Response management and damage assessment during and after environmental disasters such as the Deepwater Horizon (DWH) oil spill require an ecological baseline and a solid understanding of the main drivers of the ecosystem. During the DWH horizon event, a large fraction of the spilled oil was transported to depth via sinking phytodetritus aggregates (‘marine snow ’). This route for the oil had not been expected, and there was limited baseline knowledge of this particle export in the Northern Gulf of Mexico and how it varies spatially and temporally. We conducted a detailed assessment of the drivers of deep (~1400 m depth) particle fluxes during 2012-2016 at three contrasting sites in the Northern Gulf of Mexico: near the DWH site, at an active natural oil seep site, and from a site considered typical for background conditions.
The DWH site fluxes were strongly linked to the Mississippi nitrogen discharge, and carried clear signals of combustion products transported offshore via the Mississippi plume. The seep and reference sites were, on the other hand, strongly influenced by the open Gulf of Mexico. At the seep site, we observed “natural” marine oil snow sedimentation by three different pathways: (1) scavenging by sinking particles at depth, (2) weathering at the surface before incorporation into sinking particles, and (3) entry into the food web and subsequent sinking in form of detritus. Overall sedimentation rates at the three sites were markedly different in quality and quantity, owing to varying degrees of riverine and oceanic influence, including natural seepage and contamination by combustion products.