attributed to the oil spill. The second is a spatially coherent FLH anomaly during December 2010 and January 2011, extending from Mobile Bay to the Florida Keys (mainly between 30 and 100‐m isobaths). This anomaly appears to have resulted from unusually strong upwelling and mixing events during late fall. Available data are insufficient to support or reject a hypothesis that the subsurface oil may have contributed to the enhanced biomass during December 2010 and January 2011.