207
during August 2010 (Hu et al. 2011). A sediment trap revealed that high particle flux
rates relative to other years were due to the sinking of a large Skeletonema sp. bloom
(Yan et al. 2016), a cosmopolitan diatom that thrives under brackish conditions and
is tolerant of the presence of oil. In addition to the MOSSFA data, consideration
should also be given to the transport of OMA given the high concentration of suspended sediments in the coastal waters of the Gulf of Mexico due to sediment transport from the Mississippi River and lower than expected levels of oil reaching shore
relative to the volume spilled. In fact, after the DwH spill, sinking MOS was rich in
lithogenic material (Yan et al. 2016), which likely played a significant role in the
formation and sinking (ballasting) of MOS (Brooks et al. 2015).
Another unique feature of the DwH oil spill was the presence of persistent subsurface oil plumes that occurred primarily between 1000 and 1400 m depth (Camilli
et al. 2010; Diercks et al. 2010) (see Fig. 12.2). Marine snow, which formed at the
surface and sedimented to the seafloor, likely interacted with rising oil droplets and/
or the oil plumes at depth (Valentine et al. 2014). Surface bacteria and phytoplanktonaffiliated gene sequences were found in sediments at and below the subsurface
plume (Mason et al. 2014), validating the notion that surface-formed MOS sank to
the seafloor. MOS also may have formed in the deepwater plumes, as many bacteria
were active in those regions (Hazen et al. 2010); experiments using bacteria from
the deep plume support this finding (Baelum et al. 2012; Kleindienst et al. 2015).
Cumulative evidence suggests that the large DwH MOSSFA episode occurred
due to a nexus of events: (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 densities
were at a maximum and surface productivity rates were relatively high, and (3) there
was greater microbial mucus formation, especially in the presence of weathered oil.
12.6 MOSSFA: Unique to the Deepwater Horizon Oil Spill?
Preservation of the DwH MOSSFA event in sediments and its long-term impact on
the postdepositional chemical environment and the benthic ecosystem have been
well documented (Brooks et al. 2015; Chanton et al. 2015; Romero et al. 2015,
2017; Schwing et al. 2015, 2017, 2018; Hastings et al. 2015). Vonk et al. (2015)
performed a meta-analysis of 52 large, historical oil spills in an effort to investigate
whether MOSSFA occurred and to identify the main drivers of oil sedimentation.
Similar MOSSFA events were reported for the Tsesis, Ixtoc 1, and other oil spills
(Johansson et al. 1980; Boehm and Fiest 1980; Jernelöv and Lindén 1981; Patton
et al. 1981; Teal and Howarth 1984), and possibly even in the Santa Barbara spill in
1969 (Doyle et al. 2018), but these were not reported because sedimentation of oil
was not expected and the seafloor was not monitored. The formation of MOSSFA in
these cases is expected due to the presence of particulate matter, the proximity to
river outflow (i.e., clay minerals and phytoplankton biomass), and the presence of
EPS-producing biota (phytoplankton and oil-degrading bacteria; Vonk et al. 2015).
12 Marine Oil Snow Sedimentation and Flocculent Accumulation (MOSSFA) Events…
during August 2010 (Hu et al. 2011). A sediment trap revealed that high particle flux
rates relative to other years were due to the sinking of a large Skeletonema sp. bloom
(Yan et al. 2016), a cosmopolitan diatom that thrives under brackish conditions and
is tolerant of the presence of oil. In addition to the MOSSFA data, consideration
should also be given to the transport of OMA given the high concentration of suspended sediments in the coastal waters of the Gulf of Mexico due to sediment transport from the Mississippi River and lower than expected levels of oil reaching shore
relative to the volume spilled. In fact, after the DwH spill, sinking MOS was rich in
lithogenic material (Yan et al. 2016), which likely played a significant role in the
formation and sinking (ballasting) of MOS (Brooks et al. 2015).
Another unique feature of the DwH oil spill was the presence of persistent subsurface oil plumes that occurred primarily between 1000 and 1400 m depth (Camilli
et al. 2010; Diercks et al. 2010) (see Fig. 12.2). Marine snow, which formed at the
surface and sedimented to the seafloor, likely interacted with rising oil droplets and/
or the oil plumes at depth (Valentine et al. 2014). Surface bacteria and phytoplanktonaffiliated gene sequences were found in sediments at and below the subsurface
plume (Mason et al. 2014), validating the notion that surface-formed MOS sank to
the seafloor. MOS also may have formed in the deepwater plumes, as many bacteria
were active in those regions (Hazen et al. 2010); experiments using bacteria from
the deep plume support this finding (Baelum et al. 2012; Kleindienst et al. 2015).
Cumulative evidence suggests that the large DwH MOSSFA episode occurred
due to a nexus of events: (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 densities
were at a maximum and surface productivity rates were relatively high, and (3) there
was greater microbial mucus formation, especially in the presence of weathered oil.
12.6 MOSSFA: Unique to the Deepwater Horizon Oil Spill?
Preservation of the DwH MOSSFA event in sediments and its long-term impact on
the postdepositional chemical environment and the benthic ecosystem have been
well documented (Brooks et al. 2015; Chanton et al. 2015; Romero et al. 2015,
2017; Schwing et al. 2015, 2017, 2018; Hastings et al. 2015). Vonk et al. (2015)
performed a meta-analysis of 52 large, historical oil spills in an effort to investigate
whether MOSSFA occurred and to identify the main drivers of oil sedimentation.
Similar MOSSFA events were reported for the Tsesis, Ixtoc 1, and other oil spills
(Johansson et al. 1980; Boehm and Fiest 1980; Jernelöv and Lindén 1981; Patton
et al. 1981; Teal and Howarth 1984), and possibly even in the Santa Barbara spill in
1969 (Doyle et al. 2018), but these were not reported because sedimentation of oil
was not expected and the seafloor was not monitored. The formation of MOSSFA in
these cases is expected due to the presence of particulate matter, the proximity to
river outflow (i.e., clay minerals and phytoplankton biomass), and the presence of
EPS-producing biota (phytoplankton and oil-degrading bacteria; Vonk et al. 2015).
12 Marine Oil Snow Sedimentation and Flocculent Accumulation (MOSSFA) Events…