9.3.2: Biological Pump

The oceanic biological carbon pump (BCP) is a complex mechanism regulating the Earth carbon (C) cycle by sequestrating part of the photosynthetically fixed CO₂ into the deep ocean and the seafloor. Suppressing this mechanism would increase the current levels of atmospheric CO₂ level by about 50% (Sarmiento et al., 2004; Parekh et al., 2006). Currently, the variability in the proportion of surface primary production (PP) leaving the euphotic zone and the amount of organic carbon (OC) exported to a certain depth via the BCP are challenging to explain. This prevents robust predictions of how C cycle, and hence climate, will change in the future.

Three major forms of the BCP are distinguished: (i) gravitational, (ii) mixing and (iii) migration. The gravitational pump (i) involves downward sinking of the OC contained in various particles (POC) of planktonic origin from the surface ocean to depth due to their large size and/or high density (Siegel et al., 2016). This sinking POC is termed “marine snow” and can be made of single phytoplankton cells (DiTullio et al., 2000; Rembauville et al., 2016; Leblanc et al., 2018), various types of aggregates resulting from coagulation processes (Alldredge and Jackson, 1995; Riley et al., 2012; Laurenceau-Cornec et al., 2015) and zooplankton fecal pellets (FP) (Turner, 2002; Cavan et al., 2015). The mixing pump (ii) occurs when neutrally buoyant OC is transported to depth by downward-moving water masses and/or mixing. This OC is either dissolved (DOC) or entrained in particles which are too small in size or positively buoyant relative to seawater to sink on their own (Hansell et al., 2009; Le Moigne et al., 2016). The physical mechanisms that transport the OC downward can be either diapycnal with DOC diffusing out of the surface waters, or isopycnal, when water mass movements eddies and/or mixed layer bring DOC and/or suspended particles to depth (Hansell et al., 2009; Omand et al., 2015; Dall’Olmo et al., 2016). (iii) Diurnal vertical migration (DVM) of zooplankton and fish is a common feature in oceanic ecosystems (Longhurst, 1991; Steinberg, 2000; Landry and Calbet, 2004). In order to avoid predation, swimmers feed at night in surface waters and defecate deeper in the mesopelagic during the day. Doing so, they actively transport OC into the ocean’s interior.

The presence, absence, and dominance of each of these different export pathways are controlled by biological (Piontek et al., 2014; Agusti et al., 2015; Le Moigne et al., 2016), chemical (Edwards et al., 2015), ecological (Cavan et al., 2015; Guidi et al., 2016), and physical (Levy et al., 2013; Taucher et al., 2014; Dall’Olmo et al., 2016) processes. They all vary in time and space and at depth. Most of these processes, however, are still poorly understood which hinders mechanistic predictions of the magnitude of the downward OC fluxes. At the base of the euphotic zone, the sinking of particles (termed gravitational flux) dominate the amount of OC (Giering et al., 2014). However, in the end of winter, the OC flux is induced by the so-called mixed layer pump which is driven by the seasonal entrainment of surface waters in deeper layers (Gardner et al., 1995) and can supply a substantial amount of OC to the deep ocean. Also, while globally the proportion of OC transported by migrating zooplankton and larger organisms relative to total OC flux is <20%, this active flux can occasionally dominate the total OC flux [(Aumont et al., 2018) and references therein].

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Excerpted from:

Le Moigne, F. A. (2019). Pathways of organic carbon downward transport by the oceanic biological carbon pump. Frontiers in Marine Science, 6, 634.