Abstract
The vast majority of freshly produced oceanic dissolved organic carbon (DOC) is derived from marine phytoplankton, then rapidly recycled by heterotrophic microbes. A small fraction of this DOC survives long enough to be routed to the interior ocean, which houses the largest and oldest DOC reservoir. DOC reactivity depends upon its intrinsic chemical composition and extrinsic environmental conditions. Therefore, recalcitrance is an emergent property of DOC that is analytically difficult to constrain. New isotopic techniques that track the flow of carbon through individual organic molecules show promise in unveiling specific biosynthetic or degradation pathways that control the metabolic turnover of DOC and its accumulation in the deep ocean. However, a multivariate approach is required to constrain current carbon fluxes so that we may better predict how the cycling of oceanic DOC will be altered with continued climate change. Ocean warming, acidification, and oxygen depletion may upset the balance between the primary production and heterotrophic reworking of DOC, thus modifying the amount and/or composition of recalcitrant DOC. Climate change and anthropogenic activities may enhance mobilization of terrestrial DOC and/or stimulate DOC production in coastal waters, but it is unclear how this would affect the flux of DOC to the open ocean. Here, we assess current knowledge on the oceanic DOC cycle and identify research gaps that must be addressed to successfully implement its use in global scale carbon models.
Highlights
Containing roughly the same amount of carbon as terrestrial biomass, or CO2 in the atmosphere, the oceanic dissolved organic matter (DOM) pool represents one of Earth’s large organic carbon reservoirs (660 Pg-C; Hansell et al, 2009; Houghton, 2014)
The total dissolved organic carbon (DOC) flux from external sources is similar to the export of DOC to the mesopelagic and an order of magnitude greater than the export of DOC to the interior ocean
DOC exported to the ocean could impact the sum of DOC transferred from the surface to the interior ocean each year
Summary
Containing roughly the same amount of carbon as terrestrial biomass, or CO2 in the atmosphere, the oceanic dissolved organic matter (DOM) pool represents one of Earth’s large organic carbon reservoirs (660 Pg-C; Hansell et al, 2009; Houghton, 2014). Terrestrial and marine organic matter that is temporarily stored in coastal and continental margin sediments is laterally transported to deliver an additional 0.12–0.23 Pg-C yr−1 as DOC to open oceanic surface waters (Burdige et al, 1999; Burdige and Komada, 2015).
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