The changing carbon cycle of the coastal ocean

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The carbon cycle of the coastal ocean is a dynamic component of the global carbon budget. But the diverse sources and sinks of carbon and their complex interactions in these waters remain poorly understood. Here we discuss the sources, exchanges and fates of carbon in the coastal ocean and how anthropogenic activities have altered the carbon cycle. Recent evidence suggests that the coastal ocean may have become a net sink for atmospheric carbon dioxide during post-industrial times. Continued human pressures in coastal zones will probably have an important impact on the future evolution of the coastal ocean's carbon budget.

At a glance


  1. Processes that affect organic and inorganic carbon cycling and fluxes in the major coastal ocean subsystems.
    Figure 1: Processes that affect organic and inorganic carbon cycling and fluxes in the major coastal ocean subsystems.

    a, Natural and anthropogenic processes altering riverine carbon inputs to the coastal ocean. Inputs can be altered through changes in the water balance (precipitation and evapotranspiration) and carbon stocks and flows in watersheds. Hydrological alterations include the effects of climate change on the amount and frequency of rainfall events and temperature regulation of evapotranspiration. Land management practices such as irrigation and clearance of vegetation that alter rates of evapotranspiration can also be important. Recent studies have indicated that inputs of sulphuric acid, agricultural practices, peatland disturbance, permafrost thaw, wetland removal and reservoir construction can alter carbon stocks (biogeochemical response) and flows through varied mechanisms at the drainage-network level. Carbon flux is equal to carbon concentration (the hydrological and biogeochemical response) multiplied by discharge (precipitation minus evapotranspiration). b, Major processes affecting carbon sources and fluxes in estuaries. Estuaries contain a mixture of organic and inorganic carbon sources derived from terrestrial materials carried by fresh river water (in which the salinity is zero), marine sources carried in shelf sea water (with a salinity of more than or equal to 30), and uniquely estuarine materials. Organic carbon is lost owing to salinity-induced flocculation, sedimentation, microbial respiration and photooxidation. Estuaries can modulate the export of carbon to the shelf depending on whether the estuary is a net carbon source or carbon sink. c, A representative continental shelf at its interface with a low-salinity river or estuarine plume. Physical and biogeochemical processes control the source, transport and fate of organic carbon. Carbon is exchanged at the interface between plume and shelf waters through sorption and desorption. Organic carbon transport to the open ocean is supplemented by physical resuspension, bioturbation and mobile and fluidized mud layers. The benthic nepheloid layer contains significant amounts of suspended sediment, which may be deposited to and resuspended from depocentres. Primary production in inner shelf waters may be limited by high sediment loads in plumes, whereas regions of upwelling in outer shelf waters can lead to elevated primary production. DOC, dissolved organic carbon; POC, particulate organic carbon; DIC, dissolved inorganic carbon.

  2. Organic and inorganic carbon fluxes in the estuarine, tidal wetland and continental shelf subsystems of the coastal ocean.
    Figure 2: Organic and inorganic carbon fluxes in the estuarine, tidal wetland and continental shelf subsystems of the coastal ocean.

    Fluxes between adjacent subsystems and other components of the earth system are regulated by a number of processes (the major ones are shown here). Carbon can flux both within (values in black) and across (values in red) the boundaries of the coastal ocean. All organic carbon (OC) and inorganic carbon (IC) fluxes are presented as positive values, arrows indicate direction of flux. Particulate and dissolved OC fluxes are presented as total OC values. The balance between gross primary production (GPP) and total system respiration (both autotrophic, A, and heterotrophic, H; RAH) is net ecosystem production (NEP), with negative values indicating conversion of OC to IC. The IC burial flux takes into consideration calcification. The methods used to estimate flux values and their associated uncertainties are described in Box 1. Typical uncertainties for carbon fluxes: *95% certainty that the estimate is within 50% of the reported value; 95% certainty that the estimate is within 100% of the reported value; uncertainty greater than 100%. Units are Pg C yr−1 (1 Pg = 1015 g) rounded to ± 0.05 Pg C yr−1. Within-river fluxes and transformation of carbon are excluded from this analysis.

  3. Air-surface water CO2 exchange fluxes of different aquatic systems.
    Figure 3: Air–surface water CO2 exchange fluxes of different aquatic systems.

    a, The global areas of major aquatic systems. b, The CO2 flux and global areas of river deltas and embayments (estuary type I and II in refs 33 and 40), coastal lagoons (type III), and fjords and fjards (type IV) (note that large river plumes extending onto and across continental shelves are not included, although it is known that they are a CO2 sink for the atmosphere). c, Carbon flux and global areas of continental shelves. Continental shelves freely exchange with the open ocean in low latitudes (0–30°), temperate or mid-latitudes (30–60°) and polar or high latitudes (60–90°), upwelling systems, and enclosed shelf seas. We assigned an uncertainty of <50% to each flux term except inland waters, fjords and fjards, enclosed seas and low latitude open shelves, which have an assigned uncertainty of 50–100% because of the very sparse data coverage. Flux values and their uncertainties were derived according to the methods in Box 1.

  4. pCO2 levels, net ecosystem production and organic and inorganic carbon fluxes in pre-industrial and current continental shelves.
    Figure 4: pCO2 levels, net ecosystem production and organic and inorganic carbon fluxes in pre-industrial and current continental shelves.

    According to this model, a, the entire pre-industrial continental shelf was a source of CO2 to the atmosphere, but b, it became a CO2 sink at some point in the latter twentieth century owing to increased atmospheric pCO2. We further suggest that together with the current known CO2 uptake from the atmosphere, increased shelf export of dissolved inorganic carbon (DIC) would lead to an increased DIC inventory in the open ocean. Organic carbon, OC; net ecosystem production, NEP; CO2 flux, FCO2. All carbon fluxes including NEP have units of Pg C yr−1. Flux estimates were derived as described in Box 1. We assign an uncertainty of 100% for the pre-industrial air–sea CO2 flux, larger than the current CO2 flux uncertainty of 50–75%. *95% certainty that the estimate is within 50% of the reported value; 95% certainty that the estimate is within 100% of the reported value; uncertainty greater than 100% (Box 1). The sea surface average pCO2 value and its associated current or pre-industrial uncertainties were back-calculated from the assigned air–sea CO2 flux, known atmospheric pCO2 value, shelf area and average gas-exchange parameter2, as described in Box 1.


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  1. Aquatic Biogeochemistry Laboratory, Department of Evolution, Ecology and Organismal Biology, The Ohio State University, Columbus, Ohio 43210, USA.

    • James E. Bauer
  2. School of Marine Science and Policy, University of Delaware, Newark, Delaware 19716, USA.

    • Wei-Jun Cai
  3. School of Forestry and Environmental Studies, Yale University, New Haven, Connecticut 06511, USA.

    • Peter A. Raymond
  4. Department of Geological Sciences, University of Florida, Gainesville, Florida 32611, USA.

    • Thomas S. Bianchi
  5. Department of Marine Science, University of Georgia, Athens, Georgia 30602, USA.

    • Charles S. Hopkinson
  6. Department of Earth & Environmental Sciences, Université Libre de Bruxelles, Brussels 1050, Belgium.

    • Pierre A. G. Regnier

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