The ecosystem baseline for particle flux in the Northern Gulf of Mexico

S.L.C Giering,J Sweet,A Diercks,B Yan,V Asper,J.P Chanton,M Pitiranggon,U Passow,Laurenz Thomsen,Jody W Deming

doi:10.1525/elementa.264

Abstract

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 event, a large fraction of the spilled oil was transported to depth via sinking marine snow, a routing of spilled oil unexpected to emergency response planners. Because baseline knowledge of particle export in the Northern Gulf of Mexico and how it varies spatially and temporally was limited, we conducted a detailed assessment of the potential drivers of deep (~1400 m depth) particle fluxes during 2012–2016 using sediment traps at three contrasting sites in the Northern Gulf of Mexico: near the DWH site, at an active natural oil seep site, and at a site considered typical for background conditions. The DWH site, located ~70 km from the Mississippi River Delta, showed flux patterns that were strongly linked to the Mississippi nitrogen discharge and an annual subsequent surface bloom. Fluxes carried clear signals of combustion products, which likely originated from pyrogenic sources that were transported offshore via the Mississippi plume. The seep and reference sites were more strongly influenced by the open Gulf of Mexico, did not show a clear seasonal flux pattern, and their overall sedimentation rates were lower than those at the DWH site. At the seep site, based on polycyclic aromatic hydrocarbon data, we observed indications of three different pathways for “natural” oiled-snow sedimentation: scavenging by sinking particles at depth, weathering at the surface before incorporation into sinking particles, and 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 influences, including natural seepage and contamination by combustion products.

Highlights

The Northern Gulf of Mexico is a very dynamic and high-contrast ecosystem
To date, knowledge of this important pathway and how it varies spatially and temporally in the Northern Gulf of Mexico is very limited (Davies et al, 2010; Mienis et al, 2012; Prouty et al, 2016). This limitation became especially apparent during the Deep Water Horizon (DWH) oil spill when it was realized that sinking particles were concentrating and transporting large amounts of oil to depth (Passow et al, 2012)
We propose that particle flux at the DWH site is strongly influenced by the Mississippi River discharge through two discharge processes

Summary

Introduction

The Northern Gulf of Mexico is a very dynamic and high-contrast ecosystem. In the oligotrophic offshore regions, phytoplankton are nutrient-limited and primary production is low. The complex interaction between different waters (with differing nutrient and trophic states) and highly variable currents leads to a very heterogeneous and dynamic system This complexity likely affects particles that sink from the surface, supply carbon to the deep sea community, and play an important role in the carbon cycle. To date, knowledge of this important pathway and how it varies spatially and temporally in the Northern Gulf of Mexico is very limited (Davies et al, 2010; Mienis et al, 2012; Prouty et al, 2016) This limitation became especially apparent during the Deep Water Horizon (DWH) oil spill when it was realized that sinking particles were concentrating and transporting large amounts of oil to depth (Passow et al, 2012)

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