Australia: The Land Where Time Began
Carbon Fluxes from Land to Ocean - Anthropogenic Perturbations
Photosynthesis and chemical weathering takes up a substantial amount of atmospheric carbon on land which is subsequently transported laterally from terrestrial ecosystems in uplands along the aquatic continuum to the ocean. According to Regnier et al. estimates of global carbon budgets have so far implicitly assumed that the transformation and lateral transport along this aquatic continuum has not changed since pre-industrial times. A synthesis of published work has revealed the magnitude of lateral carbon fluxes from the land to the ocean at the present time, and the extent by which these fluxes have been altered by human activities. In this paper Regnier et al. show that the flux of carbon to inland waters may have been increased by as much as 1.0 Pg C/yr since pre-industrial time by human perturbation, mainly by the enhanced carbon export from soils. Most of this input of carbon to upstream rivers is either emitted back to the atmosphere as carbon dioxide (about 0.4 Pg C/yr) or is sequestered in sediments (about 0.5 Pg C/yr) along the continuum of freshwater bodies to estuaries to coastal waters, the remaining perturbation carbon input of about 0.1 Pg C/yr entering the open ocean. Regnier et al. say the results of their analysis indicate that about 0.9 Pg C/yr are presently stored in terrestrial ecosystems, a figure which agrees with forest inventories, though it differs significantly from the 1.5 Pg C/yr that has previously been estimated when the lateral carbon flux changes were ignored. Regnier et al. suggest that in global carbon dioxide budgets carbon fluxes along the aquatic continuum from land to ocean need to be included.
Human activities have modified greatly the exchange of carbon and nutrients between the land, atmosphere, freshwater bodies, coastal zones and the open ocean over the last 200 years (Likens et al., 1981; Mulholland & Elwood, 1982; Wollast & Mackenzie, 1989; Degens, Kempe & Richey, 1991; Smith & Hollibaugh, 1993; Stallard, 1998; Ver, Mackenzie & Lerman, 1999; Richey, 2004; Raymond, Oh, Turner & Broussard, 2008). The delivery of these elements through the aquatic continuum that connects soil water to the open ocean through rivers, streams, lakes, reservoirs, estuaries and coastal zones, have been modified by human activity that includes changes in land use, soil erosion, liming, application of fertiliser and pesticides, sewage water production, damming watercourses, water withdrawal, and human-induced climate change, have resulted in major impacts on global biogeochemical cycles (Aumont et al., 2001; Glob. Biogeochem. Cycles 19 (special issue), 2005; Mackenzie et al., 2005; Cotrim da Cunha et al., 2007; Quinton et al., 2010). Carbon is transferred laterally through the aquatic continuum across ecosystems and regional geographic boundaries, and vertically exchanged with the atmosphere, often as greenhouse gases.
The magnitude of the anthropogenic perturbation of the aquatic continuum from land to ocean, in terms of lateral carbon fluxes, has become apparent (Richey, 2004; Mackenzie et al., 2005; Cole et al., 2007; Battin et al., 2009; McLeod et al., 2011) only recently, though it has been known of for more than 20 years (Sarmiento & Sundquist, 1992). It has long been believed that the lateral transport of C from land to sea is a natural loop in the global C cycle that is not affected by anthropogenic perturbations. As a result of this the flux was neglected in assessments of the anthropogenic CO2 budgets that have been reported, as by the IPCC or Global Carbon Project (Sarmiento & Gruber, 2002; Denman et al., 2007; Le Quéré et al., 2009; Peters et al., 2012). The quantification of lateral C fluxes between land and ocean, and the implications they have on the exchange of CO2 with the atmosphere is important to further the understanding of the mechanisms that drive the natural C cycle along the aquatic continuum (Ludwig & Probst, 1998; Archer, 2005), as well as for closing the C budget of the anthropogenic perturbation that is ongoing.
The data that relates to the carbon cycle in the aquatic continuum from land to ocean is too sparse to provide global coverage; not enough water sampling, hydrology and areal extent of various ecosystems being poorly constrained, and few direct ρCO2 and other measurements that are carbon-relevant (Tranvik et al., 2009; Laruelle, Dürr, Slomp & Borges, 2010). The exploration of the magnitude of these fluxes and their anthropogenic perturbations has used global box models, but the processes have remained highly parameterised (Ver, Mackenzie & Lerman, 1999). Included among the current generation of 3-D Earth system models is the coupling between the C cycle and the physical climate system, though it ignores the lateral flows of C (and nutrients) completely (Collins et al., 2011). There are major challenges in the study of C in the aquatic continuum which include the disentangling of the anthropogenic perturbations from the transfers that are natural, identification of drivers that are responsible for the ongoing changes and, ultimately, the forecasting of their future evolution, e.g., by the incorporation of these processes in Earth system models. It is not only necessary to resolve these issues to refine the allocation of fluxes of greenhouse gas on global and regional scales, but also to establish regional budgets and mitigation strategies that are policy-relevant (Ciais et al., 2010).
In order to designate the vertical and lateral fluxes to and from inland waters only the term “boundless carbon cycle” was introduced (Battin et al., 2009). In this paper Regnier et al. extend this concept to all components of the global carbon cycle that are connected by the land-ocean continuum and discuss changes that are possible relative to the natural carbon cycle by providing new separate estimates for the present day and the anthropogenic perturbation. In some instances bulk fluxes have been compared with perturbation fluxes, such as that of the net land carbon sink of anthropogenic carbon dioxide, which may cause some confusion, which means that this distinction is important (Battin et al., 2009; Bastviken et al., 2011). In this paper Regnier et al. deal with total carbon fluxes, though do not distinguish systematically between organic and inorganic, as this is still not well known on a global scale for a number of components of the land-ocean continuum. Regnier et al. don’t, however, highlight the exact chemical composition where it is constrained sufficiently. The Supplementary Table S1 for this article is a compilation of the major flux estimates from the literature and has been estimated in this paper with a measure of confidence that involves transfer from one global domain to another. Regnier et al. also provide a brief justification of the estimates they have proposed.
Contemporary estimates of lateral carbon fluxes
In this section they derive contemporary estimates of carbon fluxes along the continuum from land to ocean aquatic systems, looking first at carbon transport that involve inland waters and then consider their links to flows of carbon through the estuaries and coastal ocean and beyond.
It has been estimated recently that at the present the bulk carbon input of natural plus anthropogenic carbon to freshwaters at 2.7-2.9 Pg C/yr, based on upscaling of local C budgets (Battin et al., 2009; Tranvik et al., 2009). There are 4 fluxes that comprise this input:
1) The first and largest of these is the carbon derived from soil that is released to inland waters, mainly in organic form, particulate and dissolved, though also as free dissolved CO2 from respiration in the soil (Ittekkot, Humborg, Rahm Nguyen, 2004). The flux is evaluated at 1.9 Pg C/yr, by subtracting, from a total medium estimate of 2.8 Pg C/yr, with the smaller contributions from the other 3 fluxes: chemical weathering (F2), sewage (F4), and net C fixation (F5). The C flux that was derived from the soil is part of the terrestrial ecosystem C cycle, which represents about 5% of the soil heterotrophic respiration (FT7). The C that is released to inland waters is neglected in current estimates of soil respiration. In order to account for the soil C that is channelled into inland freshwater systems would nevertheless remain within the uncertainty of this flux (Luyssaert et al., 2007).
2) The second flux involves the chemical weathering of continental surfaces (carbonate and silicate rocks). It is part of the inorganic (often ‘geological‘ C cycle and results in an additional ⁓0.5 Pg C/yr input to upstream sections of rivers (Garrels & MacKenzie, 1971; Holland, 1978; Gaillardet et al., 1999; Munhoven, 2002; Hartmann et al., 2009) (F2). About ⅔ of this C flux results from the removal of atmospheric CO2 in weathering reactions (F3) and the remaining fraction results from chemical weathering of the C in rocks. Nevertheless, the pathway for chemical weathering is largely indirect with most of the CO2 that is removed from the atmosphere being soil CO2, after having passed through photosynthetic fixation. C is released to the aquatic continuum as inorganic C, mainly in the form of bicarbonate, given that the average pH is in the range of 6-8 for freshwater aquatic systems (MacKenzie & Lerman, 2006). Contrasting with the organic C that is derived from soil, it is assumed that the C derived from the weathering of rock will not degas to the atmosphere during its transfer through inland waters (Kempe, 1982).
3) The 3rd flux represents the C that is dissolved in sewage water that originates from the consumption of biomass by humans and domestic animals (F4), which releases an additional ⁓0.1 Pg C/yr as an input to freshwaters (Prairie & Duarte, 2007; Mackenzie, Lerman & Ver, 2001).
4) The 4th flux involves the photosynthetic fixation of C within inland waters, which is potentially high on an aerial basis (Cole et al., 2007). As the result of decomposition within inland waters (Cole, Caraco, Kling & Kratz, 1994), a substantial fraction of this C is returned to the atmosphere, though there is percentage that remains for export and burial (Downing et al., 2008; Raymond & Bauer, 2001), and priming for the decomposition of terrestrial organic matter (Bianchi, 2011). Therefore, aquatic systems can still be autotrophic (Stets et al., 2009), though they can emit CO2 to the atmosphere. Regnier et al. estimated with low confidence that 20% of the organic C that is buried and exported from inland waters is autochthonous (F5).
Another C source to the aquatic continuum (Meybeck, 1982; Copard, Amiotte-Suchet & D-Giovanni, 2007) is represented by physical erosion of particulate inorganic C (⁓0.2 Pg C/yr) and of organic C that is resistant to mineralisation (⁓0.1 Pg C/yr). It is likely to be refractory at the continental timescale (Galey et al., 2007) and is most likely to be channelled through the inland waters and estuaries to the open ocean without significant exchange with the atmosphere, though the fate of this physically eroded C is not easy to trace. Therefore, it is treated separately in Fig 1A in the source below.
A fraction of the lateral flux that passes through inland waters is emitted to the atmosphere, mainly as CO2 (F2), during the transport of carbon to the coastal ocean. CH4 is also emitted from lakes and some rivers (F6), though this flux represents a small fraction of C flux (Bastviken et al., 2011). Estimates of the water-to-atmosphere CO2 efflux that are data-driven have been obtained for individual components of the inland freshwater continuum (Cole et al., 2007; Battin et al., 2009; Sobek, Tranvik & Cole, 2005). This efflux is sustained by CO2 that originates from respiration of soil and roots, aquatic decomposition of organic matter, dissolved and particulate, and decomposition of organic C from sewage, as has been detailed above. Also, as well as C from fringing and riparian wetlands, which is counted as C input to freshwater in Fig. 1a of Regnier et al., may also contribute significantly to freshwater CO2 outgassing (Butman & Raymond, 2011). There are now about 12,000 sampling locations of the inorganic C cycle that have been reported in databases of inland water. Calculation of ρCO2 from alkalinity and pH indicates that 96% of inland waters are oversaturated with respect to CO2 relative to the concentration in the atmosphere, while 82% have a concentration that is at least double that of the atmosphere (Global River Chemistry Database (GloRiCh), unpublished data; ref. 52).
For some regions of the Earth, such as the catchment of the Rhine River, Scandinavia and the conterminous US (Kempe, 1982; Cole, Caraco, Kling & Kratz, 1994; Butman & Raymond, 2011; Sobek, Tranvik, Prairie, Kortelainen & Cole, 2007; Humborg et al., 2010), many measurements of the CO2 efflux are available. However, lack of direct CO2 flux measurements, incomplete spatial coverage of ρCO2 sampling locations that were coupled with the difficulty in determination of the surface waters, and the scaling of the velocity of gas transfer in freshwaters, results in large uncertainties and prevents the obtaining of global-scale estimates that are robust. In particular, many rivers and lakes that are contributors of a significant fraction to the aquatic C flux have remained poorly surveyed in terms of ρCO2 (GloRiCh, unpublished data). Included among these are the rivers of Southeast Asia, tropical Africa and the Ganges and, to a lesser extent, the waters of the Amazon Basin (Richey, 2002; Melak et al., 2009), which carry disproportionately high loads of organic carbon due to their combination of high terrestrial productivity, high rates of decomposition and precipitation that is at a high uniform rate. The large uncertainty in the outgassing of CO2 from freshwaters (Richey, 2004, Cole et al., 2007; Tranvik et al., 2009; Butman & Raymond, 2011) with a range of 0.6-1.25 Pg C/yr is explained by the scarcity of direct measurements of ρCO2 and lack of knowledge of regional surface areas and velocity of gas transfer. At the higher end of the spectrum the values also include the contribution from streams and small lakes, which are typically not considered in estimates of flux (Tranvik et al., 2009). The most likely value estimated by Regnier et al. of the CO2 outgassing flux of 1.0 Pg C/yr (F7) with a medium-to-low confidence.
In freshwater sediments the burial rate has been estimated to be between ⁓0.2 and 1.6 Pg C/yr. The lower estimate refers to only the lakes, ponds and reservoirs (Cole et al., 2007; Tranvik et al., 2009) (0.2-0.6 Pg C/yr), whereas the upper one also includes sedimentation in flood plains (Stallard, 1998; Smith, Renwick, Buddemeier & Crossland, 2001; Aufdenkampe et al., 2011) (0.5-1.6 Pg C/yr). Between the higher and lower bound estimates of this burial flux the factor of 8 highlights the limited amount of observational data that are available to constrain this term at the global scale. Within this large uncertainty, Regnier et al. adopted with a low confidence a value of 0.6 Pg C/yr for the burial of C in inland freshwater sediments (F8). Part of this burial is carbon that has been transported, by erosion processes, to lake sediments and floodplains from soils.
Based on the mass balance of the C input from soils to freshwaters minus outgassing of CO2 and the fluxes of C burial in inland waters is adopted in this paper, the output represents a lateral flux that is transported downstream into estuarine systems (F9) of 1.0 Pg C/yr (Likens et al., 1981). Therefore, the estimate by Regnier et al. is close to the values based on compilation of field data (Meybeck, 1982; Meybeck, 1991) and the results of Global Nutrient Export from Watersheds model of carbon and water flows (Beusen et al., 2005), though higher values have also been suggested (Richey, 2004). Conventional partitioning among different C pools (Meybeck, 1982; Meybeck, 1991; Schlesinger & Melack, 1981; Degens, 1982) is a flux of particulate and dissolved C, each of which is equivalent to about 0.2 Pg C/yr, and a flux of dissolved inorganic C of about 0.4 Pg C/yr. If the uncertainty of each individual fluxes of inland water (weathering, outgassing, burial and export, are taken into account, it is also indicated that by the balance that C flux derived from soil (F1)( 1.9 Pg C/yr) is certainly not known any better than within ⁓±1.0 Pg C/yr.
Estuaries (total area 1.1 x 106 km2) correspond to the boundary between inland aquatic systems and the coastal ocean, which is represented mainly by the shelves of the oceans of the world, in the analysis of Regnier et al. It is indicated by recent analyses of observational data that estuaries emit CO2 to the atmosphere (Laruelle, Dürr, Slomp & Borges, 2010; Cai, 2011), within the range of 0.25 ± 0.25 Pg C/yr (F10). It is suggested by field measurements that about 10% of the CO2 outgassing from estuaries is sustained by the input from upstream freshwaters (F9) and 90% by local net heterotrophy (Borges & Abril, 2012), a significant fraction of the organic C required coming from adjacent marsh ecosystems (F11). Regnier et al. used in this paper a more conservative estimate of ⁓0.3 ± 0.1 Pg C/yr for the common estuarine vegetation of mangroves and salt marches, which was based on upscaling of a detailed regional budget for the southeastern USA (Cai, 2011), though coastal vegetated environments, salt marshes, mangroves, seagrasses, macroalgae and coral reefs, may export as much as 0.77-3.18 Pg C/yr to the coastal ocean (Cai, 2011). Also as far as Regnier et al. know there are no global estimates for the burial in all estuarine sediments, though a long term burial in mangroves and salt marshes of 0.1 ± 0.05 Pg C/yr has been proposed (McLeod et al., 2011; Breithaupt et al., 2012) (F12). According to Regnier et al. if they combined their inputs from their upstream river and vegetation with their average outgassing estimate of CO2 to the atmosphere and the 1st-order estimate for C burial in estuarine sediments and vegetated ecosystems (F12), they obtain a delivery of C from estuarine sediments of the ocean of 0.95 Pg C/yr (F13). This estimate amounts to ⁓⅓ of the initial C flux that is released from soils, rocks and sewage as input to freshwater systems.
When materials leave estuaries, they transit into the coastal ocean and beyond to the open ocean. It is suggested by recent syntheses of the air-sea CO2 fluxes in coastal waters (total area of 31 x 106 km2) (Laruelle et al., 2012) that at present between 0.22 and 0.45 Pg C/yr are taken up by the coastal ocean (Cai, Dai & Wang, 2006; Borges, Delille & Frankignoulle, 2005). Regnier et al. chose here a lower estimate of 0.2 Pg C/yr for the coastal ocean sink of CO2, that was based on a recent analysis for the global ocean (Wanninkhof et al., 2012) (F14). This value relies on the observation that, outside the nearshore environments, in the coastal regions, the net CO2 fluxes are of similar strengths and directions as those of adjacent ocean regions, i.e., the coastal regions at low latitudes tend to be sources of CO2 to the atmosphere, whereas they tend to be sinks (Cai & Wang, 2006; Borges, Delille & Frankignoulle, 2005) at high latitudes. This makes possible extrapolation of the exchange values of CO2 in the open ocean towards the coasts. The most recent estimate (0.25 Pg C/yr), that was based on upscaling from new sites with good observation coverage suggests a similar value (Cai, 2011) though this extrapolation is an oversimplification. It is nevertheless important to recognise that the limited spatial coverage of ρCO2 data in the coastal ocean and its heterogeneous nature confine the coastal action to low-to-medium. Also, the influence of the input of terrestrial C on air-sea CO2 fluxes extends considerably beyond the limit of the shelf in the discharge plumes of large tropical rivers, such as the Amazon (Cai, 2011; Liu, Atkinson, Quiñones & Talaue-McManus, 2010). Regnier et al. suggest that these plumes should be considered an integral part of the land-ocean continuum.
Between 0.2 and 0.5 Pg C/yr of organic C and calcium carbonate (Muller-Karger et al., 2005; Krumins et al., 2013), may be sequestered by coastal sediments, though significantly higher values have been reported (Dunne, Sarmiento & Gnanadesikan, 2007) (F15). Regnier et al. chose in this paper a central estimate of 0.35 Pg C/yr, of which a sediment C burial of 0.05-0.1 Pg C/yr is attributed to the seagrass meadows of the shallow coastal seas (McLeod et al., 2011). Also, the coastal sediment C pool is the most probable repository for much of the recalcitrant terrestrial C that is related to physical weathering (FR). The increase by about 0.5 Pg C/yr of dissolved inorganic carbon storage in the water column (Mackenzie, De Carlo & Lerman, 2012), may also result from the net pumping of anthropogenic CO2 from the atmosphere to the coastal waters. A direct global estimate of lateral carbon fluxes at the boundary between the coastal and open ocean, which is delineated by the shelf break (Laruelle et al., 2012), cannot be achieved solely by observational means (Mackenzie, Anderson Lerman & Ver, 2005; Liu, Atkinson, Quiñones & Talaue-McManus, 2010; Jahnke, 2010), as a result of data paucity. Therefore, based on mass-balance calculations by the use of the above estimates of flux, Regnier et al. propose with low confidence that the net organic and inorganic C export from coastal ocean to open ocean is ⁓0.75 Pg C/yr (F16).
Anthropogenic perturbation of lateral carbon fluxes
The route of the perturbed C fluxes through the global systems of inland waters to estuaries to coastal waters and beyond was traced by Regnier et al., as was the case with contemporary lateral fluxes.
Reconstructions of the historical evolution, Preindustrial, about the year 1750 to present, of the global aquatic carbon cycle and its fluxes has so far relied primarily on box models (Ver, Mackenzie & Lerman, 1999; Anderson, Mackenzie & Lerman, 2005) that were averaged globally. These models, that are highly parameterised, are driven by increasing concentrations of atmospheric CO2, changes in land use, application of nitrogen and phosphorus fertiliser, C, nitrogen and phosphorus in sewage discharge and changes in global temperature. It is suggested by model simulations that the transport of riverine C (F9) since 1750 has increased by about 20% from ⁓0.75 Pg C/yr in 1750, to 0.9-0.95 Pg C/yr at present. The available data from the literature (Wollast & Makenzie, 1989; Richey, 2004; Meybeck, 1982; Milliman & Meade, 1983) supports the existence of such an enhanced C delivery, and has been attributed to deforestation and cultivation practices that are more intensive that have increased the degree of soil degradation and erosion. An increase in the export of organic and inorganic C to aquatic systems (Raymond, Oh, Turner & Broussard, 2008) results from this. E.g., particulate organic C erosion in the range of 0.4-1.2 Pg C/yr has been reported for agricultural land alone (Stallard, 1998; Quinton, Govers, Van Oost & Bardgett, 2010; Van Oost et al., 2007). Only a percentage of this flux, however, represents the lateral transfer of anthropogenic CO2 that has been fixed by photosynthesis (Stallard, 1998; Smith, Renwick, Buddemeier & Crossland, 2001; Van Oost et al. 2007; Billings et al., 2010).
There is no estimate of the preindustrial C flux from soils to inland waters that is observationally based, and no associated CO2 outgassing and C burial fluxes in freshwater systems in preindustrial times, though budgets have been established for conditions of the present. Also, Regnier et al. say they are not aware of any model simulation that is spatially explicit of the CO2 outgassing and fluxes of C burial in inland aquatic systems during the industrial period at the global scale. In various inland aquatic systems the potential anthropic effects on the cycling of C in various inland aquatic systems have been reviewed (Cole et al., 2007), though a quantitative estimate of the anthropogenic perturbation is still to be assessed. Nevertheless, the bulk fluxes are large enough that even a small change would alter the global C budget of the anthropogenic CO2. E.g., damming and withdrawal of freshwater is highly likely to have impacted the CO2 outgassing fluxes and rates of burial of organic carbon since preindustrial times by the effect they have on surface area and residence time of inland waters (Mulholland & Elwood, 1982; Stallard, 1998; Richey, 2004). The evolution of agricultural practices, in particular, and the construction of human-made impoundments over the past century have most likely led to enhanced outgassing of CO2. Also, the flux of burial of C in the sediments in reservoirs and small agricultural ponds of 0.35 Pg C/yr has been estimated (Mulholland & Elwood, 1982; Stallard, 1998; Richey, 2004; Cole, 2007; Tranvik et al., 2009; Smith, Renwick, Buddemeier & Crossland, 2001), with the C probably coming from terrestrial and autochthonous sources.
In order to estimate the extent to which other inland water environments, such as lakes, streams and rivers have been perturbed by human activities, Regnier et al. assumed that outgassing of CO2 and burial fluxes of C in these systems linearly scale with the increase that has been estimated, about 20%, in the C derived from soil that is exported from rivers to estuaries (F9) and the coastal zone (Mackenzie, Ver & Lerman, 2002). This leads to a perturbation of ⁓0.1 Pg C/yr for the flux of CO2 outgassing and ⁓0.05 Pg C/yr for the flux of C burying. It is implied by the linear scaling assumption that the outgassing of CO2 and the sedimentation rate of C are first order processes with respect to the additional concentration of C that is derived from enhanced exports from soil in the freshwater aquatic systems. It is probable that this assumption is reasonable for the air-water flux, though it is almost certain that the change in the burial flux of carbon is more complex (Richey, 2004).
It has been inferred that inputs to upstream rivers (F4) add another 0.1 Pg C/yr to the anthropogenic perturbation, and the assumption was made by Regnier et al. that this labile organic C is entirely outgassed within inland waters. When all the contributions were combined, the budget analysis gives outgassing of CO2 (F7) and the burial fluxes of C (F8) under preindustrial conditions of 0.6 and 0.2 Pg C/yr, respectively. The extra outgassing that remains, 0.5 Pg C/yr, and the extra burial fluxes, 0.4 Pg/yr, was attributed to the anthropogenic perturbation. Also, an increase in chemical weathering of continental surfaces that is caused by climate change that is human-induced and increased levels of CO2 contributes to the enhanced export flux of riverine C that is derived from the weathering of rock (Gislason et al., 2009; Beaulieu et al., 2012) (F2). It is suggested by Regnier et al. that anthropogenic perturbation could possibly reach 0.1 Pg C/yr, mainly by enhanced dissolution of carbonate rocks (Beaulieu et al., 2012). The impact of land use change on rates of weathering may have begun 3,000 years ago (Bayon et al., 2012) though its effects on atmospheric CO2 is difficult to assess (Oh & Raymond, 2006; Hamilton et al., 2007). C that is mobilised by agricultural liming is a source of enhanced land use C fluxes (Oh & Raymond, 2006) and could result in a sink of ⁓0.05 Pg C/yr.
To sum up, the total flux from soils, bedrock and sewage to aquatic systems of 2.5 Pg C/yr of the present can be decomposed as the sum of a natural flux of ⁓1.5 Pg C/yr and an anthropogenic perturbation flux of ⁓1.0 Pg C/yr – a value that is similar to a previously published estimate (Richey, 2004). This anthropogenic perturbation (0.5 Pg C/yr) is respired back to the atmosphere in freshwater systems (F7), while the remainder contributes to enhanced levels of C burial (F8) and export to estuaries (F9) and, possibly, to the coastal ocean (F13).
The perturbation of historical drainage and conversion of salt marshes and mangroves resulting from human activities, as well as the channelisation of estuarine conduits have modified the C balance in estuaries. The total loss of C from these intertidal pools, for instance, could be, according to Regnier et al., as high as 25-50% over the past century, mainly as a result of land use changes (McLeod et al., 2011). If it is assumed that the reduced C flux to estuaries (F2) from marshes and mangrove ecosystems is proportional to the surface area reduction of these ecosystems, it was estimated by Regnier et al. that the flux of C that is transported to estuaries from coastal vegetation in preindustrial times must have been about 0.15 Pg C/yr larger than that of the present value of 0.30 Pg C/yr. Regnier et al. predicted that in estuarine sediments C burial has been reduced from preindustrial times to the present by the same relative factor, which amounts to an anthropogenic reduction of 0.05 Pg C/yr of the C burial flux (F12) in estuarine sediments. They assumed that in the absence of independent evidence the air-sea estuarine CO2 flux has remained constant since preindustrial times (F10). Closing the mass balance of preindustrial and present C budgets requires that the export of C to the coastal ocean (F13) has increased by ⁓0.1 Pg C/yr since 1750, from 0.85 to 0.95 Pg C/yr.
Coastal ocean and beyond
As there was insufficient observational evidence, process-based arguments and models were relied on to separate C fluxes for the coastal ocean of the present into preindustrial and anthropogenic components. The uptake of anthropogenic CO2 across the air-sea interface, which has been estimated to be about 0.2 Pg C/yr, is possibly the best constrained flux component, on the basis of this uptake has the same flux density as that of the mean ocean (Wanninkhof, 2012), i.e., about 6 g C/m2/yr. According to Regnier et al. this assumption is warranted as the oceanic uptake flux of anthropogenic CO2 is to first order controlled by the surface area. The degree to which the enhanced inputs of nutrient and C to the coastal ocean could have modified the air-sea CO2 balance is much less certain. It is suggested by simulations by a box model that enhanced supply of nutrients from land may have increased coastal productivity and C burial in coastal sediments (Lerman, Mackenzie & Ver, 2004), from about 0.2 Pg C/yr to 0.35 Pg C/yr, as well as contributing to a substantial increase in the air-to-sea CO2 flux, by up to 0.2-0.4. The efficiency by which the additional nutrient supply delivered to the coastal ocean is, however, actually reducing ρCO2 and enhancing the uptake of CO2 is globally uncertain. On continental shelves, e.g., the enhanced nitrogen supply (<50 Tg N/yr) (Seitzinger et al., 2005; Gruber & Galloway,2008) may stimulate a maximal additional growth of about 0.3 Pg N/yr, only a portion of which is exported to depth, and by which uptake of CO2 from the atmosphere (Jin et al., 2008) replaces less than 50%. It was estimated by Regnier et al., that an increase in the air-to-sea flux of CO2 that is no larger than about 0.1 Pg C/yr was caused by that coastal eutrophication. It has remained largely unknown what the response of the highly heterogeneous, very shallow coastal ocean, including reefs, banks and bays (<50 m. 12 x 106 km2) (Laruelle et al., 2012). It is expected, however, that it is in this region that the nutrient impact on biological productivity, organic C burial and CO2 fluxes that are area specific, is at the highest. The anthropogenic air-coastal water flux of CO2 is, therefore, known with only low confidence. Regnier et al. estimated a conservative value of 0.2 Pg C/yr for this anthropogenic flux (F14), which is significantly lower than the value of 0.5 Pg C/yr that has been suggested in recent syntheses (Liu, Atkinson, Quiñones & Talaue-McManus, 2010). It is not clear what the fate of the additional C received from the estuaries (F13) is. It is suggested that some of this C is probably sequestered in coastal sediments, as well as with some of the organic C that was produced in response to the nutrient input, which amounts to a flux that is potentially as large as 0.1-0.15 Pg C/yr (F15). The remaining C is exported to the open ocean, together with some of the anthropogenic CO2 that was taken up by the atmosphere, which amounts to a flux of approximately 0.1 Pg C/yr (F16). Again this value is significantly lower than previous estimates (Liu, Atkinson, Quiñones & Talaue-McManus, 2010), which highlights that the confidence in this numbers of Regnier et al. is very low.
Though it is still a challenge to obtain accurate quantification, it can be firmly concluded that in the industrial era, the C fluxes that are transported laterally and the vertically exchanged atmospheric CO2 fluxes that are relevant to the land-ocean aquatic continuum have been altered significantly by human activities, with land use changes being the main driver. It is suggested by the analysis of Regnier et al. that of the about 1.1 Pg C/yr of extra anthropogenic C that is delivered to the continuum of land-ocean aquatic systems (0.8 Pg C/yr from soils, 0.1 Pg C/yr from weathering, 0.1 Pg C/yr from sewage, 0.1 Pg C/yr, enhanced fixation of C in inland waters, which at present about 50 % of which is sequestered in inland water, estuarine and coastal sediments, <20% is exported to the open ocean and the remaining >30% is emitted to the atmosphere as CO2. Along the land-ocean continuum fluxes of CO2 may not only be altered directly by increased anthropogenic export of C from soil and subsequent respiration, though also indirectly by increased decomposition of autochthonous organic materials that are triggered by priming. This indirect process cannot yet be quantified, though it may be a contribution that is quantitatively relevant to the estimated fluxes and net heterotrophy of many systems that have been observed. It is the uncertainties associated with the breakdown of Regnier et al. that are large and represent a fundamental obstacle for global C assessments and a fertile avenue for future research. According to Regnier et al. their conclusions may be overruled on the quantitative value of each flux in this analysis if research in the future succeeds in narrowing down the uncertainties on the anthropogenic uncertainties, though it is not likely to affect the conclusions they reached that the anthropogenic perturbation to the C fluxes of the aquatic continuum is important in the global C budget.
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