Scholarly article on topic 'Assessing the impacts of oil-associated marine snow formation and sedimentation during and after the Deepwater Horizon oil spill'

Assessing the impacts of oil-associated marine snow formation and sedimentation during and after the Deepwater Horizon oil spill Academic research paper on "Earth and related environmental sciences"

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Abstract of research paper on Earth and related environmental sciences, author of scientific article — Kendra L. Daly, Uta Passow, Jeffrey Chanton, David Hollander

Abstract The Deepwater Horizon oil spill was the largest in US history, unprecedented for the depth and volume of oil released, the amount of dispersants applied, and the unexpected, protracted sedimentation of oil-associated marine snow (MOS) to the seafloor. Marine snow formation, incorporation of oil, and subsequent gravitational settling to the seafloor (i.e., MOSSFA: Marine Oil Snow Sedimentation and Flocculent Accumulation) was a significant pathway for the distribution and fate of oil, accounting for as much as 14% of the total oil released. Long residence times of oil on the seafloor will result in prolonged exposure by benthic organisms and economically important fish. Bioaccumulation of hydrocarbons into the food web also has been documented. Major surface processes governing the MOSSFA event included an elevated and extended Mississippi River discharge, which enhanced phytoplankton production and suspended particle concentrations, zooplankton grazing, and enhanced microbial mucus formation. Previous reports indicated that MOS sedimentation also occurred during the Tsesis and Ixtoc-I oil spills; thus, MOSSFA events may occur during future oil spills, particularly since 85% of global deep-water oil exploration sites are adjacent to deltaic systems. We provide a conceptual framework of MOSSFA processes and identify data gaps to help guide current research and to improve our ability to predict MOSSFA events under different environmental conditions. Baseline time-series data and model development are urgently needed for all levels of ecosystems in regions of hydrocarbon extraction to prepare for and respond to future oil spills and to understand the impacts of oil spills on the environment.

Academic research paper on topic "Assessing the impacts of oil-associated marine snow formation and sedimentation during and after the Deepwater Horizon oil spill"

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Title: Assessing the impacts of oil-associated marine snow formation and sedimentation during and after the Deepwater Horizon oil spill

Author: Kendra L. Daly Uta Passow Jeffrey Chanton David Hollander

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S2213-3054(16)30006-6

http://dx.doi.Org/doi:10.1016/j.ancene.2016.01.006 ANCENE 103

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Received date: Revised date: Accepted date:

20-10-2015

27-1-2016

28-1-2016

Please cite this article as: Daly, Kendra L., Passow, Uta, Chanton, Jeffrey, Hollander, David, Assessing the impacts of oil-associated marine snow formation and sedimentation during and after the Deepwater Horizon oil spill.Anthropocene http://dx.doi.org/10.1016Zj.ancene.2016.01.006

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Assessing the impacts of oil-associated marine snow formation and sedimentation during and after the Deepwater Horizon oil spill

Kendra L. Dalya, Uta Passowb, Jeffrey Chantonc, and David Hollander3

a College of Marine Science, University of South Florida, St. Petersburg, FL 33705, United States

b Marine Science Institute, University of California, Santa Barbara, CA 93106, United States c Earth, Ocean, Atmospheric Science, Florida State University, Tallahassee, FL 32306, United States

Corresponding author: K. L. Daly; kdaly@mail.usf.edu October 16, 2015

Highlights

• A significant fraction of DWH oil was transported to depth via sinking oil-associated marine snow (MOS), which formed in surface waters.

• MOS formation and sedimentation was influenced by plankton dynamics and river discharge of nutrients and suspended minerals.

• Sedimented oil on the seafloor impacted benthic organisms and sediment bio-geochemistry.

Baseline time-series data and model development are urgently needed for all levels of ecosystems in regions of hydrocarbon extraction.

• Emergency responders should consider oil sedimentation processes when planning oil spill mitigation strategies.

Abstract

The Deepwater Horizon oil spill was the largest in US history, unprecedented for the depth and volume of oil released, the amount of dispersants applied, and the unexpected, protracted sedimentation of oil-associated marine snow (MOS) to the seafloor. Marine snow formation, incorporation of oil, and subsequent gravitational settling to the seafloor (i.e., MOSSFA: Marine Oil Snow Sedimentation and Flocculent Accumulation) was a significant pathway for the distribution and fate of oil, accounting for as much as 14% of the total oil released. Long residence times of oil on the seafloor will result in prolonged exposure by benthic organisms and economically important fish. Bioaccumulation of hydrocarbons into the food web also has been documented. Major surface processes governing the MOSSFA event included an elevated and extended Mississippi River discharge, which enhanced phytoplankton production and suspended particle concentrations, zooplankton grazing, and enhanced microbial mucus formation. Previous

reports indicated that MOS sedimentation also occurred during the Tsesis and Ixtoc-I oil spills; thus, MOSSFA events may occur during future oil spills, particularly since 85% of global deep-water oil exploration sites are adjacent to deltaic systems. We provide a conceptual framework of MOSSFA processes and identify data gaps to help guide current research and to improve our ability to predict MOSSFA events under different environmental conditions. Baseline time-series data and model development are urgently needed for all levels of ecosystems in regions of hydrocarbon extraction to prepare for and respond to future oil spills and to understand the impacts of oil spills on the environment.

Keywords: Deepwater Horizon oil spill; Gulf of Mexico; marine oil snow; MOSSFA; bacteria; plankton

1. Introduction

The 2010 Deepwater Horizon (DWH) oil spill (Fig. 1), which began on April 20th and ended on July 15th, was unprecedented for several reasons: (1) the oil outflow originated from an ultra-deep well at 1,500 m, (2) the large volume of oil released (about 4.9 million barrels or 779 million liters), (3) the large amount of dispersants (Corexit EC9500A and EC9527A; about 2.1 million gallons or 7.9 million liters) released in deep water and at the sea surface (Kujawinski et al. 2011, McNutt et al. 2012, Lubchenco et al. 2012), and (4) the unexpected and protracted sedimentation event of oil-associated marine snow to the seafloor (Passow 2014, Brooks et al. 2015, Romero et al. 2015).

The Oil Budget Calculator Science and Engineering Team reported that by the end of the oil spill 17% of the South Louisiana Sweet Crude oil (MS252) had been recovered at the wellhead, 5% burned, 16% chemically dispersed, 13% naturally dispersed, 23% evaporated or dissolved, with 23% remaining in an Other category (Lehr et al. 2010, McNutt et al. 2012). The Oil Budget Calculator was designed as a response tool to inform response actions and cleanup decisions and was not intended to be a research or damage assessment tool. Consequently, oil sedimentation and its accumulation on the seafloor were not specifically considered in the oil budget calculation. However, early scientific investigations of the oil spill noted that high concentrations of oil-associated marine snow (herein marine oil snow or MOS) were observed in the vicinity of surface oil and the subsurface oil plumes (e.g., Passow et al. 2012, Daly et al. 2013). Based on satellite images, the oil spill ultimately covered up to 68,000 square miles (180,000 km2) of ocean before it was contained (Norse and Amos 2010), including coastal, deep water, and riverine influenced regions. The outflow from the Mississippi River and associated distributary channels was above climatological mean between mid-May and October of 2010, which influenced the near surface transport of oil (Kourafalou and Androulidakis 2013). Typically, the river discharges about 130 to 150 106 t yr-1 of sediment (Corbett et al. 2006, Bianchi et al. 2007); hence, riverine suspended mineral particles likely interacted with oil (Muschenheim and Lee 2002, Khelifa et al. 2005) to form sinking oil-mineral aggregations (OMAs) in addition to MOS. Significant sediment and hydrocarbon deposition to the seafloor was observed in the DeSoto Canyon region to the east of the Deepwater Horizon wellhead (Brooks et al. 2015, Romero et al. 2015) and elsewhere (White et al. 2012, Montagna et al. 2013, Valentine et al. 2014; Chanton et al. 2015). Based on combined sedimentology,

geochemical, and biological approaches, this mass deposition was primarily a product of marine snow formation and appears to have occurred over a 4 to 5 month period during and after the oil spill, far exceeding pre-spill sediment accumulation rates (Brooks et al. 2015). A high degree of patchiness in the deposition of oil on the seafloor, combined with the large geographic area of the sedimentation event (including on the upper continental slope and shelf), makes it a difficult task to estimate the total amount of oil transported to depth. Nevertheless since the oil spill ended, Valentine et al. (2014) estimated that 1.8 to 14% of the oil was transported to the seafloor using hopanes as a biomarker-tracer, while Chanton et al. (2015) estimated the amount at 0.5 to 9% using radiocarbon distributions. For comparison, Boehm and Fiest (1980) estimated 1- 3% of the oil from the Ixtoc-I spill in the southern Gulf of Mexico (1979 - 1980) reached the shallower seafloor in that region, while Jernelov and Linden (1981) estimated 25% of the Ixtoc oil sank to the seafloor.

In May 2010, BP committed $500 M for the Gulf of Mexico Research Initiative (GOMRI) to investigate the impacts of oil and dispersants on the Gulf of Mexico ecosystems. Preliminary findings reported at conferences and in discussion groups indicated that MOS led to significant oil sedimentation to the seafloor. The GOMRI funded Marine Oil Snow Sedimentation and Flocculent Accumulation (MOSSFA) Workshop was held during October 2013, with the goal to obtain community input, particularly on three topic areas: (1) factors affecting the formation and sinking of MOS in the water column, (2) the deposition, accumulation, and biogeochemical fate of MOS on the seafloor, and (3) the ecologic impacts of MOS on pelagic and benthic species and communities (MOSSFA Workshop Report 2014, Kinner et al. 2014). The purpose of this review is to raise awareness that the settling of

MOS may play a significant role in the distribution and fate of spilled oil in both shallow and deep water environments, and that sinking and sedimentary-oil deposition should be considered in future oil spill response assessments. Here, we report on the MOSSFA Workshop findings, provide a summary of the current state of knowledge related to the Deepwater Horizon marine snow-oil deposition event, present a mechanistic framework of MOSSFA processes, and offer examples of data gaps, as well as providing recommendations for future research. A comprehensive review of marine snow is outside the scope of this paper; for general background information see, for example, Alldredge and Silver (1988), Simon et al. (2002), Turner (2002), and Burd and Jackson (2009).

2. Characterization of oil-associated marine snow: processes and pathways 2.1. Marine snow distribution and sedimentation patterns

Marine snow is defined as particles > 0.5 mm to 10s of cm in size, which may consist of aggregations of smaller organic and inorganic particles, including bacteria, phytoplankton, microzooplankton, zooplankton fecal pellets and feeding structures (e.g., larvacean houses), biominerals, terrestrially-derived lithogenic components, and detritus (Alldredge and Silver 1988). Marine snow occurs throughout the world's oceans and at all depths. Marine snow is formed in near surface waters where particle abundances vary spatially and seasonally, usually between 1 - 14 particles per L-1 (range 0 - 500 particles L-1) (Alldredge and Silver 1988). In the Gulf of Mexico, suspended particle concentrations off Louisiana during April 1987 were reported to range from 10 - 60 particles L-1 on the shelf to 0.2 to > 3 particles L-1 on the slope, declining with depth and then increasing near bottom (1.6 - 3.3 particles L-1) (Gardner and Walsh 1990). Aggregates in the Mississippi

Canyon were assessed for size and settling rates below the mixed layer at 167 m depth during October 1993 (Dierks and Asper 1997). Aggregates typically ranged between 0.5 and 3.5 mm in diameter, with one aggregate at 7.3 mm. Settling rates ranged from 10 - 85 m d-1, with the largest particle having a rate similar to 1-2 mm particles. Particle abundances near coastal shelves in this region also may be elevated at midwater depths as a result of benthic resuspension by mesoscale circulation features and transport (lateral advection) of particles off the shelf and slope (Walsh and Gardner 1992, Dierks and Asper 1997). Relatively high concentrations of suspended aggregates (20 - 250 particles L-1) were observed in a bottom nepheloid layer in the Mississippi Canyon following the passage of Hurricane Isaac in August 2012, with most aggregates < 1 mm in diameter (Ziervogel et al. 2015). In comparison, within one month after the wellhead was capped (August 2010), marine snow concentrations ranged from 10.5 to 64.6 particles L-1 in the upper 20 m of off-shelf waters in the region of the oil spill, with particle sizes ranging from 0.150 to 17.6 mm in diameter (Daly et al. 2014). Oil concentrations (total petroleum hydrocarbons, TPH, and polycyclic aromatic hydrocarbons, PAH) were still elevated compared to background levels within 800 km of the wellhead during August, albeit at lower concentrations than were observed prior to capping the wellhead (Wade et al. 2015). The August 2010 maximum marine snow concentrations (64.6 particles L-1) were considerably higher in near surface waters, than during September 2011 (2 particles L-1), August 2012 (4.9 particles L-1), August 2013 (20.7 particles L-1), or August 2014 (1.5 particles L-1) at a deep-water station 50 km to the east of the DWH site. Integrated abundances (0 - 140 m) of marine snow also showed a similar pattern of maximum concentrations during August 2010 (866,862 particles m-2) at this site compared to summer concentrations during the following four

years (September 2011: 190,375, August 2012: 181,644, August 2013: 238,574, and August 2014: 74,311 particles m-2) (Fig. 2).

The dominant factors controlling gravitational settling of surface particles and aggregates vary spatially and temporally (Boyd and Trull 2007). Sinking speeds, however, are primarily a function of particle size and excess density (i.e., particle density > seawater density) (Armstrong et al. 2002, Iverson and Ploug 2010, De La Rocha and Passow 2007). Sinking speeds of marine snow range from 10s to 100s of meters per day. Slowly sinking particles are removed by microbial remineralization or grazing and, therefore, rarely reach great depths. The formation and flux of rapidly sinking marine snow is one of the primary processes by which the marine biological pump exports surface carbon and other elements to the deep ocean and seafloor.

2.2. Factors affecting MOSSFA processes

There are many factors that impact the formation and modification of MOS in the water column (Figs 3, 4, 5). Figures 3, 4, and 5 were developed as part of the MOSSFA Workshop discussions. Figure 3 focuses on surface processes, Figure 4 illustrates processes from the origin of oil outflow to final oil deposition, and Figure 5 depicts the impacts of gradients driven by riverine processes on MOS in the water column, on the seafloor, and within the sediments. These diverse factors emphasize the complex nature of the ocean environment and its interactions with oil blowouts. Factors shown in Figure 3 include (1) Riverine gradients of salinity and inputs of nutrients, dissolved organic matter, and lithogenic material influences particle aggregation (Muschenheim and Lee 2002), because coagulation

is a function of particle abundance and size (Burd and Jackson 2009). Nutrients enhance phytoplankton growth and dissolved organic matter forms gel particles, especially in salinity gradients (Wetz et al. 2009, Verdugo et al. 2004). (2) Marine biota may form or destroy marine snow (Alldredge and Silver, 1988, Burd and Jackson 2009). For example, phytoplankton and bacteria release 'sticky' exopolymeric substances (EPS) due to oil and dispersant exposure and mucus acts as glue, providing the matrix for aggregates (Passow 2014). Biological formation also includes processes such as incorporation into zooplankton feeding structures and feces and microbial generated marine snow. Alternatively, zooplankton feeding on MOS may lead to fragmentation of particles. Both microbial degradation of hydrocarbons to non-toxic forms (Kleindienst et al. 2015) and bioaccumulation into the food web (Lee et al. 2012, Chanton et al. 2012) occur. (3) Mediating measures, such as chemical dispersants and burning, affect the properties and behavior of hydrocarbons and influence the formation of MOS (Passow 2014). Dispersants break down oil into small droplets, but may inhibit microbial oil degradation (see section 2.5, Kleindienst et al. 2015b). Oil burning leaves about 5% of the original volume as burned residue such as black carbon and other pyrogenic by-products (Lehr et al. 2010). Black carbon particles efficiently absorb organic matter, including PAHs (Koelmans et al. 2006), and can stimulate transparent exopolymer particle (TEP) production and aggregation of marine snow (Mari et al. 2014). (4) Weathering, or aging, and photochemical alterations to oil similarly impact MOS properties (Bacosa et al. 2015, Passow 2014). Photo-oxidation is the process by which hydrocarbons, particularly PAHs, react with oxygen in the presence of sunlight, resulting in chemical and structural changes to oil that may lead to increased water solubility or decreased microbial biodegradation. (5) Physical processes may lead to

aggregation of particles due to collision or high turbulence may result in particle fragmentation (Hill et al. 2002, Burd and Jackson 2009). Currents, eddies, subductions, benthic resuspension and cross-shelf flow, and other physical processes impact the distribution of particles and oil pollutants in space and time (e.g. Paris et al. 2012, Smith et al. 2014). These processes all influence the dynamics of sinking MOS.

2.3. Field and lab MOS formation

Large, mucus-rich MOS particles were observed in surface oil during May, ranging in size from small compact particles to particles several centimeters in size. Large, stringy marine snow particles (<10 cm), with mucous threads resembling web-like structures, were also common (Passow et al. 2012). Sinking velocities for settling MOS particles were estimated to range from 68 - 553 m per day. Laboratory experiments elucidated some of the mechanisms leading to the formation of MOS by using roller tanks to simulate in situ conditions. Roller tanks allow marine snow to settle continuously without contact with surfaces (container walls) (Ploug et al. 2010). These experiments revealed a number of important processes, including, (1) diatom aggregates incorporated appreciable amounts of oil, either by collision and attachment of oil droplets with phytoplankton cells, or via absorption of oil to phytoplankton, (2) oil droplets were incorporated into colonies of cyanobacteria (Trichodesmium) through coagulation, and (3) bacteria mediated MOS formation in the absence of other particles, possibly through the release of high concentrations of mucus (Passow et al. 2012, Fu et al. 2014, Passow 2014). Interactions between oil components, bacteria, and natural suspended matter formed flocs (relatively large, fluffy aggregates) likely due to the formation of macro-gels, such as TEP, from EPS

released by bacteria in response to the presence of oil (Passow et al. 2012, Ziervogel et al. 2014). A complicating factor was the presence of the dispersant, Corexit, which at near in situ concentrations slowed or inhibited the formation of microbial MOS (Passow 2014). However, after longer incubations, possibly once Corexit was degraded, marine snow formed (Fu et al. 2014)

2.4. Microbial processes

Numerous microbes are able to utilize oil as a major source of carbon and energy, including 175 genera of bacteria, several haloarchaeal genera, and many Eukarya (McGenity et al. 2012). Microbial response to an oil spill is governed by many factors, including oil composition, degree of oil weathering, and environmental conditions, such as temperature and nutrient concentrations. The temporal and spatial variability in microbial populations before, during, and after the DWH oil spill, and in comparison with communities outside of the spill, showed that groups of oil-degrading bacteria responded to the presence of oil by May 2010, including alkane, PAH, and methane degraders, and nitrifying microorganisms (Dubinsky et al. 2013, Yang et al. 2014, Crespo-Medina et al. 2014, King et al. 2015). Overall, the relative importance of different taxa was governed by changes in hydrocarbon composition and supply. Surface oil communities appeared to have been more diverse than communities in the deep oil plumes (Kimes et al. 2014). By September 2010, the pre-spill community was re-established, with low background concentrations of oil degrading bacteria. In both surface and deep waters, MOS particles were colonized by heterotrophic microbes, which expressed high specific rates of enzymatic activity that were different from those of the surrounding seawater (Ziervogel et al. 2012, Arnosti et al. 2015). In

addition, several bacterial species that are noted for producing EPS had elevated concentrations in surface waters during the oil spill and formed mucus aggregates that contained oil droplets in lab experiments (Gutierrez et al. 2103). In situ MOS particles also had significant concentrations of glycoprotein (carbohydrate and protein), a primary component of EPS (Arnosti et al. 2015). Specific bacteria respond to oil in different ways, but in general, microbial communities produce mucus that acts like a biofilm, allowing a complex community to establish, which jointly utilizes the different components of oil and its metabolites (McGenity et al. 2012). Bacterial EPS also may stimulate non-oil degrading bacterial communities. Mishamandani et al. (2015) demonstrated that eukaryotic phytoplankton, such as the cosmopolitan marine diatom, Skeletonema costatum, which was abundant during the DWH oil spill (Yan et al. in review, Passow unpubl.), provide a biotope for hydrocarbonclastic bacteria, which appear to specialize in PAH degradation. While significant progress has been achieved in documenting shifts in the bacterial community in response to the oil spill, the spatial and temporal variability of microbial effects on MOS formation and degradation remain poorly known.

2.5. Plankton impacts on MOS

Microplankton, microzooplankton, and mesozooplankton were impacted by the spilled oil, which likely affected marine snow formation and sedimentation. Crude oil and the dispersant Corexit are reported to have had direct lethal and sublethal effects on bacteria, phytoplankton, microzooplankton (Garr et al. 2014, Almeda et al. 2014b, Kleindeist et al. 2015a, b) and mesozooplankton, including changes in physiology and reproduction (Ortmann et al. 2012, Almeda et al. 2013 a, b, 2014c, Cohen et al. 2014, Peiffer and Cohen

2015). The impact of dispersants, however, is being debated. Prince (2015) argues that dispersants are not significantly toxic under typical oil spill field conditions, they make oil more bioavailable for degradation, and impacts on seabirds are minimized. In contrast, a number of studies have indicated that toxic effects on pelagic organisms increased with the addition of Corexit to oil treatments, although results varied between taxa (e.g., Almeda et al. 2013b, 2014b, Cohen et al. 2014, Garr et al. 2014). Kleindienst et al. (2015b) also reported that the presence of Corexit decreased microbial degradation rates of hydrocarbons, by selecting for bacteria that degrade dispersant (i.e., Colwellia) rather than bacteria that degrade hydrocarbons (i.e., Marinobacter). These authors suggested that dispersants might stimulate hydrocarbon degradation in other marine systems that do not already have a background population of hydrocarbon-degrading bacteria. The Gulf of Mexico harbors hydrocarbon-degrading bacteria at low densities, due to chronic oil input from cold seeps.

Crustacean molts and dead zooplankton typically become part of the marine snow assemblage, sinking rapidly out of the water column. Zooplankton (e.g., dinoflagellates, gelatinous doliolids, copepods) ingest oil and egest oil in fecal pellets, which sink rapidly to the seafloor (Lee et al. 2012, Almeda et al. 2014a, c). St0rdal et al. (2015) demonstrated that despite reduced feeding activity by a North Sea copepod in the presence of oil, their fecal pellets contained oil plus oil-degrading bacteria (Rhodobacteraceae), which were indigenous to the copepods. This family of bacteria was a dominant group present after the DWH spill (Dubinsky et al. 2013, Kimes et al. 2014); hence, zooplankton may contribute to oil degradation. In addition, oil may adhere to zooplankton and be passively absorbed or

ingested; thus, contributing to bioaccumulation of PAHs (Mitra et al. 2012). Furthermore, carbon isotopic depletion in suspended particulate matter and zooplankton supports the notion that oil carbon was incorporated into the lower trophic food web through biodegradation by bacteria (Graham et al. 2010, Chanton et al. 2012; Cherrier et al. 2014).

2.6. Oil-mineral aggregates (OMAs)

Interactions between oil and suspended particulate material, or OMAs as they have been designated in the literature, have long been recognized to result in oil sedimentation in freshwater and marine systems, especially in tidal and subtidal areas (Lee 2002, Payne et al. 2003). OMAs are usually smaller (< 1 mm, often < 50 [im) than most marine snow particles (Stoffyn-Egli and Lee 2002) and form primarily in surf zones, near river outflows, melting glaciers or sea ice, and in semi-enclosed bays, where suspended lithogenic particle concentrations are relatively high (Lee and Page 1997, Payne et al. 2003). Boehm (1987) estimated that suspended particle concentrations need to be > 10 mg/L for significant deposition of oil to occur and > 100 mg/L for large deposition events. Since inorganic particles concentrations are usually < 10 mg/L in the open ocean, this pathway was considered to be relatively unimportant offshore of intertidal/subtidal regions. Offshore suspended drilling muds may be considered an exception.

A review of near-shore oil spills indicated that 1-13% of spilled oil settled to the seafloor due to OMA processes (Lee and Page 1997). The interactions between suspended particulate matter, oil weathering, adsorption, microbial processes, other marine snow particles, and grazing zooplankton impact oil packaging and MOS sedimentation (reviewed

in Muschenheim and Lee (2002). OMAs sink rapidly, in spite of their small size, because of their high mineral content. OMA formation increases the surface to volume ratio of oil, thereby extending and enhancing the weathering processes of dissolution, evaporation, and biodegradation (Lee 2002). Water turbulence (i.e. breaking waves, strong flood currents) enhances OMA formation and transport and mineral-oil flocculation rates are typically highest in low to intermediate salinity waters. Danchuk and Wilson (2011) suggested that OMA formation in the vicinity of the Mississippi River would vary depending on the time of year and type of oil. Lighter oils had a higher probability of forming OMAs during winter and spring, when there was higher sediment availability and denser, high-viscosity oil may form more OMAs during summer, when salinity was higher. OMAs form where oil and suspended minerals (including clay-sized particles) co-occur, whereas lithogenic material from, for example, a deep nepheloid layer may be scavenged by sinking marine snow passing through that layer. Since the DWH platform was about 75 km offshore directly southeast from the Mississippi River, OMAs may have played a role in DWH oil deposition.

2.7. MOS sedimentation

The role of MOS in DWH oil sedimentation is supported by data from a sediment trap deployed in late August 2010 about five km southwest of the DWH platform, 140 m above the seafloor at 1,400 m depth. The trap results showed exceptionally high POC sedimentation rates relative to other years, which were due to the sinking of a large diatom bloom that was almost entirely composed of Skeletonema sp. (Yan et al. in review, Passow

unpubl.), a cosmopolitan taxa that thrives under brackish conditions and is tolerant of the presence of oil (Parsons et al. 2014). Sedimentation of oil continued for > 5 months after the spill ended, characterized by numerous smaller events (Yan et al. in review, Passow unpubl.). Lithogenic minerals (i.e., silts and clays) co-settled with organic particles and constituted on average 60% per weight of the settled material. Biogenic silica from diatom frustules and organic carbon composed other significant portion, whereas little calcium carbonate was observed in trap material.

3. Temporal and spatial patterns of MOS

Although the overall processes related to the formation of MOS and OMAs are reasonably well know, less is known about the temporal and spatial variability of their formation and deposition, which are generally regulated by physical processes. Wind forcing was the primary mechanism governing surface oil drift (Le Henaff et al. 2012). In addition, freshwater discharge from the nearby Mississippi River, which is the world's third largest river, often results in strong stratification and enhanced wind-driven coastal jets (Morey et al. 2003, Jochens and DiMarco 2008). During April 2010, freshwater diversions along the lower Mississippi River were opened with the intent to minimize the impact of the oil spill on estuaries and wetlands (Bianchi et al. 2011). Kourafalou and Androulidakis (2013) concluded that in addition to wind forcing, Mississippi River induced circulation also significantly influenced near surface oil transport based on satellite and in situ data and numerical simulations. Winds blowing from the south reduced the interaction of surface oil with the Loop Current, consequently only a small amount of oil was trapped in a spin off

eddy (Eddy Franklin) (Le Hénaff et al. 2012). A number of processes, which likely influenced weathering of MC252 crude oil, including physical agitation, wave conditions, evaporation, photo-oxidation, and dispersability, were summarized in Daling et al. (2014). Three storm events during the oil spill, of which two were named (e.g. Hurricane Alex, Tropical Storm Bonnie), led to a change in surface oil extent and deep mixing (Goni et al. 2015), but the storm impacts on the oil spill and oil deposition are for the most part unknown. Previous studies during hurricanes observed significant vertical variations in currents and internal waves (Shay and Elsberry 1987, Keen and Allen 2000.) Storm induced resuspension of flocculent material deposited on the seafloor during the DWH event was shown to lead to increased microbial activities (Ziervogel et al. 2015). In addition, the Loop Current, eddies, cross-shelf flows, and mesoscale circulation are important physical processes in the NE Gulf of Mexico, which impact biological production and oil transport (Nababan et al. 2011, Olascoaga et al. 2013, Goni et al. 2015, Jones and Wiggert 2015, Smith et al. 2014).

Biological factors were another major influence on the spatial - temporal patchiness of the formation and fate of MOS. During August 2010, satellite data (MODIS Fluorescence Line Height) showed unusually high chlorophyll concentrations compared to the mean of an eight-year time-series for the NE Gulf of Mexico. This chlorophyll maximum was located to the east of the DWH wellhead and covered an area >11,000 km2 (Hu et al. 2011). These elevated phytoplankton concentrations appeared to be related to the oil spill since the high river flow anomalies did not correlate with the spatial area of the chlorophyll anomaly. Whereas exposure to high concentrations of oil is lethal to most phytoplankton, sensitivity

to hydrocarbons varies and some species thrive under low concentrations of oil (Parsons et al. 2014, Gonzales et al. 2013). In addition to the region of the elevated chlorophyll anomaly, nutrients from the Mississippi River fueled phytoplankton blooms during 2010 (Hu et al. 2011), which are typically observed every year in the vicinity of the river plume (Lohrenz et al. 1997). Large mucus-rich marine snow particles were observed floating in surface waters during May, but had disappeared by June (Passow et al. 2012). Other types of marine snow, however, remained in relatively high concentrations at least into August (Daly et al. 2014). Similarly, Patton et al. (1981) noted that pancakes and flakes of mousse or weathered oil occurred at the surface during the Ixtoc-I oil spill. In addition, large numbers of dead gelatinous zooplankton (Pyrosoma sp.) were observed floating in an area covering a two mile radius on14 June 2010, about three nautical miles southwest of the DWH spill site (R. Amon pers. comm.), suggesting that thalacians (e.g., pyrosome, salp, or doliolid zooplankton) also may have contributed to MOS flux.

4. Deep oil plumes

After the DWH explosion, persistent subsurface oil plumes (Fig. 4) were detected by early May and later verified by chemical analyses to have occurred primarily between 1,000 and 1,400 m depth (Camilli et al. 2010, Diercks et al. 2010, Spier et al. 2013). Ryerson et al. (2012) estimated that >30% of the hydrocarbon mass released may have gone into the deep plumes. The subsurface oil plumes contained a complex mixture of soluble and insoluble liquid hydrocarbons, including alkanes, monoaromatic hydrocarbons (e.g., BTEX), PAHs, and gaseous C1-C4 hydrocarbons including methane, ethane, propane and butane

(Spier et al. 2013, Reddy et al. 2012). The application of dispersants at the wellhead in deep water was intended to decrease the mean oil droplet size, which may have decreased the droplet rising velocity and increased the residence time of oil in the water column and impacted its transport (Socolofsky et al. 2015). In contrast, high-pressure experimental studies of oil droplet size by Aman et al. (2015) suggest that the sub-surface plume would have formed even without dispersant application. Marine snow particles, which formed at the surface and sank to the seafloor, likely interacted with rising oil droplets and/or the oil plumes at depth (Valentine et al. 2014). Surface bacteria and phytoplankton-affiliated gene sequences were found in sediments at and below the subsurface plume (Mason et al. 2014, Brooks et al. 2015), substantiating the notion of transport of surface-formed MOS to depths. MOS likely formed in the deep-water plumes as well, as many bacteria were active in these regions (Hazen et al. 2010, Redmond and Valentine 2012, Dubinsky et al. 2013) and because MOS formed in experiments using bacteria from the deep plume (Baelum et al. 2012, Kleindienst et al. 2015b).

The temporal and spatial dynamics of the deep oil plumes were complex and variable. A coupled 3-D hydrodynamic and oil particle tracking model indicated that local topography and hydrodynamic processes, such as eddies, were important in governing the DWH oil distribution and that use of deep dispersants may have led to a deeper vertical distribution of suspended oil (Paris et al. 2012). The relatively narrow plumes occurred at multiple depths and extended both to the southwest and to the northeast of the wellhead, with the maximum oil concentrations in the plume generally flowing along the 1,200 m isobath. By the time the wellhead was capped, the plumes were estimated to have reached up to 300

km in extent, with oil concentrations ranging from 5 - 500 ppb, and had interacted with the slope sediments of the Mississippi and Desoto canyons (Paris et al. 2012, Lindo-Atichati et al. 2014, Romero et al. 2015).

Socolofsky et al. (2011) demonstrated that the deep subsurface oil plumes derived from a stratification-dominated multiphase plume, which was characterized by multiple subsurface intrusions of dissolved gas and oil, as well as small droplets of liquid oil. Simulations by North et al. (2015) emphasized the importance of oil droplet size and rates of biodegradation in understanding factors controlling oil transport. Based on a model comparison study, DWH oil droplet sizes were predicted to have ranged from 0.3 - 6 mm without the addition of dispersants and between 0.01 - 0.8 mm with the addition of dispersants (Socolofsky et al. 2015). Ryerson et al. (2012) estimated that the majority of oil droplets that arrived at the sea surface were millimeters in diameter based on hydrocarbon mass transport calculations. Aman et al. (2015) reported that droplets <40 [im formed the deep lateral oil intrusions, whereas larger droplets >100 [im rapidly rose to the surface, based on results from high-pressure laboratory experiments and model simulations (Paris et al. 2012). The simulations further suggested that the application of dispersants might not have played a significant role in the partitioning of surface and subsea oil, in that the amount of oil reaching the surface may have only been reduced by 3%. Details on the impacts of droplet size and pathways for the formation of MOS await further investigations.

Microbial degradation rates of oil and gas hydrocarbons in the deep-water plumes are the subject of debate. Ziervogel and Arnosti (2013) observed higher microbial rates of protein and carbohydrate hydrolysis in the deep oil plume compared to waters outside the plume, suggesting EPS and oil degradation products enhanced bacterial metabolism in the plume. Hazen et al. (2010) suggested that degradation rate in these deep cold waters (5° C) were relatively high (turnover rates on order of days) due to several factors: (1) the MC252 oil was a light (volatile) crude and thus more easily degraded, (2) the deep oil plumes included accessible small oil droplets, and (3) oil from natural seeps in the Gulf of Mexico maintained an oil-adapted deep-sea microbial community. Camilli et al. (2010), however, reported relatively slow rates of hydrocarbon degradation on the order of months. The observed differences in rate measurements are likely related to variability in the temporal and spatial response of the microbial community to oil and Corexit input. Degradation rates also may have been depressed by dispersants (Kleindienst et al. 2015b). Recent topic reviews suggest that the initial microbial response to the DWH oil spill was driven by light hydrocarbons (Valentine et al. 2010) and that overall the microbial response was rapid and robust (Kimes et al. 2014, King et al. 2015). Although little methane emitted at the wellhead made it to the sea surface (Yvon-Lewis et al. 2011), the timing of methane degradation is controversial (Kessler et al. 2011, Joye et al. 2011, Crespo-Medina et al. 2014). In addition, the rates of hydrocarbon degradation under high-pressure conditions remain poorly understood (Gutierrez and Aitken 2014, Joye et al. 2014, King et al. 2015), even though pressure affects reaction rates of dissolved gasses (Bowles et al. 2011). Metabolic function in some bacterial strains which degrade hydrocarbons were recently shown to be inhibited by pressure levels typical of the deep oil plumes (Schedler et al.

2014), emphasizing that pressure must be considered when evaluating degradation rates in deep water. In contrast, methane oxidation, which is a first order kinetic process, occurs at significantly higher rates under deep-sea pressures, preventing methane supersaturation (Bowles et al. 2011). Nonetheless, early concerns raised about the onset of hypoxic conditions as a result of the large methane releases in the deep sea (Joye et al. 2011) were not realized, as dissolved oxygen concentrations did not decrease substantially. Instead, the methanotroph community appears to have consumed methane relatively rapidly, transforming it into particulate organic carbon (Redmond and Valentine 2012, King et al. 2015, Cherrier et al. 2014). Overall microbial processing of oil components has been estimated to account for 40 - 60% of the discharged hydrocarbons (gas and oil) (Joye

2015). Because marine snow particles are considered hot spots of microbial activity (Azam and Long 2001), with greatly enhanced enzyme activities and degradation rates compared to surrounding seawater (Smith et al. 1992), MOS and OMAs likely contributed to the high microbial degradation rates of oil in the deep oil plumes.

5. Oil accumulation rates and fate at the seafloor

Factors affecting the long-term sediment accumulation rates in the northeast GOM and during the DWH marine snow-oil event are shown in Figures 3, 4, and 5. The Mississippi River typically dominates sediment transport and composition on Mississippi, Louisiana, and Texas shelves, as well as the offshore Mississippi Deep-Sea Fan (Balsam and Beeson 2003). The Apalachicola River to the east on the Florida Gulf coast affects sediment transport and composition too. Figure 5 illustrates the large gradients of ocean properties

driven by riverine processes. Sediment deposition from riverine sources is a function of discharge rate, grain size, clay minerology, sediment resuspension, cross-shelf transport, and slumping (Gardner and Walsh 1990, Corbett et al. 2006, Bianchi et al. 2007). River additions of nutrients also fuels marine phytoplankton blooms of diatoms (Lohrenz et al. 1997, Dagg and Breed 2003). Thus, sources of organic matter accumulated on the seafloor include anthropogenic and terrestrially-derived biogenic substances associated with river discharge, marine detritus, phytoplankton, microbial, and petrochemical substances. The relative importance of these materials will vary with distance from rivers and continental shelves and water depth. Except for regions influenced by the Mississippi River Plume, Loop Current, or eddies, offshore areas in the central Gulf of Mexico typically have lower productivity than shelf regions and phytoplankton communities are dominated by small cells, primarily cyanobacteria (Wawrik and Paul 2004, Muller-Karger et al. 2015). While Figure 5 shows a simplified pattern of onshore to offshore gradients in properties and productivity, the off-shelf region of the oil spill in the NE Gulf of Mexico is much more environmentally complex and generally has higher productivity than other regions of the Gulf of Mexico (Nababan et al. 2010). Sedimentation rates coupled to sediment grain-size and organic content are the major controls on the rates and depths of oxidation-reduction (redox) reactions in sediments (Hastings et al. 2015). Redox reactions, in turn, will influence porewater chemistry, microbial community structure/function, dissolved and solid-phase metal concentrations, degradation rates, and decomposition and transformation of petrochemicals. Furthermore, the impacts of oil deposition on benthic fauna via smothering, shoaling of the oxic-anoxic boundary, and toxic effects of oil will

affect the depth and occurrence of sediment bioturbation and oil and organic matter degradation rates.

MOS deposition was observed in bottom sediments within 256 km of the DWH platform, as an approximately 1 cm-thick sedimentary layer (Montagna et al. 2013, Mason et al. 2014, Valentine et al. 2014, Brooks et al. 2015). The Brooks et al. (2015) study area extended up to 100 nautical miles (185 km) northeast of the DWH wellhead where thorium inventories of sediment cores indicated that the deposition occurred within four to five months, through the summer and fall of 2010. Mass accumulation rates over the past ca. 100 years ranged from 0.05-0.16 g/cm2/yr. In contrast, the 2010 deposition rate was four-fold higher (0.48 to 2.40 g/cm2/yr) compared to rates before 2010 or after the oil spill during 2011 and 2012. Sedimentation at these sites was principally due to marine snow, with minor input from the extended Mississippi River discharge (Brooks et al. 2015). Petrographic investigations, biomarker analyses, and gene sequencing indicated that the amorphous aggregates in sediments were derived mainly from surface-derived diatoms, coccolithophores, and cyanobacteria sources, which co-occurred with pyrogenic PAHs from surface oil burning (Brooks et al. 2015, Romero et al. 2015). In addition, Lincoln et al. (2015) observed increased carbohydrate concentrations associated with relatively large particles in sediments. These authors utilized scanning confocal laser microscopy and fluorescently-labeled lectins (that bind to specific glycoconjugates common in marine exopolysaccharides) to show that the labeled carbohydrates occurred as blebs with distinct drape and stringer morphologies and that their collapsed appearance suggested a water column origin as opposed to formation in situ by hydrocarbon-degrading bacteria. These

microscopy results provide further evidence that the carbohydrate source was from surface-derived EPS in marine snow.

The amount and distribution of DWH oil in sediments was estimated by Chanton et al. (2015) to be 0.5 to 9% of the total oil from the wellhead, using radiocarbon distributions. Their results indicated that oil was deposited within the upper 1 cm of the sediments over an area of 8,400 km2, mostly to the southwest and as far as 190 km to the southwest of the wellhead (Chanton unpubl.). Valentine et al. (2014), using hopane as a tracer, reported that 1.8 - 14% of the oil from DWH was in the upper 1 cm of sediments over an area of 3,200 km2 around the wellhead. DWH PAH concentrations in sediment cores from the same sites reported in Brooks et al. (2015) were significantly elevated during 2010 (up to two orders of magnitude greater than background), ranging from 70 to 524 ng g-1 and diagnostic markers were consistent with a MC252 oil origin (Romero et al. 2015). The PAHs appeared to be a mix of petrogenic and pyrogenic (burn-derived) material. In addition, these authors point out that the deep oil plumes likely impinged on the continental slope and acted as another mechanism for oil deposition on the seafloor. Mason et al. (2014) evaluated 64 sediment cores collected primarily south-southwest of the DWH site and observed the highest PAH concentrations (19,258 ng g-1) within five km of the wellhead and the lowest concentrations (18 ng g-1) further away. Valentine et al. (2014) tabulated hopane concentrations for 534 locations, noting that the highest concentrations occurred within 40 km of the wellhead, primarily to the southwest. These authors argued that there is a mismatch between the location of the surface oil slick and where oil was observed on the seafloor, implying that surface oil was not the primary source of the seafloor oil. Instead,

they suggested that the majority of the oil stemmed from the deep plume, rather than from the surface layer, based on the composition of oil residues at the seafloor. In addition, Kolian et al. (2015) reported continued deposition of oil near the wellhead and in coastal waters of Mississippi, suggesting that the wellhead may have continued to leak until at least May 2012. Thus, a number of different processes may have contributed over different time and space scales resulting in the heterogeneous distribution of oil on the seafloor.

Oil deposition in these deep-sea sediments resulted in enhanced microbial response and an intensification of redox reactions and anaerobic conditions relative to unoiled sediments (Mason et al. 2014, Hastings et al. 2015, King et al. 2015). Both aerobic and anaerobic processes were detected in surface sediments, with shifting community compositions. The relatively high abundance of microbial genes for hydrocarbon degradation suggested that sediment surface microbes could rapidly deplete oil in that environment. Ziervogel et al. (2014), however, observed only a moderate increase in microbial rates in sediments in response to the MOSSFA event several months (November and December 2010) after the well-head was capped, leading these authors to hypothesize that this oil may have a long residence time on the seafloor. Furthermore, Hastings et al. (2015) recognized, through the analysis of redox sensitive metals (e.g., Re and Mn), that the intensity of redox reactions and anaerobic conditions increased for up to three years after the DWH event, likely a result of excess organic matter and hydrocarbon burial and decomposition. These redox changes over time and space were associated with dramatic shifts in the benthic community composition (Schwing et al. 2015). While the fate of DWH oil on the seafloor remains uncertain, Soto et al. (2014) reported that during the Ixtoc-I oil spill total

hydrocarbon concentrations in sediments had returned to background levels two years after the release of 3 million barrels of oil.

6. Ecosystem effects

It is uncertain to what extent the MOSSFA event impacted Gulf of Mexico ecosystems. The MOSSA Workshop participants developed a conceptual model (Fig. 6) to convey the complexity of an ecosystem response to a perturbation and the need for information on all levels: genes, organisms, populations, communities, and ecosystem, for both water column and seafloor habitats. All levels are connected through food web and environmental processes, with impacts transmitted from genes up to ecosystem scales and vice versa with ecosystems impacting changes in populations and ultimately genetic adaptation. Connectivity through currents with unimpacted regions is an important factor in recovery following severe disturbances in marine systems. However, ecosystem response to disturbances and long-term changes may involve threshold effects and non-linear dynamics (Folke et al. 2004). Thus, the complexity of these interactions at all temporal and spatial scales makes it difficult to assess the impacts of disturbances, such as the DWH oil spill and MOSSFA, on marine ecosystems.

Baseline data, which are essential for assessing the impacts of disturbances, unfortunately, are not available for most components of the food web in the NE Gulf of Mexico. Bernhardt and Leslie (2013) recommend obtaining time-series data for the underlying ecological components of food webs that are important for resilience, i.e., population size, species

diversity, and functional groups. Information also is needed on response patterns to MOS sedimentation for key species, such as those that are ecologically or economically important. Ecologically important species should include those that are representative of specific habitats, or foundation species, at all depths of the water column and in the benthos. Laboratory mesocosm studies would provide information for the many species that were not adequately studied during the DWH spill. Furthermore, laboratory studies would permit data collection on lethal and sublethal exposure and impacts to different flux rates of MOS and concentrations of oil and dispersants.

Several studies have shown that MOSSFA due to the oil spill impacted the marine ecosystem (Fischer et al. 2014, Schwing et al. 2015). Direct and indirect impacts of MOS may have occurred through ingestion, microbial activity, smothering, suboxic and anoxic conditions, transfer through the marine food web, and immunotoxicity through gills and transdermal exposure and/or bioaccumulation. Bioaccumulation of hydrocarbons through the planktonic food web would increase exposure of higher-trophic-level organisms (Meador 2003). Zooplankton ingested oil droplets directly (Lee et al. 2012, Almeda et al. 2014c) or may have ingested oil-containing marine snow, as many zooplankton are known to feed on marine snow (Alldredge and Silver 1988). Typically 70 - 90% of marine snow particles sinking from surface waters are ingested (repackaged) or fragmented by mesopelagic zooplankton or remineralized by bacteria (Guidi et al. 2008). It is not known what percent of the sinking MOS was deposited on the seafloor or the fate of the mesopelagic community during and after the oil spill. Carbon and nitrogen stable isotope analyses and natural abundance radiocarbon analysis have indicated that oil entered

particulate organic carbon and the planktonic food web in surface waters (Graham et al. 2010, Chanton et al. 2012, Mitra et al. 2012, Cherrier et al. 2014, Fernandez et al. in review) and mesopelagic fish and shrimp in deep water (> 600 m) likely through trophic transfer (Sammarco et al. 2013, Quinana-Rizzo et al. 2015). Pelagic fish that swim with their mouths open to maintain a continuous current of water across their gills (e.g., scombrid fishes; Klinger et al. 2015) could experience increased oil exposure from suspended or sinking MOS. There is evidence that coastal macrofauna ingested petrocarbon as well (Sammarco et al. 2013, Wilson et al. 2015). The mesopelagic community primarily feeds on zooplankton and vertically migrates into the upper 200 m at night, so they may have encountered subsurface oil plumes and food sources enriched in oil both at depth and at the surface. In turn, surface and mesopelagic fish are eaten by higher trophic animals, such as tuna, seabirds, and dolphins. An unusual mortality event has been reported for bottlenose dolphins (Tursiops truncatus) in coastal waters of Louisiana, Mississippi, and Alabama starting in 2010 and continuing into 2014 (Venn-Watson et al. 2015). This is the longest marine mammal die-off in recorded history of the Gulf of Mexico. Although the majority of strandings are bottlenose dolphins, spinner dolphins (Stenella longirostris), Atlantic spotted dolphins (Stenella frontalis), and melon-headed whales (Peponocephala electra) also have stranded. While the factors contributing to the high mortalities are still being investigated, dolphins in Barrataria Bay, Louisiana have exhibited moderate to severe lung disease and evidence of hypoadrenocorticism since 2010 consistent with immunotoxic effects of oil.

The large sedimentation of MOS clearly had a negative impact on benthic organisms, including injuries, mortality, and changes in community composition. The Gulf of Mexico has a high benthic species diversity, with maximum diversity on the mid to upper continental slope between 1,200 to 1,600 m depth (Wei et al. 2010), which coincides with the depth of the DWH well site. Montagna et al. (2013) and Baguley et al. (2015) reported on the results of benthic fauna sampled at 170 stations after the oil spill, of which 68 stations were located 0.5 to 125 km from the wellhead. These authors observed a severe to moderate impact on benthic meiofauna, with the highest impact within 3 km of the wellhead and a moderate impact within 60 km to the southwest. Community changes in severely impacted areas included an increase in nematode worms (an indicator of organic pollution), a decline in harpacticoid copepod abundance, and low meiofauna and macrofauna diversity compared to unimpacted sites. Overall, copepods had the largest population decline (10-fold), followed by polychaete worms, ostracods, and kinorhynchs. The dramatic increase in nematode abundance could be due to organic enrichment from sinking MOS and/or a higher tolerance to hydrocarbons. Moreover, Schwing et al. (2015) determined that there was an 80 to 93% decline in benthic foraminifera following the oil spill up to 111 km from the wellhead, likely due to suboxic conditions (Hastings et al. 2015). Using principal component analysis, Schwing et al. (2015) strongly suggested that oil exposure coupled to the onset of suboxic-anoxic conditions in the sediments were the factors controlling foraminifera decline. A year later there was evidence of recovery at some, but not all sites including the site 111 km away. This is consistent with the protracted and increasing intensity of sediment redox conditions for up to three years after MOSSFA (Hasting et al. 2015). In addition, deep-water coral communities were impacted.

Oil-containing flocculent material was observed on 90% of the corals within 6 km of the wellhead with less impact on communities up to 22 km to the southwest (White et al. 2012, Fisher et al. 2014). Corals that were covered with brown flocculent material (MOS) showed evidence of stress (e.g., excess mucus production, sclerite enlargement, tissue loss) and mortality (DeLeo et al. 2015). Shallow water mesophotic corals at the Pinnacles Reef offshore of Alabama were impacted as well, with corals having bare skeletons and broken or missing branches (Silva et al. 2015). Coral branches that were exposed to higher concentrations of settling MOS have not recovered to date (Hsing et al. 2013). Prouty et al. (2014) reported that corals had incorporated oil carbon up to 30 km from the spill site. Experiments using toxicological assays of coral fragments demonstrated that all three coral species tested exhibited more severe health decline (e.g., polyp retraction, mucous discharge, exposed skeleton, mortality) when exposed to dispersant and oil-dispersant mixtures than to oil alone (Deleo et al. 2015). Since these corals are very slow growing and may live > 600 yrs, they are highly vulnerable to disturbance and may be very slow to recover (Prouty et al. 2014). Some fish and sharks use deep corals as spawning grounds; thus, impacts to corals could lead to larger ecosystem impacts.

Sedimentation of MOS also impacted bottom dwelling fish. An assessment of offshore fishes between 2011 and 2012 revealed red snapper (Lutjanus campechanus), king snake eel (Ophichthus rex), and golden tilefish (Lopholatilus chamaeleonticeps) had elevated PAH and metabolite concentrations, with a composition similar to that from the DWH oil (Murawski et al. 2014, Snyder et al. 2015). Elevated PAHs were detected in fish bile, but not in muscle tissue, at sites near the wellhead and extending to the west Florida shelf. Skin lesions were

present on up to 9% of the fish. Although skin lesions may be a short-term consequence of acute PAH contamination, PAH exposure may result in a variety of immunotoxicity population-level effects, including impaired growth, increased disease susceptibility, reduced larval survival, and reduced net population fecundity. Red snapper and king snake eels showed signs of recovery during 2012 and 2013, while golden tilefish, which burrows into sediments and likely had a longer exposure to PAHs, still had elevated biliary PAH metabolites during 2013 (Snyder et al. 2015). The recovery of the deep-sea ecosystem is uncertain given the spatial complexity of the sedimentation event and lower biodegradation rates of oil in sediments (Mason et al. 2014, Ziervogel et al. 2014).

7. Proposed framework for MOSSFA investigations and data gaps

MOSSFA Workshop participants created a conceptual diagram (Fig. 4) to (1) better define the processes that impact MOS formation and sedimentation from the point of the deep-water oil discharge to MOS sedimentation on the seafloor, (2) identify significant research questions, (3) determine the space and time scales of processes and events, and (4) identify data gaps that may be important for further defining and modeling of MOSSFA events. Examples of data gaps and questions relating to the MOSSFA processes are provided below to help guide future research. The MOSSFA Workshop report (2014) provides a more comprehensive list of questions and discussion of data gaps. The letter-designated sections below correspond to topics illustrated in Figure 4.

7.1. Physical and chemical characteristics of oil droplets and associated hydrates (A)

The characteristics of oil released at a point source are determined by the type of oil released, water temperature, water depth (ambient pressure), and the pressure within the petroleum reservoir. Laboratory experiments and models analyses of oil droplet size distributions are required over a range of conditions (pressure, turbulence) to predict the fate of bubbles of gas hydrates and oil droplets as they rise to the surface. During the DWH oil spill, chemical dispersants were applied directly to the deep subsea oil jet to promote smaller oil droplet size and longer residence time in the water column. The consequences of this action need further investigation.

7.2. Formation of subsurface oil plumes: droplet and bubble size (B)

Deep horizontal intrusions of dissolved gas and oil formed at mid-water depths, which impacted the mesopelagic environment, sedimentation of oil, and sediments where plumes impinged on the continental shelf and slope. In addition, MOS particles may have formed in the plumes and marine snow particles that formed at the surface may have entrained additional oil as they sank through the plumes. Many questions remain relating to the characteristics of oil in the plumes, the role of the physical environment in plume formation and spatial extent, the plume's role in the MOSSFA event, and an evaluation of the conditions under which subsurface plumes may form during future oil well blowouts.

7.3. Important processes in the surface mixed layer (C)

All surface ocean processes that impacted MOS formation and sedimentation varied spatially and temporally. Atmosphere-ocean interactions and submesoscale physical

processes influenced the spatial distribution of oil, marine biota, and MOS. Winds and current shear spread the surface oil slick. Breaking waves disperses oil into the water column resulting in oil droplets of different sizes. The impacts of large storms are unknown. Continued model development is needed to better understand the role of physical processes in oil dispersion and MOS aggregation and sedimentation, as well as the recovery (replacement via advection) of the marine food web.

7.3.1. Interaction of oil droplets with euphotic zone plankton (C1)

Oil that rose to the surface encountered high concentrations of phytoplankton fueled by nutrients from river discharge. Dispersants also were applied at the surface. Laboratory experiments demonstrated that MOS particles were formed by interactions of oil with bacteria, phytoplankton, and zooplankton. Did the oil spill contribute to the observed enhanced phytoplankton bloom? How did variation in oil and dispersant concentrations change microbial mucus formation and zooplankton behavior and survival?

7.3.2. Transformation of oil floating at the atmosphere-ocean interface (C2)

UV light acts to weather oil and evaporation of more volatile constituents acts to change oil composition and density over time. Both evaporation and emulsification increases the density and viscosity of the surface slick. Why does weathered oil enhance MOS aggregation in experiments? Microbial activity was shown to create large flocs early during the oil spill. Why did this phenomena change over time? How did dispersants impact MOS formation?

7.3.3. New sources of material at the surface (C3)

Oil was burned at the surface and pyrogenic particles were observed in the sediments. The role of aerosols and burning oil by-products in MOS formation and sedimentation, however, are unknown. Did Saharan dust input interact with the sea surface microlayer to influence MOS formation? What was the role of soot (i.e., charred combustion products of burning oil) in particle formation and sedimentation?

7.3.4. Sinking of oil-derived material from the surface (C4)

Increasing the density of marine snow was key to forming sinking particles. Although the general physical and biological factors contributing to MOS formation and sedimentation are known, there is little information on the composition of MOS particles and the factors controlling its spatial and temporal variation and sinking rates. Lithogenic deposition was evident in a sediment trap deployed five km SW of the DWH site and on the seafloor shortly after the oil spill. However, the role of OMAs (mineral ballast) in oil sedimentation and whether OMAs aggregated with MOS particles over the broad area of the oil spill is not known.

7.4. Processes impacting MOS during sedimentation (D)

Microbial activity, turbulent disruption, and zooplankton fragmentation transform sinking aggregates. Currents laterally advect sinking particles. There is evidence that a subsurface nepheloid layer occurred off the shelf slope during the oil spill. It is not known how surface-formed sinking particles interacted with suspended sediment layers and/or the deep subsurface oil plumes, or to what extent particles were laterally advected. Clay

mineralogy, the size range of clay particles, and oil emulsions likely controlled whether oil droplet/particles are buoyant or sink, as clay particles may stabilize emulsified droplets (Sullivan and Kilpatrick 2002). The extent to which sinking aggregates scavenged organisms, organic detritus, inorganic particles, and other oil droplets is uncertain.

7.5. Processes impacting MOS on the seafloor (E)

Although it is clear that a significant percentage of DWH oil sedimented to the sea floor, an accurate assessment of the amount of oil and its spatial distribution remain to be determined. The dominant factors controlling the persistence and degradation of hydrocarbons in sediments needs further investigation. What are the roles of microbial communities, redox interactions, and sediment oxygen demand in the degradation of hydrocarbons? In particular, long-term studies are needed to determine microbial rates of hydrocarbon degradation over time and to assess the recovery rate of benthic communities.

7.6. Processes impacting resuspension of oiled sediments (F)

The role of water motion in resuspension, disaggregation, and re-aggregation of oiled sediments in benthic boundary layers is poorly known. What is the role of storm-generated deep currents in resuspending oiled sediments? How do these processes impact benthic communities? There is little information on the role of natural oil seeps in accumulation and re-suspension of oiled sediments. Are marine snow particles produced near natural seeps?

8. Conclusions

The DWH MOSSFA event was unexpected, but now recognized to be a significant pathway for the distribution and fate of oil. The contribution to sedimentation and interactions between MOS and OMA particles will vary with distance from shore and river water plumes. Evidence suggests that the prolonged DWH sedimentation event may have been comprised of numerous smaller sedimentation events during and after the spill, with the overall accumulation at the seafloor of as much as 14% of the total oil released, thereby relocating oil to mesopelagic and deep benthic ecosystems. Clearly, understanding the impact of MOS processes on the transport and fate of oil spilled into the environment is important and should be considered for future oil spill assessments. Consequences of the MOSSFA event include the fact that MOS particles in surface waters and aggregates sinking through the water column were remineralized by bacteria and/or ingested by zooplankton. Assimilation of petrocarbon into food web biomass would be a more favorable pathway for marine food webs than bioaccumulation of oil in organisms, which negatively impact higher trophic levels. In addition, the accumulation of DWH oil on the seafloor will have a relatively long residence time, with continued metabolism of toxic and carcinogenic hydrocarbons by microbial communities and, therefore, protracted exposure by ecologically sensitive benthic animals and economically and recreationally important fish. Responders to future oil incidents should consider the possibility of a MOSFFA event when evaluating appropriate actions to limit the impacts of oil spills.

The large DWH MOSSFA incident occurred due to a nexus of events in a similar manner to the events which created Superstorm Sandy on the Atlantic coast of the USA in 2012: (1)

the DWH site was located in one of the most productive regions of the Gulf of Mexico governed by Mississippi River outflow, (2) the spill occurred during spring and summer when bacteria and phytoplankton concentrations were at a maximum and surface community activity rates were relatively high, (3) the large Mississippi River discharge enhanced phytoplankton production and suspended particle concentrations, and (4) there was enhanced microbial mucus formation, especially in the presence of weathered oil. Nevertheless, the DWH event was not unique as several reports indicate that oil sedimentation was observed during the Tsesis, Ixtoc-I, and other oil spills (Johansson et al. 1980, Jernelov and Linden 1981, Patton et al. 1981, Teal and Howarth 1984, Vonk et al. 2015). Indeed, such observations suggest that MOSSFA events have a high probability of occuring during future oil spills in coastal and deep-water sites, particularly since 85% of deep- water oil exploration sites worldwide are adjacent to deltaic systems (Weimer and Pettingill 2007). Despite the large academic and government response to the oil spill, there remain significant data gaps in understanding the dominant factors controlling the temporal and spatial variability of the DWH MOSSFA event. In this paper, we provide a conceptual framework for understanding MOSSFA processes to help guide current research on the DWH oil spill and which could be applied to other locations. Mechanistic models of MOS formation and sedimentation coupled to circulation models need to be developed to use as a tool to manage risk and improve our ability to predict the extent of MOSSFA events under different conditions and environments. Furthermore, baseline time-series data are urgently needed for all levels of the ecosystem in regions of hydrocarbon extraction to prepare for and respond to oil spills and to understand the impacts of oil spills on the environment, particularly in sensitive ecosystems such as the Arctic.

Acknowledgements

The MOSSFA workshop was supported by a grant from the Gulf of Mexico Research Initiative. Research support was provided by the University of South Florida Division of Sponsored Research and Florida Institute of Oceanography (FIO)/BP to Daly, and grants from the Gulf of Mexico Research Initiative through its consortia: Center for Integrated Modeling and Analysis of Gulf Ecosystems (C-IMAGE) to Daly, Hollander, and Chanton, Deep-C to Chanton and Hollander, and Ecosystem Impacts of Oil & Gas Inputs to the Gulf (ECOGIG) to Passow and Chanton. We thank the workshop participants (http://deep-c.org/mossfa/) for their significant contributions to the workshop discussions. We are especially grateful to Dr. Nancy Kinner for facilitating the workshop and Dr. Peter Kinner for organizing and editing the final report. Meredith Fields and Kathy Mandsager provided excellent logistical support. A. Remsen and K. Kramer provided analyses of marine snow. We also thank the reviewers for thoughtful comments and suggestions. Data are publically available through the Gulf of Mexico Research Initiative Information and Data Cooperative (GRIIDC) at http://data.gulfresearchinitiative.org (doi:10.7266/N78P5XFP, http://dx.doi.org/10.7266/N76T0TKS).

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Figures

90° W 87° W 84° W

Cell Average H 0.02 - 0.10 0.19-0.29 0.44 -0.61 0.84 - 1.13 H 1.56 - 2.28 Vol (m3/km2) ■0.11 -0.18 0.30 -0.43 0.62 - 0.83 ■ 1.14 - 1.55 ■ 2.29 - 4.06 Footprint Average Volume (m3): 22619 Footprint Average Area (km2): 11804

93° W 90° W 87° W 84° W

Figure 1. The surface distribution of Deepwater Horizon oil. Average volume of surface oil (m3/km2) in 5 x 5 km gridded cells between 24 April and 3 August 2010, based on SAR satellite data (after MacDonald et al. 2015).

Figure 2. Particle size spectra of marine snow and examples of marine snow images observed by the SIPPER imaging system during August 2010, September 2011, August 2012, August 2013, and August 2014. The particle size distribution data are from a station 50 km to the east of the DWH site.

Figure 3. Image depicts the factors that affect the formation and modification of MOS. The blue triangle represents the area of the oil spill. MOS is affected by the ocean environment, the state (degree of weathering) and composition of oil, spatial overlap with riverine and shelf processes, the type of marine biota present and their related processes, and the timing and location of spill mediation measures (e.g., application of dispersants and burning).

Figure 4. Conceptual diagram of MOS related processes from the source of oil discharge to the fate of hydrocarbons in sediments. (A) Shows the release of oil at the wellhead and application of dispersants and (B) represents rising oil droplets and gas bubbles and the formation of a deep oil plume. (C1-C4) Shows surface processes influencing the formation of MOS: (C1) illustrates wind impacts, a diatom bloom, and application of surface dispersants, (C2) shows oil transformation due to UV light and evaporation, (C3) depicts the role of aerosols and oil burning in creating new material sources, and (C4) shows processes impacting sinking MOS particles in surface waters and as particles sink through (D) a benthic nepheloid layer and deep oil plumes. (E) Shows benthic sedimentation of MOS and flocculation onto corals, and (F) represents resuspension of oiled sediments due to turbulence. See the text for amore detailed explanation of the figure.

Figure 5. Diagram illustrates the environmental gradients of material properties and fluxes associated with a point source of oil released in regions influenced by river outflow compared to offshore regions not influenced by riverine processes. Gradient shifts include the concentration and composition of suspended particles (clays to carbonate), the magnitude of particulate organic carbon (POC) and petrochemical fluxes to the seafloor, the depth of the sediment redoxcline, and the tolerance of benthic organisms, such as foraminifera, to different oxygen levels in sediments. Oil-mineral aggregations (OMAs) may sediment separately or in association with marine oil-snow (MOS). These environmental gradients overlap and interact with gradients generated by oil spills, e.g. oil and dispersant distributions, causing a complex temporal-spatial distribution of interactive effects.

Figure 6. Conceptual model illustrating the complexity of ecosystem response to a disturbance, such as an oil spill. In order to understand the impact and system response, information is needed at all levels from genes to species, populations, communities, and ecosystems. The green arrows depict ecosystem impacts flowing down to impact genetic adaptation and red arrows show impacts of genetic variability on changes at the species, population, and community levels. Impacts may vary for different parts of an ecosystem and can be positive, negative, or no observable change.