9.3.3: Export from the Upper Ocean to the Depths and Sediments

The gravitational flux of OC occurs in the form of various particles of planktonic origin sinking from the surface ocean due to their large size and/or high density. The gravitational pump includes particles that are single phytoplankton cells, various types of aggregates, and zooplankton dejections termed fecal pellets.

Fecal Pellets Flux

Specific OC flux associated with zooplankton fecal pellets is relatively easy to measure. Fecal pellets are compact and have distinctive shapes and thus can easily identified and picked out from samples collected using different types of sediment traps (Refaeli et al., 2008; Turner, 2015). However, some pellets can experience degradation in moored sediment traps cups and are not being counted as such (Wilson et al., 2008). This may result in underestimation of the flux of fecal pellets.

Phytoplankton Cells Flux

Turbulent mixing and buoyancy regulation tend to retain single phytoplankton single cells tend in the surface ocean. However, despite of their small Reynolds number [describes the nature of the surrounding flow and its fall velocity (Stokes, 1851)], some intact cells (not grazed or aggregated) are found in sediment traps. This can happen to single live or dead phytoplankton cells, resting spores or cysts, and colonies.

Phytoplankton cells sink into the ocean’s interior as single cells which are live, senescent and/or dead. DiTullio et al. (2000) observed single cells of Phaeocystis antarctica in the lower mesopelagic zone (500–600 m) in the Ross sea and concluded that they were exported rapidly and alive following a large Phaeocystis antarctica bloom. However, the mechanism delivering these cells down to the mesopelagic zone is unclear. Although physical processes could entrain surface cells to depth (see following sections), the cells observed at 500 m were still photosynthesizing, which suggests that their export to depth was likely too rapid to be induced by a physical process. Phytoplankton cells also sink dead. For example (Salter et al., 2007), observed dead cells of large diatoms Eucampia antarctica being selectively exported relative to the surface phytoplankton community in the Southern Ocean. These diatoms were exported rapidly owing to their heavily silicified, and thus dense, frustule (Salter et al., 2007). Phytoplankton resting spores and/or cysts are also exported from the surface ocean down to bathypelagic/abyssal depths (>1000 m). They are formed strategically by some phytoplankton in response to nutrient limitation and contain significant amounts of OC. In sediment traps located within an iron-fertilized bloom in the Southern Ocean near the Kerguelen Islands, Rembauville et al. (2016) measured OC fluxes associated with resting spores of diatoms Chaetoceros Hyalochaete or Thalassiosira Antarctica. The resting spores of a single species can account for up to 90% of the total gravitational OC flux (Rembauville et al., 2016). Acantharian cysts (unicellular organisms related to Radiolaria producing celestite, the densest biomineral in the ocean) were abundant in deep sediment trap material in the Iceland Basin (Martin et al., 2010) and the Southern Ocean (Belcher et al., 2018). High cyst flux was restricted to only 2 weeks but contributed up to half of the total gravitational OC flux during this period (Martin et al., 2010; Decelle et al., 2013). The Acantarian cysts are made of dense celestite which ensures their fast sedimentation to the bathypelagic depths. Phytoplankton cell colonies and chains are also observed in sediment traps samples. Laurenceau-Cornec et al. (2015) found chains of the pennate diatoms Fragilariopsis sp. in the Southern Ocean, while Pabortsava et al. (2017) collected intact colonies of nitrogen-fixing Trichodesmium species in the deep traps (3000 m) collecting in the North Atlantic gyre.

Aggregate Flux

Evaluating OC flux associated with aggregates is challenging as their structure is often fragile and erratic (Alldredge, 1998). In moored sediment traps, marine aggregates lose their shapes. Unlike fecal pellets (see section above), aggregates in the bulk sediment trap material are clumped and thus difficult to pick out individually.

The Mixing Flux

Several ocean physical processes transport slow-sinking, neutrally buoyant or dissolved OC (DOC) from the surface where they are produced to the mesopelagic zone. Recently, “mixing fluxes” gained more importance (see following references). This is mainly due the development of semi-automated platforms and devices which enabled high frequency sizing of particles (Picheral et al., 2010; Claustre et al., 2011). Large-scale ocean circulation is also one of the processes transporting particles produced in the surface to ocean’s interior. For instance, regions of deep water formation are hotspots for such transport (Levy et al., 2013).

The mixed layer pump is another process that redistributes dissolved OC, neutrally buoyant or slow-sinking organic matter down the water column (Dall’Olmo et al., 2016). This process occurs during transient stratifications at the start of the productive season when suspended POC first accumulates in surface, and then redistributed throughout the water column by deep mixing. Globally, the mixed layer pump is responsible for a flux of 0.5 Gt C yr^–1. This process supplies POC mainly to locations with deep winter mixing such as the North Atlantic Ocean or the Southern Ocean (Dall’Olmo et al., 2016).

The water mass subduction induced by eddies also pumps POC into the mesopelagic zone. Omand et al. (2015) quantified the amount of suspended POC sinking following eddy driven subduction in the North Atlantic. In essence, suspended POC produced in surface sink along tilted isopycnals. Globally, eddy-driven subduction of POC is responsible for an export flux of ∼1 Gt C yr^–1 (Omand et al., 2015). Altogether, these processes may export up to 2 Gt of POC yr^–1, nearly a third of the gravitational flux (Boyd et al., 2019).

Large-scale circulation, mixing and eddies subduction also transport DOC beneath the surface ocean. Examining large scale variability of DOC concentrations, Hansell et al. (2009) estimated that DOC export represents 20% that of gravitational POC export flux in the global ocean. However, the amount of DOC transported by processes such as eddy driven subduction and mixed layer pump (see references above) have poor spatial coverage both on local and global scales. Using an artificial neural network, Roshan and DeVries (2017) reconstructed global DOC concentrations distribution. Here, DOC production and export fluxes were estimated by coupling reconstructed DOC concentrations to a global ocean circulation model (DeVries and Primeau, 2011). Large annual DOC export rates were shown in the tropics (50–80 mg of OC m^–2 d^–1) and low in high latitudes (0–40 mg of OC m^–2 d^–1), contrary to findings by Dall’Olmo et al. (2016) and Omand et al. (2015). Their model, however, extrapolated DOC concentrations measured during summer to a global scale. This may have limited the effect of the aforementioned physical processes. More work is required to estimate the annual flux of DOC from the surface ocean.

The Active Migration Flux

The concept of active flux by migrating organisms (also known as “swimmers”) in the mesopelagic is based on widespread observations of DVM of zooplankton and fish caught in nets (Longhurst, 1991; Steinberg, 2000; Landry and Calbet, 2004). In essence, to avoid predation swimmers feed at night in surface waters and defecate deeper in the mesopelagic during the day (Steinberg and Landry, 2017). Inverted DVM and desynchronized vertical migration do exist (Neill, 1990; Cohen and Forward, 2016), however, typical DVM as described above are more common in marine systems. This actively supplies OC to depth with minimizing decomposition. To date, estimates of DVM have mostly been focused on mesozooplankton because they are readily caught in multiple opening/closing towed nets (Tarling et al., 2002). DVM is a difficult process to model (Hansen and Visser, 2016), however, this field has made significant progresses recently (Aumont et al., 2018; Archibald et al., 2019; Gorgues et al., 2019).

The active transport of OC by migrating mesozooplankton can contribute up to 40% of the gravitational flux (Stukel et al., 2013) with global estimates ranging from 0.2 to 71 mg m^–2 d^–1 (Aumont et al., 2018). Most studies of mesozooplankton associated OC flux were conducted in coastal and upwelling regions, while data from high latitudes and polar waters are scarce. The largest flux is recorded in the Benguela upwelling system while the smallest are observed near the Antarctic peninsula. Large DVM fluxes in upwelling system is counterintuitive given that these waters are productive but often poorly oxygenated below surface. However, some migrating organisms can survive with little oxygen concentrations by adopting adaptive metabolic strategies (Bianchi et al., 2013, 2014; Kiko et al., 2015). Excluding the extreme values of 0.2 and 71 mg m^–2 d^–1, the OC fluxes associated with migratory organisms appear to be relatively homogeneous and in the order of 9.8 ± 7.3 mg C m^–2 d^–1.

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