Budgeting sinks and sources of CO2 in the coastal ocean: Diversity of ecosystems counts
TL;DR: In this article, the authors upscaled air-water CO2 fluxes to take into account the latitudinal and ecosystem diversity of the coastal ocean, based on an exhaustive literature survey.
Abstract: [1] Air-water CO2 fluxes were up-scaled to take into account the latitudinal and ecosystem diversity of the coastal ocean, based on an exhaustive literature survey. Marginal seas at high and temperate latitudes act as sinks of CO2 from the atmosphere, in contrast to subtropical and tropical marginal seas that act as sources of CO2 to the atmosphere. Overall, marginal seas act as a strong sink of CO2 of about −0.45 Pg C yr−1. This sink could be almost fully compensated by the emission of CO2 from the ensemble of near-shore coastal ecosystems of about 0.40 Pg C yr−1. Although this value is subject to large uncertainty, it stresses the importance of the diversity of ecosystems, in particular near-shore systems, when integrating CO2 fluxes at global scale in the coastal ocean.
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TL;DR: In this article, the authors combine geophysics, microbial ecology and organic geochemistry to show geophysical opportunity and microbial capacity to enhance the net heterotrophy in streams, rivers and estuaries.
Abstract: Rivers may be efficient environments for metabolizing terrestrial organic carbon that was previously thought to be recalcitrant, owing to pockets that provide geophysical opportunities by retaining material for longer, and to the adaptation of microbial communities, which has enabled them to exploit the energy that escapes upstream ecosystems. Metabolism of terrestrial organic carbon in freshwater ecosystems is responsible for a large amount of carbon dioxide outgassing to the atmosphere, in contradiction to the conventional wisdom that terrestrial organic carbon is recalcitrant and contributes little to the support of aquatic metabolism. Here, we combine recent findings from geophysics, microbial ecology and organic geochemistry to show geophysical opportunity and microbial capacity to enhance the net heterotrophy in streams, rivers and estuaries. We identify hydrological storage and retention zones that extend the residence time of organic carbon during downstream transport as geophysical opportunities for microorganisms to develop as attached biofilms or suspended aggregates, and to metabolize organic carbon for energy and growth. We consider fluvial networks as meta-ecosystems to include the acclimation of microbial communities in downstream ecosystems that enable them to exploit energy that escapes from upstream ecosystems, thereby increasing the overall energy utilization at the network level.
1,246 citations
Cites background from "Budgeting sinks and sources of CO2 ..."
...Estuarine respiration and notably net heterotrophy decline (that is, NEP increases) relative to streams and rivers, which puts the role of estuaries in terrestrial carbon cyclin...
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TL;DR: The sources, exchanges and fates of carbon in the coastal ocean and how anthropogenic activities have altered the carbon cycle are discussed.
Abstract: 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.
1,091 citations
Université libre de Bruxelles1, University of Exeter2, ETH Zurich3, University of Antwerp4, University of Hamburg5, University of California, San Diego6, University of Bristol7, University of North Carolina at Chapel Hill8, University of Liège9, Centre national de la recherche scientifique10, Geophysical Institute, University of Bergen11, Bjerknes Centre for Climate Research12, Max Planck Society13, University of Bern14, University of Southern California15, Vrije Universiteit Brussel16, Yale University17, University of East Anglia18, Helmholtz Centre for Environmental Research - UFZ19
TL;DR: This article showed that anthropogenic perturbation may have increased the flux of carbon to inland waters by as much as 1.0 Pg C yr−1 since pre-industrial times, mainly owing to enhanced carbon export from soils.
Abstract: A substantial amount of the atmospheric carbon taken up on land through photosynthesis and chemical weathering is transported laterally along the aquatic continuum from upland terrestrial ecosystems to the ocean. So far, global carbon budget estimates have implicitly assumed that the transformation and lateral transport of carbon along this aquatic continuum has remained unchanged since pre-industrial times. A synthesis of published work reveals the magnitude of present-day lateral carbon fluxes from land to ocean, and the extent to which human activities have altered these fluxes. We show that anthropogenic perturbation may have increased the flux of carbon to inland waters by as much as 1.0 Pg C yr−1 since pre-industrial times, mainly owing to enhanced carbon export from soils. Most of this additional carbon input to upstream rivers is either emitted back to the atmosphere as carbon dioxide (~0.4 Pg C yr−1) or sequestered in sediments (~0.5 Pg C yr−1) along the continuum of freshwater bodies, estuaries and coastal waters, leaving only a perturbation carbon input of ~0.1 Pg C yr−1 to the open ocean. According to our analysis, terrestrial ecosystems store ~0.9 Pg C yr−1 at present, which is in agreement with results from forest inventories but significantly differs from the figure of 1.5 Pg C yr−1 previously estimated when ignoring changes in lateral carbon fluxes. We suggest that carbon fluxes along the land–ocean aquatic continuum need to be included in global carbon dioxide budgets.
948 citations
TL;DR: It is demonstrated here that CO2 release in estuaries is largely supported by microbial decomposition of highly productive intertidal marsh biomass, thus leading to more dissolved inorganic carbon export to the ocean.
Abstract: Estuaries are a major boundary in the land-ocean interaction zone where organic carbon (OC) and nutrients are being processed, resulting in a high water-to-air carbon dioxide (CO2) flux (approximately 0.25 Pg C y(-1)). The continental shelves, however, take up CO2 (approximately 0.25 Pg C y(-1)) from the atmosphere, accounting for approximately 17% of open ocean CO2 uptake (1.5 Pg Cy(-1)). It is demonstrated here that CO2 release in estuaries is largely supported by microbial decomposition of highly productive intertidal marsh biomass. It appears that riverine OC, however, would bypass the estuarine zone, because of short river-transit times, and contribute to carbon cycling in the ocean margins and interiors. Low-latitude ocean margins release CO2 because they receive two-thirds of the terrestrial OC. Because of recent CO2 increase in the atmosphere, CO2 releases from low latitudes have become weaker and CO2 uptake by mid- and high-latitude shelves has become stronger, thus leading to more dissolved inorganic carbon export to the ocean.
682 citations
Cites background or methods from "Budgeting sinks and sources of CO2 ..."
...Borges et al. (2005) calculated that the average CO2 efflux from low-latitude (0–30◦) and midlatitude (30–60◦) estuaries is, respectively, 17 and 46 mol m−2 y−1....
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...Jiang et al. (2008a) showed that the annual and area-averaged CO2 degassing flux in the Altamaha River estuary was as high as 36 mol m−2 y−1, only moderately lower than the global average given in Borges et al. (2005)....
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...The global estuarine CO2 degassing flux estimated in Borges (2005) and Borges et al. (2005) is probably inaccurate (most likely too high) for the above reasons....
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...Borges (2005) and Borges et al. (2005) were the first to compile all available CO2 flux data from world estuaries, and they derived a global overall estuarine CO2 degassing flux of 0.34–0.45 Pg C y−1 (Figure 1)....
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...Using all available continental shelf sea-air CO2 flux data and a scaling method, Borges and colleagues (Borges 2005, Borges et al. 2005, Chen & Borges 2009) derived a global continental shelf sea-air CO2 flux between 0.3 and 0.4 Pg C y−1 (Figure 1)....
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TL;DR: Takahashi et al. as discussed by the authors showed that continental marginal seas play a significant role in biogeochemical cycles of carbon, as they receive huge amounts of upwelled and riverine inputs of carbon and nutrients, sustaining a disproportionate large biological activity compared to their relative surface area.
Abstract: Despite their moderately sized surface area, continental marginal seas play a significant role in the biogeochemical cycles of carbon, as they receive huge amounts of upwelled and riverine inputs of carbon and nutrients, sustaining a disproportionate large biological activity compared to their relative surface area. A synthesis of worldwide measurements of the partial pressure of CO2 (pCO2) indicates that most open shelves in the temperate and high-latitude regions are under-saturated with respect to atmospheric CO2 during all seasons, although the low-latitude shelves seem to be over-saturated. Most inner estuaries and near-shore coastal areas on the other hand are over-saturated with respect to atmospheric CO2. The scaling of air–sea CO2 fluxes based on pCO2 measurements and carbon massbalance calculations indicate that the continental shelves absorb atmospheric CO2 ranging between 0.33 and 0.36 Pg C yr � 1 that corresponds to an additional sink of 27% to � 30% of the CO2 uptake by the open oceans based on the most recent pCO2 climatology [Takahashi, T., Sutherland, S.C., Wanninkhof, R.,
557 citations
Cites background from "Budgeting sinks and sources of CO2 ..."
...Table 1 compiles available air–water CO2 fluxes in near-shore ecosystems (inner estuaries, salt marshes and mangrove surrounding waters) and updates previous global compilations by Abril and Borges (2004), Borges (2005) and Borges et al. (2005), with an increase of almost 50% in available data....
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...Borges et al. (2005); 4....
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...Despite some uncertainties (Borges, 2005; Borges et al., 2005; Cai et al., 2006), mounting evidence based on pCO2 measurements and mass-balance calculations (Chen et al....
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...The present work updates previous compilations of pCO2 measurements in coastal environments (Borges, 2005; Borges et al., 2005; Cai et al., 2006) and attempts to reconcile long-lived opposing views on C cycling in marginal seas, either as net heterotrophic and potential sources of CO2 to the atmosphere (e....
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...The present work updates previous compilations of pCO2 measurements in coastal environments (Borges, 2005; Borges et al., 2005; Cai et al., 2006) and attempts to reconcile long-lived opposing views on C cycling in marginal seas, either as net heterotrophic and potential sources of CO2 to the…...
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References
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TL;DR: In this paper, the influence of variability in wind speed on the calculated gas transfer velocities and the possibility of chemical enhancement of CO2 exchange at low wind speeds over the ocean is illustrated using a quadratic dependence of gas exchange on wind speed.
Abstract: Relationships between wind speed and gas transfer, combined with knowledge of the partial pressure difference of CO2 across the air-sea interface are frequently used to determine the CO2 flux between the ocean and the atmosphere. Little attention has been paid to the influence of variability in wind speed on the calculated gas transfer velocities and the possibility of chemical enhancement of CO2 exchange at low wind speeds over the ocean. The effect of these parameters is illustrated using a quadratic dependence of gas exchange on wind speed which is fit through gas transfer velocities over the ocean determined by the natural-14C disequilibrium and the bomb-14C inventory methods. Some of the variability between different data sets can be accounted for by the suggested mechanisms, but much of the variation appears due to other causes. Possible causes for the large difference between two frequently used relationships between gas transfer and wind speed are discussed. To determine fluxes of gases other than CO2 across the air-water interface, the relevant expressions for gas transfer, and the temperature and salinity dependence of the Schmidt number and solubility of several gases of environmental interest are included in an appendix.
4,187 citations
"Budgeting sinks and sources of CO2 ..." refers methods in this paper
...from the k parameterization of Wanninkhof and McGillis [1999], from Friederich et al....
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...2 flux values converted to the k parameterization of Wanninkhof [1992] using conversion...
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...2 using the k parameterization of Wanninkhof [1992], based on data compiled from Kelly and...
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...2 using the k parameterization of Wanninkhof [1992], from DeGranpre et al....
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...5 flux values converted to the k parameterization of Wanninkhof [1992] using conversion...
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TL;DR: In this paper, the Wanninkhof dependence of the CO2 gas transfer velocity has been used to estimate the global ocean CO2 flux in the mean non-El Nino conditions for a reference year 1995.
Abstract: Based on about 940,000 measurements of surface-water pCO2 obtained since the International Geophysical Year of 1956–59, the climatological, monthly distribution of pCO2 in the global surface waters representing mean non-El Nino conditions has been obtained with a spatial resolution of 4°×5° for a reference year 1995. The monthly and annual net sea–air CO2 flux has been computed using the NCEP/NCAR 41-year mean monthly wind speeds. An annual net uptake flux of CO2 by the global oceans has been estimated to be 2.2 (+22% or ?19%) Pg C yr?1 using the (wind speed)2 dependence of the CO2 gas transfer velocity of Wanninkhof (J. Geophys. Res. 97 (1992) 7373). The errors associated with the wind-speed variation have been estimated using one standard deviation (about±2 m s?1) from the mean monthly wind speed observed over each 4°×5° pixel area of the global oceans. The new global uptake flux obtained with the Wanninkhof (wind speed)2 dependence is compared with those obtained previously using a smaller number of measurements, about 250,000 and 550,000, respectively, and are found to be consistent within±0.2 Pg C yr?1. This estimate for the global ocean uptake flux is consistent with the values of 2.0±0.6 Pg C yr?1 estimated on the basis of the observed changes in the atmospheric CO2 and oxygen concentrations during the 1990s (Nature 381 (1996) 218; Science 287 (2000) 2467). However, if the (wind speed)3 dependence of Wanninkhof and McGillis (Res. Lett. 26 (1999) 1889) is used instead, the annual ocean uptake as well as the sensitivity to wind-speed variability is increased by about 70%. A zone between 40° and 60° latitudes in both the northern and southern hemispheres is found to be a major sink for atmospheric CO2. In these areas, poleward-flowing warm waters meet and mix with the cold subpolar waters rich in nutrients. The pCO2 in the surface water is decreased by the cooling effect on warm waters and by the biological drawdown of pCO2 in subpolar waters. High wind speeds over these low pCO2 waters increase the CO2 uptake rate by the ocean waters. The pCO2 in surface waters of the global oceans varies seasonally over a wide range of about 60% above and below the current atmospheric pCO2 level of about 360 ?atm. A global map showing the seasonal amplitude of surface-water pCO2 is presented. The effect of biological utilization of CO2 is differentiated from that of seasonal temperature changes using seasonal temperature data. The seasonal amplitude of surface-water pCO2 in high-latitude waters located poleward of about 40° latitude and in the equatorial zone is dominated by the biology effect, whereas that in the temperate gyre regions is dominated by the temperature effect. These effects are about 6 months out of phase. Accordingly, along the boundaries between these two regimes, they tend to cancel each other, forming a zone of small pCO2 amplitude. In the oligotrophic waters of the northern and southern temperate gyres, the biology effect is about 35 ?atm on average. This is consistent with the biological export flux estimated by Laws et al. (Glob. Biogeochem. Cycles 14 (2000) 1231). Small areas such as the northwestern Arabian Sea and the eastern equatorial Pacific, where seasonal upwelling occurs, exhibit intense seasonal changes in pCO2 due to the biological drawdown of CO2.
1,637 citations
TL;DR: Denitrification occurs in essentially all river, lake, and coastal marine ecosystems that have been studied as discussed by the authors, and the major source of nitrate for denitrification in most river and lake sediments underlying an aerobic water column is nitrate produced in the sediments, not nitrate diffusing into the overlying water.
Abstract: Denitrification occurs in essentially all river, lake, and coastal marine ecosystems that have been studied. In general, the range of denitrification rates measured in coastal marine sediments is greater than that measured in lake or river sediments. In various estuarine and coastal marine sediments, rates commonly range between 50 and 250 µmol N m−2 h−1, with extremes from 0 to 1,067. Rates of denitrification in lake sediments measured at near-ambient conditions range from 2 to 171 µmol N m−2 h−1. Denitrification rates in river and stream sediments range from 0 to 345 µmol N m−2 h−1. The higher rates are from systems that receive substantial amounts of anthropogenic nutrient input. In lakes, denitrification also occurs in low oxygen hypolimnetic waters, where rates generally range from 0.2 to 1.9 µmol N liter−1 d−1. In lakes where denitrification rates in both the water and sediments have been measured, denitrification is greater in the sediments.
The major source of nitrate for denitrification in most river, lake, and coastal marine sediments underlying an aerobic water column is nitrate produced in the sediments, not nitrate diffusing into the sediments from the overlying water. During the mineralization of organic matter in sediments, a major portion of the mineralized nitrogen is lost from the ecosystem via denitrification. In freshwater sediments, denitrification appears to remove a larger percentage of the mineralized nitrogen. N2 fluxes accounted for 76–100% of the sediment-water nitrogen flux in rivers and lakes, but only 15–70% in estuarine and coastal marine sediments. Benthic N2O fluxes were always small compared to N, fluxes.
The loss of nitrogen via denitrification exceeds the input of nitrogen via N2 fixation in almost all river, lake, and coastal marine ecosystems in which both processes have been measured.
Denitrification is also important relative to other inputs of fixed N in both freshwater and coastal marine ecosystems. In the two rivers where both denitrification measurements and N input data were available, denitrification removed an amount of nitrogen equivalent to 7 and 35% of the external nitrogen loading. In six lakes and six estuaries where data are available, denitrification is estimated to remove an amount of nitrogen equivalent to between 1 and 36% of the input to the lakes and between 20 and 50% of the input to the estuaries.
1,571 citations
TL;DR: In this article, the authors focus on the transport and transformations of land-derived organic matter in the ocean, highlighting recent research on the patterns and processes involved in the degradation of terrestrial organic matter.
1,335 citations
1,263 citations