The influences of phytoplankton species, mineral particles and concentrations of dispersed oil on the formation and fate of marine oil-related aggregates

Ingrid A Henry,Roman Netzer,Emlyn Davies,Odd Gunnar Brakstad

doi:10.1016/j.scitotenv.2020.141786

Abstract

The formation and fallout of oil-related marine snow have been associated with interactions between dispersed oil and small marine particles, like phytoplankton and mineral particles. In these studies, the influences of phytoplankton species, mineral particle concentration, and oil concentration on the aggregation of oil in seawater (SW) were investigated. The experiments were performed in a low-turbidity carousel incubation system, using natural SW at 13 °C. Aggregation was measured by silhouette camera analyses, and oil compound group distribution and depletion by gas chromatography (GC-FID or GC–MS). Aggregates with median sizes larger than 500 μm in diameter were measured in the presence of dispersed oil and the phytoplankton species Thalassiosira rotula, Phaeocystis globosa, Skeletonema pseudocostatum, but not with the microalgae Micromonas pusilla. When mineral particles (diatomaceous earth) were incubated at different concentrations (5–30 mg/L) with dispersed oil and S. pseudocostatum, the largest aggregates were measured at the lower mineral particle concentration (5 mg/L). Since dispersed oil rapidly dilutes in the marine water column, experiments were performed with oil concentrations of from 10 mg/L to 0.01 mg/L in the presence of S. pseudocostatum and diatomaceous earth. Aggregates larger than 500 μm was measured only at the highest oil concentrations (10 mg/L). However, oil attachment to the marine particles were also measured at low oil concentrations (≤1 mg/L). Depletion of oil compound groups (n-alkanes, naphthalenes, PAHs, decalins) were measured at all oil concentrations, both in aggregate and water phases, with biodegradation as the expected main depletion process. These results showed that oil concentration may be important for oil-related marine snow formation, but that even oil droplets at low concentrations may attach to the particles and be transported by prevailing currents.

Highlights

The formation and fate of aggregates associated with oil spills, often called marine oil snow (MOS) or Marine Oil Snow Sedimentation & Flocculent Accumulation (MOSSFA), has received considerable attention after the Deepwater Horizon (DWH) oil spill (Passow et al, 2012; Ziervogel et al, 2012; Daly et al, 2016; Passow, 2016)
Depletion of oil compound groups (n-alkanes, naphthalenes, polycyclic aromatic hydrocarbons (PAH), decalins) were measured at all oil concentrations, both in aggregate and water phases, with biodegradation as the expected main depletion process. These results showed that oil concentration may be important for oil-related marine snow formation, but that even oil droplets at low concentrations may attach to the particles and be transported by prevailing currents
We investigated the impacts of different phytoplankton species, mineral particle concentrations, oil concentrations on formation and fate of oil-related aggregates (ORAs) in temperate SW relevant for summer conditions in the North Sea and Norwegian Sea, an area with extensive offshore oil production and transport

Summary

Introduction

The formation and fate of aggregates associated with oil spills, often called marine oil snow (MOS) or Marine Oil Snow Sedimentation & Flocculent Accumulation (MOSSFA), has received considerable attention after the Deepwater Horizon (DWH) oil spill (Passow et al, 2012; Ziervogel et al, 2012; Daly et al, 2016; Passow, 2016). These processes were used to explain why considerable amounts of the Macondo oil were sedimented on the seabed during the oil spill (Valentine et al, 2014; Stout and Payne, 2016). OMA generation has been estimated to initiate at concentrations of suspended sediment >10 mg/L for significant oil deposition and > 100 mg/L for large deposition events (Boehm, 1987; Daly et al, 2016)

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