205
It is uncertain how the presence of oil impacted zooplankton’s role in the formation and alteration of marine snow. Zooplankton contribute to the formation of
marine snow through the release of fecal pellets, crustacean molts, dead bodies, and
feeding structures, such as larvaceans’ houses, and may break up or feed on marine
snow as it sinks through the water column (Alldredge and Silver 1988). Zooplankton
grazing may fragment marine snow, breaking it into smaller particles with lower
sinking velocities (Dilling and Alldredge 2000). Some zooplankton taxa (e.g., dinoflagellates, gelatinous doliolids, copepods) ingest oil and egest oil compounds
within fecal pellets, which sink rapidly to the seafloor (Lee et al. 2012b; Almeda
et al. 2014a, c). In addition, oil may adhere to zooplankton and be passively absorbed
or ingested, thus contributing to bioaccumulation of PAHs to upper trophic-level
animals (Mitra et al. 2012). Carbon isotopic depletion in suspended particulate matter and zooplankton indicated that oil carbon was also incorporated into the lower
trophic food web through biodegradation by bacteria (Graham et al. 2010; Chanton
et al. 2012). Oil and dispersants may have lethal and sublethal effects on some zooplankton (Lee et al. 1985; Almeda et al. 2014b; Buskey et al. 2016), while other
zooplankton, such as Noctiluca spp. (heterotrophic dinoflagellates), may increase in
abundance after oil spills (Févre 1979) and ingest oil, depending on the oil concentration (Almeda et al. 2014a). Indeed, relatively high abundances of Noctiluca spp.
were observed during the DwH spill in the water column and on MOS particles
(Remsen et al. 2015).
12.5 MOS: Sedimentation and Flocculent Accumulation
Gravitational settling removes marine snow and MOS from the upper ocean and transports them to depth. Sinking velocities are generally described as a function of aggregate size, excess density of component particles, and aggregate porosity. The relatively
large size of marine snow (>500 μm) thus makes marine snow and MOS important
vehicles for vertical flux (De La Rocha and Passow 2007; Ploug et al. 2008).
Measurements of laboratory-made MOS and marine snow indicate that sinking velocities of MOS do not appear lower than of similarly sized marine snow. Possibly the oil
droplets allow for tighter packaging, so that an increased number of dense particles
compensate for the low density of the oil in MOS (Passow et al. 2019).
During and for months after the DwH spill, sedimentation of MOS was likely
spatially and temporally very variable, as different types of marine snow formed and
incorporated oil compounds during their formation or sinking (Passow and Hetland
2016). Flux attenuation due to grazing, bacterial degradation, and physical fragmentation was likely high, and only a small fraction of the MOS (~10%) sinking out
of the euphotic zone would have reached the seafloor at >1000 m. Once at the seafloor, horizontal redistribution, especially downhill, resuspension, and the associated enhanced degradation of oil compounds and snow would have changed the
composition and drastically reduced the amount of oil compounds found weeks to
months after the spill. Estimates of sedimented oil based on sediment cores are thus
12 Marine Oil Snow Sedimentation and Flocculent Accumulation (MOSSFA) Events…
It is uncertain how the presence of oil impacted zooplankton’s role in the formation and alteration of marine snow. Zooplankton contribute to the formation of
marine snow through the release of fecal pellets, crustacean molts, dead bodies, and
feeding structures, such as larvaceans’ houses, and may break up or feed on marine
snow as it sinks through the water column (Alldredge and Silver 1988). Zooplankton
grazing may fragment marine snow, breaking it into smaller particles with lower
sinking velocities (Dilling and Alldredge 2000). Some zooplankton taxa (e.g., dinoflagellates, gelatinous doliolids, copepods) ingest oil and egest oil compounds
within fecal pellets, which sink rapidly to the seafloor (Lee et al. 2012b; Almeda
et al. 2014a, c). In addition, oil may adhere to zooplankton and be passively absorbed
or ingested, thus contributing to bioaccumulation of PAHs to upper trophic-level
animals (Mitra et al. 2012). Carbon isotopic depletion in suspended particulate matter and zooplankton indicated that oil carbon was also incorporated into the lower
trophic food web through biodegradation by bacteria (Graham et al. 2010; Chanton
et al. 2012). Oil and dispersants may have lethal and sublethal effects on some zooplankton (Lee et al. 1985; Almeda et al. 2014b; Buskey et al. 2016), while other
zooplankton, such as Noctiluca spp. (heterotrophic dinoflagellates), may increase in
abundance after oil spills (Févre 1979) and ingest oil, depending on the oil concentration (Almeda et al. 2014a). Indeed, relatively high abundances of Noctiluca spp.
were observed during the DwH spill in the water column and on MOS particles
(Remsen et al. 2015).
12.5 MOS: Sedimentation and Flocculent Accumulation
Gravitational settling removes marine snow and MOS from the upper ocean and transports them to depth. Sinking velocities are generally described as a function of aggregate size, excess density of component particles, and aggregate porosity. The relatively
large size of marine snow (>500 μm) thus makes marine snow and MOS important
vehicles for vertical flux (De La Rocha and Passow 2007; Ploug et al. 2008).
Measurements of laboratory-made MOS and marine snow indicate that sinking velocities of MOS do not appear lower than of similarly sized marine snow. Possibly the oil
droplets allow for tighter packaging, so that an increased number of dense particles
compensate for the low density of the oil in MOS (Passow et al. 2019).
During and for months after the DwH spill, sedimentation of MOS was likely
spatially and temporally very variable, as different types of marine snow formed and
incorporated oil compounds during their formation or sinking (Passow and Hetland
2016). Flux attenuation due to grazing, bacterial degradation, and physical fragmentation was likely high, and only a small fraction of the MOS (~10%) sinking out
of the euphotic zone would have reached the seafloor at >1000 m. Once at the seafloor, horizontal redistribution, especially downhill, resuspension, and the associated enhanced degradation of oil compounds and snow would have changed the
composition and drastically reduced the amount of oil compounds found weeks to
months after the spill. Estimates of sedimented oil based on sediment cores are thus
12 Marine Oil Snow Sedimentation and Flocculent Accumulation (MOSSFA) Events…