Exploring the ecology of the mesopelagic biological pump

The oceans’ biological pump (BP) exports large amounts of particulate organic carbon (POC) to the mesopelagic zone (base of the euphotic zone – 1000 m depth). The efficiency at which POC is transferred through the mesopelagic zone determines the size of the deep ocean carbon store. Few observational BP studies focus on the mesopelagic, often leading to the need to oversimplify the representation of processes within this depth horizon in numerical models. In this review, we identify and describe three interlinked biological processes that act to regulate and control the transfer efficiency of POC through the mesopelagic zone; (1) direct sinking of phytoplankton cells and aggregates, (2) zooplankton community structure and (3) the microbial loop and associated carbon pump. We reveal previously unidentified relationships between planktonic community structure and POC transfer efficiency for specific regions. We also compare mesopelagic POC remineralisation depth (a proxy for POC transfer efficiency) with the permanent thermocline in different regions. Our analysis shows that even when mesopelagic POC transfer efficiency is low, such a transfer efficiency does not necessarily mean low carbon sequestration if the permanent thermocline is shallow, and we define a carbon sequestration ratio (Cseq, the remineralisation depth divided by the permanent thermocline) to highlight this. Low latitude regions typically have a higher Cseq than temperate and polar regions, and thus could be more important in transferring carbon on long timescales than previously thought. POC transfer efficiency should be regularly discussed in the context of the physical water properties such as the permanent thermocline, to truly assess an oceanic region’s ability to sequester carbon. Improved understanding of mesopelagic ecological processes and links to surface processes will better constrain ecosystem models and improve projections of the future global carbon cycle

Marine Oil Snow Sedimentation and Flocculent Accumulation (MOSSFA) Events: Learning from the Past to Predict the Future

Despite interest as early as in the 1880s, it was not until 1953 that Tokimi Tsujita (Seikai Fisheries Research Laboratory, Japan) was able to carefully collect and describe the matrix of microorganisms embedded in suspended organic matter (Tsujita, J Oceanogr Soc Jpn 8:1–14, 1953) that today we call marine snow. Subsequent studies reported that marine snow consisted of phytoplankton, small zooplankton, fecal material, and other particles (Nishizawa et al., Bull Fac Fish, Hokkaido Univ. 5:36–40, 1954). Across the ocean, Riley (Limnol Oceanogr 8:372–381, 1963) called this material “organic aggregates” which in addition to the organic material included nonliving material that was a “substrate for bacterial growth.” More than a decade later, Silver et al. (Science 201:371–373, 1978) quantified the abundance of marine snow, and its contribution to the total community in situ, and showed that marine snow particles were “metabolic hotspots,” with concentrations of microorganisms 3–4 orders of magnitude greater than those in the surrounding seawater. Alldredge and Cohen (Science 235:689–691, 1987) emphasized the importance of marine snow as unique chemical and physical microhabitats. The importance of transparent exopolymer particles (TEP), which form the matrix that embeds the individual component particles of marine snow, were described and quantified in the early 1990s (Alldredge et al., Deep-Sea Res I 40: 1131–1140, 1993; Passow and Alldredge, Mar Ecol Prog Ser 113:185–198, 1994; Passow et al., Deep-Sea Res Oceanogr Abstr 41:335–357, 1994). The long-held belief that marine snow was both a specialized habitat and potential food source for those living in the deep ocean was also demonstrated at that time (Silver and Gowing, Prog Oceanogr 26:75–113, 1991). More recently it was confirmed that marine snow does indeed contribute significantly to the metabolism of the deep sea and provides hotspots of microbial diversity and activity at depth (e.g., Burd et al., Deep-Sea Res II 57:1557–1571, 2010; Bochdansky et al., Sci Rep 6:22633, 2016). Moreover, marine snow is now considered a transport vehicle for its biota and associated particulate matter (Volk and Hoffert, The carbon cycle and atmospheric CO: natural variations archean to present. American Geophysical Union, Washington, D.C., pp. 99–110, 1985; Alldredge and Gotschalk, Limnol Oceanogr 33:339–351, 1988). Rapidly sinking marine snow is important in the marine carbon cycle as it is responsible for vertical (re)distribution and remineralization of carbon. The transport of carbon from the surface to the deep sea is known as the “biological carbon pump” (De La Rocha and Passow, Deep Sea Res II 54:639–658, 2007; De La Rocha and Passow, Treatise on Geochemistry. Vol. 8, Elsevier, Oxford, 2014). This pump, which leads to the uptake and sequestration of atmospheric CO2 (e.g., Volk and Hoffert, The carbon cycle and atmospheric CO: natural variations archean to present. American Geophysical Union, Washington, D.C., pp. 99–110, 1985; Finkel et al., J Plankton Res 32:119–137, 2010; Zetsche and Ploug, Mar Chem 175:1–4, 2015), also plays an important role in the biogeochemical cycling of elements (e.g., Quigg et al., Nature 425:291–294, 2003; Quigg et al., Proc R Soc: Biol Sci 278:526–534, 2011). How climate change will change these processes is the subject of intense interest but beyond the scope of this chapter