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Carbon sequestration in wetland dominated coastal systems — a global sink of rapidly diminishing magnitude
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Carbon sequestration in wetland dominated coastal systems — a global sink of rapidly diminishing magnitude Charles S Hopkinson, Wei-Jun Cai and Xinping Hu Coastal vegetated wetlands have recently been identified as very important global C sinks but vulnerable to degradation by direct human alteration of their habitats. While their expanse is small globally, areal rates of C burial, or sequestration, are among the highest of Earth’s ecosystems. There is considerable uncertainty in the magnitude of total global sequestration in these systems for two reasons: poor estimates of their global areas and high variability and uncertainty in areal rates of burial between systems. The magnitude of C burial in vegetated coastal systems has been decreasing rapidly over the past century due primarily to human disturbances such as dredging, filling, eutrophication, and timber harvest. These systems continue to be lost globally at rates ranging from 1% to 7% annually. We find that climate change including global warming, human engineering of river systems, continued agricultural expansion, and sea level rise will also negatively impact C burial of coastal vegetated wetlands. A decrease in global C burial in these systems will ultimately exacerbate CO2 emissions, and further contribute to climate change in the future. Address Department of Marine Sciences, University of Georgia, Athens, GA 30602, USA Corresponding author: Hopkinson, Charles S ([email protected])
Current Opinion in Environmental Sustainability 2012, 4:186–194 This review comes from a themed issue on Carbon and nitrogen cycles Edited by Chen-Tung Arthur Chen and Dennis Peter Swaney Available online 25th April 2012 1877-3435/$ – see front matter # 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cosust.2012.03.005
Introduction With the evidence mounting rapidly for substantial changes to the global climate over the next century as a result of rising levels of atmospheric carbon dioxide (CO2) (IPCC), there has been considerable research over the past several decades to identify the natural carbon (C) sinks within the biosphere, both terrestrial and oceanic. Sinks that actually mitigate anthropogenic emissions of CO2 are the most important in arresting climate change. To mitigate emissions however sink strength must increase over time. Sink strength can increase either because the areal extent of those systems is increasing Current Opinion in Environmental Sustainability 2012, 4:186–194
over time or the absolute rate of sequestration increases over time. Furthermore, a reduced source due to anthropogenic or climate cause can also be considered as an increase in sink strength. Consequently much additional research has focused on understanding the feedbacks between sink strength and either climate change or other anthropogenic activities, such as land use change. One C sink that has received considerable attention over the past 10 years or so is the so-called coastal ‘blue carbon’ sink associated with organic carbon (OC) burial in seagrass meadows and intertidal wetlands (mangroves and intertidal marshes) [2,3,4,5,6,7]. There is strong agreement in all these studies that C sequestration rates per unit area are extremely high and attributable exclusively to C burial in sediments. Extrapolated to their global areal extent, C burial is estimated in excess of 100 Tg C yr1, which is almost 20 times greater than that of deep sea burial. It is similar in magnitude to C burial in depositional areas in non-vegetated portions of estuaries and the continental shelf [3] and comparable to that of terrestrial forest systems [8]. In recognition of the critical importance of coastal vegetated systems in global C sequestration, the recent ‘blue carbon’ papers have made strong pleas to restore and preserve these fragile systems, as historically they are being lost (converted to other types of systems) at rates between about 0.7% and 7% annually [6,9,10,11,12,13]. In this paper, we review the processes or C fluxes that are the basis of C sequestration in coastal vegetated wetlands and the mechanistic controls of those fluxes. We consider how climate change, sea level rise (SLR), and selected anthropogenic activities will likely impact those fluxes in the future. We do not discuss direct threats to these systems via land reclamation, filling, diking, conversion to other types of systems and the implications for C stores at the time of conversion, as these important issues have been covered well in other recent reviews (e.g. [5,6]).
The study system This treatise focuses on one community within estuarine ecosystems — the rooted macrophytes living intertidally or subtidally: mangroves, intertidal fresh, brackish and salt marshes and seagrass beds. It is important to put this community in a global perspective in order to appreciate their importance in the biosphere. Estuarine ecosystems cover less than 1% of the earth’s surface, but they are among the most productive [14]. They can be considered an ecotone — a transition zone www.sciencedirect.com
Carbon sequestration in coastal vegetated wetlands Hopkinson, Cai and Hu 187
Figure 1
Land
Estuary
Ocean Atmospheric Exchange
GPP Mangroves Tidal marshes Seagrass beds Animals Microbes Other Plants Terrestrial DIC & OC Sediment, particles, dissolved phase
Oceanic DIC & OC Sediment, particles, dissolved phase
Inorganic nutrients
Organic carbon - water column and sediments
Burial in sediments Current Opinion in Environmental Sustainability
Conceptual diagram of major C pools and fluxes in the coastal environment highlighting the burial flux of carbon produced by the autotrophic mangrove, tidal marsh, and seagrass bed subsystems. While some of the autotrophic C is buried, a large fraction is respired in the aquatic subsystem, which is often a site of intense carbon release to the atmosphere (see [15]).
between terrestrial/riverine systems and the ocean (Figure 1). Being at the mouths of rivers they have among the highest C and N loading rates on Earth, even compared to intensely fertilized agricultural systems. Terrestrial total OC flux has been estimated with various methods by several people as 460 Tg C yr1 (as high as 800 Tg C yr1) [15,16–22]. Rivers export their freshwater and associated OC to the ocean with a latitudinal distribution: about two thirds is supplied to lower latitude coastal oceans (0–308 — see [23,24]). Estuarine systems are not only deluged with OC from terrestrial systems, but they are also sites with exceptionally high rates of primary production, precisely because of where they lie at the interface between land and ocean: tidal mixing, high rates of N-loading, subtidal, intertidal and emergent primary producer organisms. Duarte et al. [3] estimated areal rates of gross primary production (GPP) to range between about 100 and 4000 gC m2 yr1. With the addition of rooted macrophyte production as a component of total estuarine production, estimates of net ecosystem production (NEP) are strongly autotrophic, ranging up to 1600 gC m2 yr1. Scaled to the globe, Duarte et al. [3] conclude that estuarine NEP is about 3000 Tg yr1. The fate of this carbon has yet to be clearly www.sciencedirect.com
resolved, but it is safe to conclude that only a small portion is buried within coastal sediments, while the majority of the excess is respired in downstream estuaries and the coastal ocean resulting in high rates of CO2 degassing [15]. It is reasonable to assume that burial of OC in sediments of coastal systems depends on the magnitude of NEP, therefore processes that affect NEP, such as GPP and R, regulate burial as well. Both mangroves and marshes are found in intertidal regions of estuaries between about mean sea level (MSL) and high, high water [25]. Seagrass beds are located subtidally in estuaries typically between mean lower low water and a depth corresponding to about the 1% light level [14,26]. Collectively these systems are found from arctic through tropical regions, with mangroves and marshes transitioning in the nearly subtropical region, as killing frosts limit the poleward extension of mangroves. Mangroves, marshes and seagrass beds are typically the most productive systems within estuaries [14]; they are autotrophic systems, with excess NEP either stored locally in sediments or exported to adjacent tidal creeks, bays and shelf or even the open ocean [15,27,28]. Here Current Opinion in Environmental Sustainability 2012, 4:186–194
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Figure 2
Progradation - →
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Long-term pattern of intertidal wetland development associated with sea level rise and estuarine basin infilling as depicted by Redfield [1]. With increasing rates of sea level rise and diminished supply of sediments from river runoff and armoring of upland-estuary borders, we expect to see a long-term decline in the areal extent of intertidal wetlands over the next century.
we focus on the excess OC that is buried in the root zones and sediments of these macrophyte-dominated subsystems of estuaries. The development of intertidal wetlands in estuaries is dependent on rates of SLR, sediment supply and the ability to accrete vertically relative to SLR. The development of intertidal wetlands began only about 4–5kyr or so ago, when the rate of SLR slowed to about 5 mm yr1 following the last glacial period [29]. Rapid expansion occurred when SLR slowed to about 3.5 mm yr1 [30]. As described by Redfield in his classic study of Barnstable marsh in Massachusetts, USA. [1,31], wetlands, including mangroves and marshes, first formed along the intertidal fringe of estuaries and then transgressed landward in conjunction with sea level that was flooding ever higher into terrestrial systems (see Figure 2). At the same time, wetlands accreted vertically at a rate proportional to SLR through peat accumulation and trapping of mineral sediments from flood tide waters. Wetlands prograded into open water areas of estuaries as land or ocean derived sediments infilled bay bottoms to levels approaching MSL. The extent of seagrass beds in estuaries reflects fine sediment substrate availability and water shallow enough and clear enough to allow sufficient light to reach plants. SLR affects the areal extent of seagrass beds as it relates to light penetration. Seagrass beds can accrete vertically through peat and sediment accumulation and laterally as bay bottoms infill with oceanic or terrestrial sediments.
length worldwide. Their paper says it ‘would be surprising if estimates derived in this way were accurate within 50%’. While there have been some better estimates made in the last decade or so, most notably for mangroves [32], uncertainty in areal extent of these systems may be a factor of 2. As indicated earlier, the worldwide coverage by these three systems has decreased substantially over the past 100 yr. While rates of loss are apparently lower now than they were 50 yr ago, the global extent of these systems is still declining rapidly.
Current best estimates of C sequestration Estimates of long-term carbon burial in coastal vegetated systems span a very large range, reflecting huge differences in carbon burial from system to system and large differences in estimates of areal extent. Here we report the results of recent syntheses of carbon burial rate [6,15], which average out the large range seen in individual reports in the literature (Table 2). For instance, the range in carbon burial rate reported for fairly large expanses of salt marsh is from about 29 gC m2 yr1 [36] to 210 gC m2 yr1 [2], almost an order of magnitude. Order of magnitude ranges in carbon burial rate are also seen for mangroves and seagrass beds [6].
Calculating C sequestration: the processes leading to carbon burial and their controls Most estimates of C burial are simply the product of sediment carbon density and sedimentation rate (see e.g. Table 1
Estimates of the areal extent of mangroves, intertidal marshes and seagrass beds have a high degree of uncertainty (Table 1). Many papers still rely on and cite first approximations of areal extent made by Woodwell’s team in the mid-1960s [33]. These authors made quick estimations of areal extent by extrapolating a few fine scale estimates for a unit length of shoreline to total shoreline Current Opinion in Environmental Sustainability 2012, 4:186–194
Estimates of global area of coastal vegetated ecosystems System Mangroves Intertidal marshes Seagrass beds
Areal extent (km2)
Reference
138 000–200 000 km 2 200 000 km2 400 000 km 2 177 000–600 000 km 2
[3,7,25,32] [33] [7] [7,34,35]
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Table 2 Estimates of global carbon burial in coastal vegetated ecosystems System
Global area (km2)
Carbon burial rate (gC m2 yr1)
Global carbon burial (Tg C yr1)
Reference for burial rate
Mangroves Intertidal marshes Seagrass beds
138 000–200 000 200 000–400 000 177 000–600 000
226 39 57 6–218 24 138 38
31.0–45.2 11.4–87.0 24.4–82.8
[6] [6,15] [6]
[2]). Neither of these terms is straightforward to measure and interpret however. C burial is the net result of multiple processes operating in concert. There are strong gradients in carbon stocks and density throughout a sediment profile reflecting the processes of in situ organic matter (root and rhizome) production, surface deposition from flood tide waters, organic matter decomposition, compaction and erosion. Temperature, solar radiation, water level, flooding frequency and duration, and wave/current energy are factors that can influence rates of above processes. Sedimentation rate and the rate of increase in wetland/ seagrass bed elevation are also challenging to measure and differ greatly depending on approach. Short-term rates (daily/weekly) can be measured by surface deposition on plates, intermediate rates can be measured by sediment erosion tables and marker horizons, and longterm rates can be measured by isotopic profiles (e.g. 137Cs from 1963 on, 210Pb for the past 100 yr, and 14C for the past 10 000 yr). Typically, the longer the integration period, the lower the apparent sedimentation rate or the slower the rate of elevation rise. Perhaps the overriding factors controlling elevation increase in wetlands are crustal movements, compaction, and the rate of SLR. Water level and the rate of SLR set an upper limit on the distribution of intertidal wetlands. Once at their upper limits in elevation wetlands can only increase as fast as SLRs, otherwise they are no longer wetlands. It is equally challenging to match appropriate density and sedimentation measures. Using exclusively surface measures of density and short-term measures of sedimentation results in the highest rates of C burial, while rates based on total C mass accumulated over a 1000-yr interval (as indicated by a dated horizon) are often more than an order of magnitude lower (e.g. for seagrass bed C burial is estimated at about 138 gC m2 yr1 over the short term, but only 7–27 gC m2 yr1 over the past 1000 yr [37]).
Vulnerability of vegetated coastal systems to climate change and SLR Until recently, the major factor influencing C sequestration in coastal vegetated wetlands has been the decline in their areal extent as a result of human activities such as dredging, diking and filling (e.g. much of Boston, Massachusetts, USA was originally wetland). Conversion for www.sciencedirect.com
mariculture has also played a large role in decreasing the global wetland area. For seagrass beds a major factor affecting their areal extent is eutrophication and other processes that decrease water clarity and hence light availability. Three other factors are likely to play an increasingly important role in affecting C sequestration in wetlands in the future: SLR, a rise in global temperature, and armoring of estuarine shorelines. SLR
SLR will directly affect rates of wetland accretion, transgression, and progradation. SLR will play a role in the distribution of seagrass beds also, as they will be limited to waters shallow enough to provide adequate light. Transgression shoreward will depend on their ability to invade areas that are currently intertidal or colonized with intertidal wetland vegetation. According to the Redfield model (Figure 2) wetlands prograde into the estuary with rising sea level if sediment inputs are sufficient to allow infilling of open water areas to intertidal depths near or above MSL. Most rivers today are heavily controlled by dams, and sediment export is a fraction of what it was a century or more ago [38–41]. Consequently, rising sea level will likely increase wave energy (function of depth and fetch) and result in wetland shoreline erosion in the future, as opposed to progradation (Figure 3). The Redfield model also shows rising water level to result in shoreward transgression of wetlands. Only wetland plants can survive under tidal flooding regimes and saline waters, so as water level rises, wetland plants will transgress into adjacent terrestrial habitats. Increasingly however we find that estuarine shorelines are armored, especially in densely populated areas, and the rate of armoring is likely to increase rapidly as sea level continues to increase (Figure 4). Consequently wetlands will not be able to expand into what are now terrestrial lands as SLRs because armored shorelines prevent that migration. Increased rates of SLR may compromise the ability of tidal freshwater marshes to accrete vertically, which could result in inundation, loss of area and decreased C sequestration. As in saline tidal wetlands, accretion is driven primarily by deposition of sediments (watershed-derived) and autochthonous organic matter produced by wetland plants themselves [42,43,44]. Salinity stress of freshCurrent Opinion in Environmental Sustainability 2012, 4:186–194
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Figure 3
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Sept 2011
Oct 2011
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Photomosaic of the potential effects of SLR on progradation. With excessive rates of SLR and inadequate sediment supplies increased wave energy from deepening open waters will result in creekbank erosion, slumping, death, and decomposition of past stores of wetland vegetation carbon. Mosaic and interpretation courtesy of James Morris & Karen Sundberg, University of South Carolina.
water plants will decrease primary production and organic matter accumulation [45,46]. Saltwater intrusion will also alter major anaerobic organic matter remineralization pathways and the release of the potent greenhouse gas, CH4 [47] as microbes shift from a primarily methanogenic pathway to sulfate reduction. Craft [36] showed a positive relationship between salinity and decomposition in freshwater wetland sediments. Weston et al. [47,48] showed that fluxes of both CO2 and CH4 actually increased following salinity intrusion, which results in a positive feedback to the climate system. There is increased evidence that salinity intrusion also influences N cycling in oligohaline marshes, influencing rates of NH4+ release, denitrification and dissimilatory nitrate reduction (DNRA) [49,50]. It remains to be seen whether salinity intrusion also influences the release of N2O, a potent greenhouse gas, from freshwater sediments. Enhanced flux of greenhouse gases is a positive feedback to the climate system. Thus salinity intrusion into freshwater tidal wetlands will alter the C balance of these Current Opinion in Environmental Sustainability 2012, 4:186–194
systems through decreased plant production, as well as increased OC remineralization. The enhanced fluxes of CH4 and potential N2O will be a positive feedback on climate change. Recent simulation modeling research has shown a complex series of interactions and nonlinear feedbacks between marsh elevation, flooding depth, plant growth, sediment supply, tidal range and the rate of SLR that control intertidal wetland accretion and the ability of intertidal wetlands to adjust to changes in rates of SLR and to persist over thousands of years [51]. It has been shown experimentally and through monitoring that wetland plant growth is greater during years with above average sea level and that sediment trapping and organic C accretion are similarly enhanced [42]. Morris found experimentally that marsh plant productivity is defined by a parabolic relation with water level. Provided that marsh elevation is within a supraoptimal range of the vegetation, marsh elevation is in a dynamic equilibrium www.sciencedirect.com
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Figure 4
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Normal and arrested transgression. The picture on left shows the normal pattern of intertidal wetlands invading adjacent low-lying terrestrial habitats in conjunction with rising sea level. Many urbanized regions around the world, however, are armoring their shorelines to prevent rising sea levels from flooding their properties. As a result intertidal wetlands will no longer be able to transgress. This will result in a long-term decline in area of intertidal wetlands around the globe. Photos courtesy of Karen Sundberg, University of South Carolina.
Global temperature change That coastal vegetated wetlands can sequester C ultimately depends on the positive balance between gross www.sciencedirect.com
primary production and community respiration — or positive NEP [14]. Here we ask how the NEP balance of vegetated wetlands will be affected by one aspect of global climate change — warming. There have been few experimental field-warming studies examining the comparative response of production and respiration Figure 5
Threshold relative SLR rate (mm/yr)
with sea level [52]. Kirwan et al. [51] codified Morris’s plant growth–supraoptimal water level relation and added the additional variables sediment supply (concentration) and tidal range to further explore limits on the adaptability of coastal wetlands to rising sea level. Simulation results suggest that there is a maximum rate of SLR conducive to wetland persistence that varies by tidal range and suspended sediment availability (Figure 5). Critical rates of SLR vary directly with sediment concentration. At low sediment availability, even low rates of SLR will result in wetland failure. There is also a positive relation between tidal range and the threshold rate of SLR. Macrotidal marshes can adapt much better than microtidal ones, but the former are more sensitive to sediment supply. Thus wetlands with high tidal ranges and suspended sediments are extremely resilient to SLR (e.g. the Yangtze River delta) while wetlands with small tidal ranges and low suspended solids are highly vulnerable (e.g. interior wetlands of the Mississippi River delta). In a future where the rate of SLR could exceed 20 mm/yr if there is significant contribution from ice-sheet melting [53], Kirwan’s modeling suggests that wetlands will survive only in macrotidal (>3 m) regions with sediment concentrations in excess of 30 mg/l. These results agree with observations of rapid wetland failure in coastal Louisiana and the peaty intertidal wetlands in Plum Island Sound, the largest wetland-dominated estuary in New England (Massachusetts, USA).
140 1m 2m 3 m tidal range
120 100 80 60 40 20 0
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Predicted threshold rates of sea level rise for a variety of suspended sediment concentrations and tidal ranges above which intertidal wetlands will not persist. For a given tidal range, systems with higher suspended sediment loads will be able to persist at higher rates of SLR. For a given sediment concentration, wetland systems with higher tidal ranges will be better able to persist at higher rates of SLR. However, macrotidal wetlands are more sensitive to sediment supply than microtidal ones.Adapted from Kirwan et al. [51]. Current Opinion in Environmental Sustainability 2012, 4:186–194
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components of metabolism in coastal systems, thus we must draw on theoretical considerations to predict response. Drawing on understanding developed from analyses of terrestrial, oceanic and estuarine systems, as well as freshwater mesocosm warming experiments, we see strong evidence that warming should decrease potential C storage in all types of ecosystems. Recent integrated empirical and modeling work suggests that the general metabolic theory of ecology provides a theoretical foundation for predicting the effects of rising temperature on all aspects of metabolism, from organism physiology, to community level processes, and even to the metabolic balance of oceans [54,55,58]. The basic metabolic theory of ecology model can be written as: P ¼ ½RM 2=3 eðE=kT Þ ; where P is production or respiration, R is reactants, M is mass of organism, and e(E/kT) is the Boltzmann factor or Van’t Hoff-Arrhenius equation [59]. E is the activation energy, k is the Boltzmann constant and T is absolute temperature [60]. What is interesting for our analysis is that photosynthetic reactions have activation energies that are substantially less than those for aerobic respiration. While gradients in organism size will affect overall patterns along latitudinal gradients [54], most metabolisms in vegetated wetlands are either autotrophic (little gradient in size of wetland plants) or microbial (no latitudinal gradients in size). In a model that predicted C turnover in three important pools (autotrophs, decomposers, labile soil C from a variety of terrestrial and aquatic systems), it was shown that rates controlled by respiration (such as decomposition) would increase 16 over a 0–308C temperature range due to activation energies of ATP synthesis reactions, while rates controlled by photosynthesis (such as NPP) would increase only 4 over the same temperature range because of the activation energy associated with Rubisco carboxylation reaction [54]. Warming experiments with freshwater mesocosms confirmed theoretical predictions derived from the MTE and suggest that NEP will be reduced 13% under the A1B IPCC 2007 warming scenario [57]. These results are important because they suggest that because of the difference between the temperature effect on primary production and respiration, global warming will quantitatively decrease whole ecosystem CO2 flux, NEP, labile C storage in sediments and by inference C burial. The theoretical basis for these suggestions is robust and well grounded with empirical observations that hold across a variety of terrestrial and aquatic systems. We lack models to predict the carbon sink capacity of vegetated coastal ecosystems [61], thus these theoretical predictions of the differential effect of temperature on P and R and hence NEP are fundamental to understanding how the importance of these systems as a future global C sink may change. Current Opinion in Environmental Sustainability 2012, 4:186–194
Scaling to the globe and considering current and future distribution of temperature over latitudinal gradients, model predictions show that a 18C increase in temperature over the growing season will have a 4 larger effect in high latitude wetlands (T near 08C) than in tropical mangroves and seagrasses (T near 258C) [54,56]. Thus we can expect C burial and sequestration to decrease as a result of global warming in all vegetated coastal wetland systems, but more so in boreal and arctic regions than in tropical systems.
Land use change and population growth There is a direct positive relation between population growth and conversion of forest land to urban or agricultural land and nutrient loading of coastal waters [62,63]. Sediment erosion and export to the coastal zone is directly related to agricultural land area [62]. With the world’s population continuing to rapidly grow, we can expect increased demand for agricultural land and productivity. The implications of these trends for coastal wetland C sequestration are unclear as damming of rivers for flood control and water supply could negate the expected increases in sediment erosion from expanded agricultural lands that could help offset increasing rates of SLR. The increased nutrient runoff from food production and human waste is likely to further coastal eutrophication, which severely limits water transparency and growth and distribution of seagrass beds. It is difficult to imagine population growth and the resultant land use change not severely impacting overall C sequestration by coastal wetland vegetation.
Summary While there are great uncertainties in the absolute magnitude of the present day C sequestration rate of coastal vegetated wetlands (largely due to accuracy of area estimates and inadequate sampling of C burial within and across systems of the world), it is clear that C sequestration in these systems is great, rivals that of C burial associated with sediment deposition in the coastal zone [3], and is a significant and important term in the global C budget [5]. However, it is equally clear that the magnitude of this sink has decreased substantially over the past century [7] and with climate change and global population growth is likely to decrease even further. Declines in sink strength are primarily the result of intertidal wetlands inability to keep up with increasing rates of SLR, temperature increase negatively affecting NEP, and coastal armoring. For seagrass beds estuarine eutrophication decreases light availability and areal expanse of seagrass beds declines. Considerable additional research is critically needed to refine and better understand the relationships between wetland persistence and global change. Most research on intertidal wetland persistence has been conducted in wetlands of North America and Europe. The extent to www.sciencedirect.com
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which models of marsh persistence apply to tropical mangrove systems remains to be determined. Finally global C models need to explicitly recognize the importance of C sequestration and that the trend in decreasing rates of sequestration that began over 100 yr ago will only accelerate in the future. This will represent an additional flux of C to the atmosphere.
Acknowledgements This work was supported by NSF grants BCS-0709685, OCE-0423565 and OCE-1058747 to CSH, NASA grant NNX10AU06G to W-JC and CSH, and NSF grant OCE-0752110 to W-JC.
References and recommended reading Papers of particular interest, published within the period of review, have been highlighted as: of special interest of outstanding interest 1.
Redfield AC: The ontogeny of a salt marsh estuary. In Estuaries. Edited by Lauff G. AAAS; 1967. Publ. No. 83.
2.
Chmura GL, Anisfeld SC, Cahoon DR, Lynch JC: Global carbon sequestration in tidal, saline wetland soils. Global Biogeochem Cycles 2003, 17:1111. Perhaps the first paper to examine the important role that tidal, saline wetlands play in global carbon cycles. There was an attempt to quantify C sequestration in these systems, but was compromised by inadequate information on the areal extent of wetlands around the globe.
3.
Duarte CM, Middelburg JJ, Caraco N: Major role of marine vegetation on the oceanic carbon cycle. Biogeosciences 2005, 2:1-8. An important, seminal paper integrating new estimates of C sequestration in vegetated coastal wetlands with geochemical budgets of global rates of sediment and C export from land and burial in the coastal and open ocean.
4.
Kristensen E, Bouillon S, Dittmar T, Marchand C: Organic carbon dynamics in mangrove ecosystems: a review. Aquat Bot 2008, 89:201-219. A paper that reviews a variety of processes that affect the production, decomposition, export and transformations of organic carbon in mangrove systems. Important in considering the possible response of mangroves to global change.
5. Laffoley D, Grimsditch G: The Management of Natural Coastal Carbon Sinks. Gland, Switzerland: IUCN; 2009. One of the first compilation of articles quantifying C sink strength of various coastal systems and the importance of managing these systems in the future in order to preserve their importance in the global C cycle. McLeod E, Chmura GL, Bouillon S, Salm R, Bjo¨rk M, Duarte CM, Lovelock CE, Schlesinger WH, Silliman BR: A blueprint for blue carbon: toward an improved understanding of the role of vegetated coastal habitats in sequestering CO2. Front Ecol Environ 2011, 9:552-560. One of the more recent estimations of C sequestration by vegetated wetland that describes some of the most important processes affecting the magnitude of C burial.
6.
Nellemann C, Corcoran E, Duarte C, Valde´s L, Young CD, Fonseca L, Grimsditch G: Blue Carbon — The Role of Healthy Oceans in Binding Carbon. GRID-Arendal: United Nations Environment Programme; 2009. Another paper from 2009 that highlights the role of the ocean ecosystems in maintaining our climate. This work was to assist policy makers to develop an ocean agenda for responding to global change.
7.
8.
Schlesinger W: Biogeochemistry: An Analysis of Global Change. edn 2. San Diego, CA: Academic Press; 1997.
9.
Alongi DM: Present state and future of the world’s mangrove forests. Environ Conserv 2002, 29:331-349.
10. FAO: The World’s Mangroves 1980–2005. Rome, Italy: FAO; 2007. www.sciencedirect.com
11. Spalding M, Kainuma M, Collins L: World Atlas of Mangroves. London: EarthScan; 2010. 12. Valiela I, Bowen JL, York JK: Mangrove forests: one of the world’s threatened major tropical environments. Bioscience 2001, 51:807-815. 13. Waycott M, Duarte CM, Carruthers TJB, Orth RJ, Dennison WC, Olyarnik S, Calladine A, Fourqurean JW, Heck KL, Hughes AR et al.: Accelerating loss of seagrasses across the globe threatens coastal ecosystems. Proc Natl Acad Sci U S A 2009, 106:12377-12381. A major examination of the downward trajectory for C sequestration by the world’s seagrass systems. 14. Odum EP: Fundamentals of Ecology. Saunders, NY: WB Saunders Co.; 1971. 15. Cai W-J: Estuarine and coastal ocean carbon paradox: CO2 sinks or sites of terrestrial carbon incineration? Ann Rev Mar Sci 2011, 3:123-145. A study that highlights the interconnectedness of coastal systems, transport of organic and inorganic carbon from land to the sea, and how that affects CO2 sink-source strength of the various components. 16. Duarte CM, Cebria´n J: The fate of marine autotrophic production. Limnol Oceanogr 1996, 41:1758-1766. 17. Meybeck M: Carbon, nitrogen, and phosphorus transport by world rivers. Am J Sci 1982, 282:401-450. 18. Meybeck M: C, N, P and S in rivers: from sources to global inputs. In Interaction of C, N, P and S Biogeochemical Cycles and Global Change. Edited by Wallast R, Mackenzie FT, Chou L. Springer-Verlag; 1993:163-193. 19. Rickey J, Victoria R, Mayorga E, Martinelli L: Integrated analysis in a humid tropical region — the Amazon Basin. In Vegetation, Water, Humans, and the Climate. Edited by Kabat P, Claussen M, Dirmeyer PA, Gash JHC, Bravo de Guenni L, Meybeck Sr M, Pielke R, Vo¨ro¨smarty CJ, Hutjes RWA, Lu¨tkemeier S. Springer; 2004: 415-428. 20. Seitzinger SP, Harrison JA, Dumont E, Beusen AHW, Bouwman AF: Sources and delivery of carbon, nitrogen, and phosphorus to the coastal zone: An overview of Global Nutrient Export from Watersheds (NEWS) models and their application. Global Biogeochem Cycles 2005, 19:GB4S01. 21. Smith SV, Hollibaugh JT: Coastal metabolism and the oceanic organic carbon balance. Rev Geophys 1993, 31:75-89. 22. Smith SV, Swaney DP, Talaue-McManus L, Bartley JD, Sandhei PT, McLaughlin CJ, Dupra VC, Crossland CJ, Buddemeier RW, Maxwell BA et al.: Humans, hydrology, and the distribution of inorganic nutrient loading to the ocean. Bioscience 2003, 53:235-245. 23. Ludwig W, Probst J-L, Kempe S: Predicting the oceanic input of organic carbon by continental erosion. Global Biogeochem Cycles 1996, 10:23-41. 24. Borges AV, Delille B, Frankignoulle M: Budgeting sinks and sources of CO2 in the coastal ocean: diversity of ecosystems counts. Geophys Res Lett 2005, 32:L14601. 25. Hogarth P: The Biology of Mangroves. Oxford: Oxford University Press; 1999. 26. Mann KH: Ecology of Coastal Waters, A Systems Approach. Berkeley, CA: University of California Press; 1982. 27. Pennings S, Alber M, Alexander C, Booth M, Burd A, Cai W-J, Craft C, DePratter C, Iorio DD, Hopkinson C et al.: South Atlantic tidal wetlands. In Wetland Habitats of North America. Edited by Batzer D, Baldwin A. University of California Press; 2011: 45-62. 28. Hopkinson CS: Patterns of organic carbon exchange between coastal ecosystems: The mass balance approach in salt marsh ecosystems. In Coastal-Offshore Ecosystem Interactions. Edited by Jansson B-O. Springer-Verlag; 1988:122-154. [Lecture Notes on Coastal and Estuarine Studies, vol 22.] 29. Redfield AC: The ontogeny of a salt marsh. Science 1965, 147:50-55. Current Opinion in Environmental Sustainability 2012, 4:186–194
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30. McKee KL, Cahoon DR, Feller IC: Caribbean mangroves adjust to rising sea level through biotic controls on change in soil elevation. Global Ecol Biogeogr 2007, 16:545-556. This paper shows that mangroves adjust to sea level mainly through subsurface accumulation of refractory roots. This paper is relevant for predicting the effects of sea level rise on mangrove ecosystems. 31. Redfield AC: Development of a New England salt marsh. Ecol Monogr 1972, 42:210-237. 32. Giri C, Ochieng E, Tieszen LL, Zhu Z, Singh A, Loveland T, Masek J, Duke N: Status and distribution of mangrove forests of the world using earth observation satellite data. Global Ecol Biogeogr 2011, 20:154-159. 33. Woodwell GM, Rich PH, Mall CSA: Carbon in estuaries. In Carbon in the Biosphere. Edited by Woodwell GM, Pecari EV. U.S. AEC, 22, l. 240; 1973. 34. Charpy-Roubaud C, Sournia A: The comparative estimation of phytoplanktonic and microphytobenthic production in the oceans. Mar Microbial Food Webs 1990, 4:31-57.
implications of relative sea-level rise. Estuaries Coasts 2007, 30:214-225. A paper that explores controls on mechanisms controlling marsh productivity and survival under climate change and sea level rise. 47. Weston N, Vile M, Neubauer S, Velinsky D: Accelerated microbial organic matter mineralization following salt-water intrusion into tidal freshwater marsh soils. Biogeochemistry 2011, 102:135-151. An important paper examining the effects of sea level rise and salinity on greenhouse gas emissions from tidal freshwater marsh systems. 48. Weston NB, Dixon RE, Joye SB: Ramifications of increased salinity in tidal freshwater sediments: geochemistry and microbial pathways of organic matter mineralization. J Geophys Res 2006, 111:G01009. 49. Giblin A, Weston N, Banta G, Tucker J, Hopkinson C: The effects of salinity on nitrogen losses from an oligohaline estuarine sediment. Estuaries Coasts 2010, 33:1054-1068.
35. Green EP, Short FT: World Atlas of Seagrasses. Berkeley: University of California Press; 2003.
50. Weston N, Giblin A, Banta G, Hopkinson C, Tucker J: The effects of varying salinity on ammonium exchange in estuarine sediments of the parker river, Massachusetts. Estuaries Coasts 2010, 33:985-1003.
36. Craft CB: Freshwater input structures soil properties, vertical accretion, and nutrient accumulation of Georgia and U.S. tidal marshes. Limnol Oceanogr 2007, 52:1220-1230. This paper describes regional patterns of vertical accretion and C storage in fresh and brackish water marshes of the U.S. It looks at the effects of various aspects of climate change, such as saltwater intrusion, on rates.
51. Kirwan ML, Guntenspergen GR, D’Alpaos A, Morris JT, Mudd SM, Temmerman S: Limits on the adaptability of coastal marshes to rising sea level. Geophys Res Lett 2010:37. A modeling examination on how coastal marshes will adapt to different rates of sea level rise and the role of biological process feedbacks that influence their physical environment.
37. Kennedy H, Beggins J, Duarte CM, Fourqurean JW, Holmer M, Marba` N, Middelburg JJ: Seagrass sediments as a global carbon sink: Isotopic constraints. Global Biogeochem Cycles 2010, 24:GB4026.
52. Morris JT: Ecological engineering in intertidial saltmarshes. In Lagoons and Coastal Wetlands in the Global Change Context: Impacts and Management Issues. Edited by Viaroli P, Lasserre P, Campostrini P. Netherlands: Springer; 2007:161-168. [Dumont HJ (Series Editor): Developments in Hydrobiology, vol 192.]
38. Bianchi TS, Allison MA: Large-river delta-front estuaries as natural ‘recorders’ of global environmental change. Proc Natl Acad Sci U S A 2009, 106:8085-8092. 39. Milliman JD, Haq BU: Sea-Level Rise and Coastal Subsidence: Causes, Consequences and Strategies. Amsterdam: Kluwer; 1996. 40. Milliman JD, Meade R: World-wide delivery of river sediments to the oceans. J Geol 1983, 91:1-21. 41. Syvitski JPM, Kettner AJ, Overeem I, Hutton EWH, Hannon MT, Brakenridge GR, Day J, Vorosmarty C, Saito Y, Giosan L et al.: Sinking deltas due to human activities. Nat Geosci 2009, 2:681-686. 42. Morris JT, Sundareshwar PV, Nietch CT, Kjerfve B, Cahoon DR: Responses of coastal wetlands to rising sea level. Ecology 2002, 83:2869-2877. One of the seminal papers where Morris describes an optimal relationship between water level and tidal marsh grass productivity that is a homeostatic mechanism whereby salt marshes are able to keep up with sea level rise (within limits). 43. Neubauer SC: Contributions of mineral and organic components to tidal freshwater marsh accretion. Estuarine Coastal Shelf Sci 2008, 78:78-88. A paper that explores some of the mechanisms contributing to accretion in tidal freshwater marshes and the relative importance of sediment and organic matter in contributing to accretion. 44. Reed DJ: The response of coastal marshes to sea-level rise: survival or submergence? Earth Surf Process Landforms 1995, 20:39-48. A global perspective that focuses on natural cycles of growth and decline of river delta systems and their intertidal marshes and bays. This is especially relevant to coastal Louisiana and management of the Mississippi River. 45. McKee KL, Mendelssohn IA: Response of a freshwater marsh plant community to increased salinity and increased water level. Aquat Bot 1989, 34:301-316. Paper discusses some of the mechanistic responses of freshwater plants to variations in salinity and water level. A critical paper for understanding how systems will respond to global change. 46. Spalding E, Hester M: Interactive effects of hydrology and salinity on oligohaline plant species productivity:
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53. Rahmstorf S: A semi-empirical approach to projecting future sea-level rise. Science 2007, 315:368-370. 54. Allen AP, Gillooly JF, Brown JH: Linking the global carbon cycle to individual metabolism. Funct Ecol 2005, 19:202-213. 55. Harris L, Duarte C, Nixon S: Allometric laws and prediction in estuarine and coastal ecology. Estuaries Coasts 2006, 29:340-344. A paper examining the comparative effects of temperature increase on ecosystem GPP, respiration, and net ecosystem production. Important for considering the effects of global warming on NEP and the potential for C sequestration in estuarine systems in the future. 56. Lo´pez-Urrutia A´, San Martin E, Harris RP, Irigoien X: Scaling the metabolic balance of the oceans. Proc Natl Acad Sci U S A 2006, 103:8739-8744. 57. Yvon-Durocher G, Jones JI, Trimmer M, Woodward G, Montoya JM: Warming alters the metabolic balance of ecosystems. Philos Trans R Soc B: Biol Sci 2010, 365:2117-2126. 58. Yvon-Durocher G, Allen AP, Montoya JM, Trimmer M, Woodward G: The temperature dependence of the carbon cycle in aquatic ecosystems. Adv Ecol Res 2010, 43:267-315. 59. Arrhenius S: Quantitative Laws in Biological Chemistry. London: G. Bell & Sons; 1915. 60. Gillooly JF, Brown JH, West GB, Savage VM, Charnov EL: Effects of size and temperature on metabolic rate. Science 2001, 293:2248-2251. 61. Duarte CM, Kennedy H, Marba` N, Hendriks I: Assessing the capacity of seagrass meadows for carbon burial: Current limitations and future strategies. Ocean Coast Manage, in press. 62. Hopkinson CS, Vallino J: The nature of watershed perturbations and their influence on estuarine metabolism. Estuaries 1995, 18:598-621. 63. Howarth RW, Anderson D, Church T, Greening H, Hopkinson C, Huber W, Marcus N, Naiman R, Segerson K, Sharpley A et al.: Clean Coastal Waters: Understanding and Reducing the Effects of Nutrient Pollution: Committee on the Causes Management of Eutrophication Ocean Studies Board Water Science Technology Board National Research Council. The National Academies Press; 2000.
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Carbon sequestration in wetland dominated coastal systems — a global sink of rapidly diminishing magnitude Charles S Hopkinson, Wei-Jun Cai and Xinping Hu Coastal vegetated wetlands have recently been identified as very important global C sinks but vulnerable to degradation by direct human alteration of their habitats. While their expanse is small globally, areal rates of C burial, or sequestration, are among the highest of Earth’s ecosystems. There is considerable uncertainty in the magnitude of total global sequestration in these systems for two reasons: poor estimates of their global areas and high variability and uncertainty in areal rates of burial between systems. The magnitude of C burial in vegetated coastal systems has been decreasing rapidly over the past century due primarily to human disturbances such as dredging, filling, eutrophication, and timber harvest. These systems continue to be lost globally at rates ranging from 1% to 7% annually. We find that climate change including global warming, human engineering of river systems, continued agricultural expansion, and sea level rise will also negatively impact C burial of coastal vegetated wetlands. A decrease in global C burial in these systems will ultimately exacerbate CO2 emissions, and further contribute to climate change in the future. Address Department of Marine Sciences, University of Georgia, Athens, GA 30602, USA Corresponding author: Hopkinson, Charles S ([email protected])
Current Opinion in Environmental Sustainability 2012, 4:186–194 This review comes from a themed issue on Carbon and nitrogen cycles Edited by Chen-Tung Arthur Chen and Dennis Peter Swaney Available online 25th April 2012 1877-3435/$ – see front matter # 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cosust.2012.03.005
Introduction With the evidence mounting rapidly for substantial changes to the global climate over the next century as a result of rising levels of atmospheric carbon dioxide (CO2) (IPCC), there has been considerable research over the past several decades to identify the natural carbon (C) sinks within the biosphere, both terrestrial and oceanic. Sinks that actually mitigate anthropogenic emissions of CO2 are the most important in arresting climate change. To mitigate emissions however sink strength must increase over time. Sink strength can increase either because the areal extent of those systems is increasing Current Opinion in Environmental Sustainability 2012, 4:186–194
over time or the absolute rate of sequestration increases over time. Furthermore, a reduced source due to anthropogenic or climate cause can also be considered as an increase in sink strength. Consequently much additional research has focused on understanding the feedbacks between sink strength and either climate change or other anthropogenic activities, such as land use change. One C sink that has received considerable attention over the past 10 years or so is the so-called coastal ‘blue carbon’ sink associated with organic carbon (OC) burial in seagrass meadows and intertidal wetlands (mangroves and intertidal marshes) [2,3,4,5,6,7]. There is strong agreement in all these studies that C sequestration rates per unit area are extremely high and attributable exclusively to C burial in sediments. Extrapolated to their global areal extent, C burial is estimated in excess of 100 Tg C yr1, which is almost 20 times greater than that of deep sea burial. It is similar in magnitude to C burial in depositional areas in non-vegetated portions of estuaries and the continental shelf [3] and comparable to that of terrestrial forest systems [8]. In recognition of the critical importance of coastal vegetated systems in global C sequestration, the recent ‘blue carbon’ papers have made strong pleas to restore and preserve these fragile systems, as historically they are being lost (converted to other types of systems) at rates between about 0.7% and 7% annually [6,9,10,11,12,13]. In this paper, we review the processes or C fluxes that are the basis of C sequestration in coastal vegetated wetlands and the mechanistic controls of those fluxes. We consider how climate change, sea level rise (SLR), and selected anthropogenic activities will likely impact those fluxes in the future. We do not discuss direct threats to these systems via land reclamation, filling, diking, conversion to other types of systems and the implications for C stores at the time of conversion, as these important issues have been covered well in other recent reviews (e.g. [5,6]).
The study system This treatise focuses on one community within estuarine ecosystems — the rooted macrophytes living intertidally or subtidally: mangroves, intertidal fresh, brackish and salt marshes and seagrass beds. It is important to put this community in a global perspective in order to appreciate their importance in the biosphere. Estuarine ecosystems cover less than 1% of the earth’s surface, but they are among the most productive [14]. They can be considered an ecotone — a transition zone www.sciencedirect.com
Carbon sequestration in coastal vegetated wetlands Hopkinson, Cai and Hu 187
Figure 1
Land
Estuary
Ocean Atmospheric Exchange
GPP Mangroves Tidal marshes Seagrass beds Animals Microbes Other Plants Terrestrial DIC & OC Sediment, particles, dissolved phase
Oceanic DIC & OC Sediment, particles, dissolved phase
Inorganic nutrients
Organic carbon - water column and sediments
Burial in sediments Current Opinion in Environmental Sustainability
Conceptual diagram of major C pools and fluxes in the coastal environment highlighting the burial flux of carbon produced by the autotrophic mangrove, tidal marsh, and seagrass bed subsystems. While some of the autotrophic C is buried, a large fraction is respired in the aquatic subsystem, which is often a site of intense carbon release to the atmosphere (see [15]).
between terrestrial/riverine systems and the ocean (Figure 1). Being at the mouths of rivers they have among the highest C and N loading rates on Earth, even compared to intensely fertilized agricultural systems. Terrestrial total OC flux has been estimated with various methods by several people as 460 Tg C yr1 (as high as 800 Tg C yr1) [15,16–22]. Rivers export their freshwater and associated OC to the ocean with a latitudinal distribution: about two thirds is supplied to lower latitude coastal oceans (0–308 — see [23,24]). Estuarine systems are not only deluged with OC from terrestrial systems, but they are also sites with exceptionally high rates of primary production, precisely because of where they lie at the interface between land and ocean: tidal mixing, high rates of N-loading, subtidal, intertidal and emergent primary producer organisms. Duarte et al. [3] estimated areal rates of gross primary production (GPP) to range between about 100 and 4000 gC m2 yr1. With the addition of rooted macrophyte production as a component of total estuarine production, estimates of net ecosystem production (NEP) are strongly autotrophic, ranging up to 1600 gC m2 yr1. Scaled to the globe, Duarte et al. [3] conclude that estuarine NEP is about 3000 Tg yr1. The fate of this carbon has yet to be clearly www.sciencedirect.com
resolved, but it is safe to conclude that only a small portion is buried within coastal sediments, while the majority of the excess is respired in downstream estuaries and the coastal ocean resulting in high rates of CO2 degassing [15]. It is reasonable to assume that burial of OC in sediments of coastal systems depends on the magnitude of NEP, therefore processes that affect NEP, such as GPP and R, regulate burial as well. Both mangroves and marshes are found in intertidal regions of estuaries between about mean sea level (MSL) and high, high water [25]. Seagrass beds are located subtidally in estuaries typically between mean lower low water and a depth corresponding to about the 1% light level [14,26]. Collectively these systems are found from arctic through tropical regions, with mangroves and marshes transitioning in the nearly subtropical region, as killing frosts limit the poleward extension of mangroves. Mangroves, marshes and seagrass beds are typically the most productive systems within estuaries [14]; they are autotrophic systems, with excess NEP either stored locally in sediments or exported to adjacent tidal creeks, bays and shelf or even the open ocean [15,27,28]. Here Current Opinion in Environmental Sustainability 2012, 4:186–194
188 Carbon and nitrogen cycles
Figure 2
Progradation - →
Transgression - ←
HW3 Sediment +/or
HW2
HIGH MARSH PEAT L
UPLAND
R
TIDA
INTE
T PEA
HW1 HW0
SAND
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Long-term pattern of intertidal wetland development associated with sea level rise and estuarine basin infilling as depicted by Redfield [1]. With increasing rates of sea level rise and diminished supply of sediments from river runoff and armoring of upland-estuary borders, we expect to see a long-term decline in the areal extent of intertidal wetlands over the next century.
we focus on the excess OC that is buried in the root zones and sediments of these macrophyte-dominated subsystems of estuaries. The development of intertidal wetlands in estuaries is dependent on rates of SLR, sediment supply and the ability to accrete vertically relative to SLR. The development of intertidal wetlands began only about 4–5kyr or so ago, when the rate of SLR slowed to about 5 mm yr1 following the last glacial period [29]. Rapid expansion occurred when SLR slowed to about 3.5 mm yr1 [30]. As described by Redfield in his classic study of Barnstable marsh in Massachusetts, USA. [1,31], wetlands, including mangroves and marshes, first formed along the intertidal fringe of estuaries and then transgressed landward in conjunction with sea level that was flooding ever higher into terrestrial systems (see Figure 2). At the same time, wetlands accreted vertically at a rate proportional to SLR through peat accumulation and trapping of mineral sediments from flood tide waters. Wetlands prograded into open water areas of estuaries as land or ocean derived sediments infilled bay bottoms to levels approaching MSL. The extent of seagrass beds in estuaries reflects fine sediment substrate availability and water shallow enough and clear enough to allow sufficient light to reach plants. SLR affects the areal extent of seagrass beds as it relates to light penetration. Seagrass beds can accrete vertically through peat and sediment accumulation and laterally as bay bottoms infill with oceanic or terrestrial sediments.
length worldwide. Their paper says it ‘would be surprising if estimates derived in this way were accurate within 50%’. While there have been some better estimates made in the last decade or so, most notably for mangroves [32], uncertainty in areal extent of these systems may be a factor of 2. As indicated earlier, the worldwide coverage by these three systems has decreased substantially over the past 100 yr. While rates of loss are apparently lower now than they were 50 yr ago, the global extent of these systems is still declining rapidly.
Current best estimates of C sequestration Estimates of long-term carbon burial in coastal vegetated systems span a very large range, reflecting huge differences in carbon burial from system to system and large differences in estimates of areal extent. Here we report the results of recent syntheses of carbon burial rate [6,15], which average out the large range seen in individual reports in the literature (Table 2). For instance, the range in carbon burial rate reported for fairly large expanses of salt marsh is from about 29 gC m2 yr1 [36] to 210 gC m2 yr1 [2], almost an order of magnitude. Order of magnitude ranges in carbon burial rate are also seen for mangroves and seagrass beds [6].
Calculating C sequestration: the processes leading to carbon burial and their controls Most estimates of C burial are simply the product of sediment carbon density and sedimentation rate (see e.g. Table 1
Estimates of the areal extent of mangroves, intertidal marshes and seagrass beds have a high degree of uncertainty (Table 1). Many papers still rely on and cite first approximations of areal extent made by Woodwell’s team in the mid-1960s [33]. These authors made quick estimations of areal extent by extrapolating a few fine scale estimates for a unit length of shoreline to total shoreline Current Opinion in Environmental Sustainability 2012, 4:186–194
Estimates of global area of coastal vegetated ecosystems System Mangroves Intertidal marshes Seagrass beds
Areal extent (km2)
Reference
138 000–200 000 km 2 200 000 km2 400 000 km 2 177 000–600 000 km 2
[3,7,25,32] [33] [7] [7,34,35]
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Table 2 Estimates of global carbon burial in coastal vegetated ecosystems System
Global area (km2)
Carbon burial rate (gC m2 yr1)
Global carbon burial (Tg C yr1)
Reference for burial rate
Mangroves Intertidal marshes Seagrass beds
138 000–200 000 200 000–400 000 177 000–600 000
226 39 57 6–218 24 138 38
31.0–45.2 11.4–87.0 24.4–82.8
[6] [6,15] [6]
[2]). Neither of these terms is straightforward to measure and interpret however. C burial is the net result of multiple processes operating in concert. There are strong gradients in carbon stocks and density throughout a sediment profile reflecting the processes of in situ organic matter (root and rhizome) production, surface deposition from flood tide waters, organic matter decomposition, compaction and erosion. Temperature, solar radiation, water level, flooding frequency and duration, and wave/current energy are factors that can influence rates of above processes. Sedimentation rate and the rate of increase in wetland/ seagrass bed elevation are also challenging to measure and differ greatly depending on approach. Short-term rates (daily/weekly) can be measured by surface deposition on plates, intermediate rates can be measured by sediment erosion tables and marker horizons, and longterm rates can be measured by isotopic profiles (e.g. 137Cs from 1963 on, 210Pb for the past 100 yr, and 14C for the past 10 000 yr). Typically, the longer the integration period, the lower the apparent sedimentation rate or the slower the rate of elevation rise. Perhaps the overriding factors controlling elevation increase in wetlands are crustal movements, compaction, and the rate of SLR. Water level and the rate of SLR set an upper limit on the distribution of intertidal wetlands. Once at their upper limits in elevation wetlands can only increase as fast as SLRs, otherwise they are no longer wetlands. It is equally challenging to match appropriate density and sedimentation measures. Using exclusively surface measures of density and short-term measures of sedimentation results in the highest rates of C burial, while rates based on total C mass accumulated over a 1000-yr interval (as indicated by a dated horizon) are often more than an order of magnitude lower (e.g. for seagrass bed C burial is estimated at about 138 gC m2 yr1 over the short term, but only 7–27 gC m2 yr1 over the past 1000 yr [37]).
Vulnerability of vegetated coastal systems to climate change and SLR Until recently, the major factor influencing C sequestration in coastal vegetated wetlands has been the decline in their areal extent as a result of human activities such as dredging, diking and filling (e.g. much of Boston, Massachusetts, USA was originally wetland). Conversion for www.sciencedirect.com
mariculture has also played a large role in decreasing the global wetland area. For seagrass beds a major factor affecting their areal extent is eutrophication and other processes that decrease water clarity and hence light availability. Three other factors are likely to play an increasingly important role in affecting C sequestration in wetlands in the future: SLR, a rise in global temperature, and armoring of estuarine shorelines. SLR
SLR will directly affect rates of wetland accretion, transgression, and progradation. SLR will play a role in the distribution of seagrass beds also, as they will be limited to waters shallow enough to provide adequate light. Transgression shoreward will depend on their ability to invade areas that are currently intertidal or colonized with intertidal wetland vegetation. According to the Redfield model (Figure 2) wetlands prograde into the estuary with rising sea level if sediment inputs are sufficient to allow infilling of open water areas to intertidal depths near or above MSL. Most rivers today are heavily controlled by dams, and sediment export is a fraction of what it was a century or more ago [38–41]. Consequently, rising sea level will likely increase wave energy (function of depth and fetch) and result in wetland shoreline erosion in the future, as opposed to progradation (Figure 3). The Redfield model also shows rising water level to result in shoreward transgression of wetlands. Only wetland plants can survive under tidal flooding regimes and saline waters, so as water level rises, wetland plants will transgress into adjacent terrestrial habitats. Increasingly however we find that estuarine shorelines are armored, especially in densely populated areas, and the rate of armoring is likely to increase rapidly as sea level continues to increase (Figure 4). Consequently wetlands will not be able to expand into what are now terrestrial lands as SLRs because armored shorelines prevent that migration. Increased rates of SLR may compromise the ability of tidal freshwater marshes to accrete vertically, which could result in inundation, loss of area and decreased C sequestration. As in saline tidal wetlands, accretion is driven primarily by deposition of sediments (watershed-derived) and autochthonous organic matter produced by wetland plants themselves [42,43,44]. Salinity stress of freshCurrent Opinion in Environmental Sustainability 2012, 4:186–194
190 Carbon and nitrogen cycles
Figure 3
Aug 2009 May 2009
Sept 2011
Oct 2011
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Photomosaic of the potential effects of SLR on progradation. With excessive rates of SLR and inadequate sediment supplies increased wave energy from deepening open waters will result in creekbank erosion, slumping, death, and decomposition of past stores of wetland vegetation carbon. Mosaic and interpretation courtesy of James Morris & Karen Sundberg, University of South Carolina.
water plants will decrease primary production and organic matter accumulation [45,46]. Saltwater intrusion will also alter major anaerobic organic matter remineralization pathways and the release of the potent greenhouse gas, CH4 [47] as microbes shift from a primarily methanogenic pathway to sulfate reduction. Craft [36] showed a positive relationship between salinity and decomposition in freshwater wetland sediments. Weston et al. [47,48] showed that fluxes of both CO2 and CH4 actually increased following salinity intrusion, which results in a positive feedback to the climate system. There is increased evidence that salinity intrusion also influences N cycling in oligohaline marshes, influencing rates of NH4+ release, denitrification and dissimilatory nitrate reduction (DNRA) [49,50]. It remains to be seen whether salinity intrusion also influences the release of N2O, a potent greenhouse gas, from freshwater sediments. Enhanced flux of greenhouse gases is a positive feedback to the climate system. Thus salinity intrusion into freshwater tidal wetlands will alter the C balance of these Current Opinion in Environmental Sustainability 2012, 4:186–194
systems through decreased plant production, as well as increased OC remineralization. The enhanced fluxes of CH4 and potential N2O will be a positive feedback on climate change. Recent simulation modeling research has shown a complex series of interactions and nonlinear feedbacks between marsh elevation, flooding depth, plant growth, sediment supply, tidal range and the rate of SLR that control intertidal wetland accretion and the ability of intertidal wetlands to adjust to changes in rates of SLR and to persist over thousands of years [51]. It has been shown experimentally and through monitoring that wetland plant growth is greater during years with above average sea level and that sediment trapping and organic C accretion are similarly enhanced [42]. Morris found experimentally that marsh plant productivity is defined by a parabolic relation with water level. Provided that marsh elevation is within a supraoptimal range of the vegetation, marsh elevation is in a dynamic equilibrium www.sciencedirect.com
Carbon sequestration in coastal vegetated wetlands Hopkinson, Cai and Hu 191
Figure 4
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Normal and arrested transgression. The picture on left shows the normal pattern of intertidal wetlands invading adjacent low-lying terrestrial habitats in conjunction with rising sea level. Many urbanized regions around the world, however, are armoring their shorelines to prevent rising sea levels from flooding their properties. As a result intertidal wetlands will no longer be able to transgress. This will result in a long-term decline in area of intertidal wetlands around the globe. Photos courtesy of Karen Sundberg, University of South Carolina.
Global temperature change That coastal vegetated wetlands can sequester C ultimately depends on the positive balance between gross www.sciencedirect.com
primary production and community respiration — or positive NEP [14]. Here we ask how the NEP balance of vegetated wetlands will be affected by one aspect of global climate change — warming. There have been few experimental field-warming studies examining the comparative response of production and respiration Figure 5
Threshold relative SLR rate (mm/yr)
with sea level [52]. Kirwan et al. [51] codified Morris’s plant growth–supraoptimal water level relation and added the additional variables sediment supply (concentration) and tidal range to further explore limits on the adaptability of coastal wetlands to rising sea level. Simulation results suggest that there is a maximum rate of SLR conducive to wetland persistence that varies by tidal range and suspended sediment availability (Figure 5). Critical rates of SLR vary directly with sediment concentration. At low sediment availability, even low rates of SLR will result in wetland failure. There is also a positive relation between tidal range and the threshold rate of SLR. Macrotidal marshes can adapt much better than microtidal ones, but the former are more sensitive to sediment supply. Thus wetlands with high tidal ranges and suspended sediments are extremely resilient to SLR (e.g. the Yangtze River delta) while wetlands with small tidal ranges and low suspended solids are highly vulnerable (e.g. interior wetlands of the Mississippi River delta). In a future where the rate of SLR could exceed 20 mm/yr if there is significant contribution from ice-sheet melting [53], Kirwan’s modeling suggests that wetlands will survive only in macrotidal (>3 m) regions with sediment concentrations in excess of 30 mg/l. These results agree with observations of rapid wetland failure in coastal Louisiana and the peaty intertidal wetlands in Plum Island Sound, the largest wetland-dominated estuary in New England (Massachusetts, USA).
140 1m 2m 3 m tidal range
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Predicted threshold rates of sea level rise for a variety of suspended sediment concentrations and tidal ranges above which intertidal wetlands will not persist. For a given tidal range, systems with higher suspended sediment loads will be able to persist at higher rates of SLR. For a given sediment concentration, wetland systems with higher tidal ranges will be better able to persist at higher rates of SLR. However, macrotidal wetlands are more sensitive to sediment supply than microtidal ones.Adapted from Kirwan et al. [51]. Current Opinion in Environmental Sustainability 2012, 4:186–194
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components of metabolism in coastal systems, thus we must draw on theoretical considerations to predict response. Drawing on understanding developed from analyses of terrestrial, oceanic and estuarine systems, as well as freshwater mesocosm warming experiments, we see strong evidence that warming should decrease potential C storage in all types of ecosystems. Recent integrated empirical and modeling work suggests that the general metabolic theory of ecology provides a theoretical foundation for predicting the effects of rising temperature on all aspects of metabolism, from organism physiology, to community level processes, and even to the metabolic balance of oceans [54,55,58]. The basic metabolic theory of ecology model can be written as: P ¼ ½RM 2=3 eðE=kT Þ ; where P is production or respiration, R is reactants, M is mass of organism, and e(E/kT) is the Boltzmann factor or Van’t Hoff-Arrhenius equation [59]. E is the activation energy, k is the Boltzmann constant and T is absolute temperature [60]. What is interesting for our analysis is that photosynthetic reactions have activation energies that are substantially less than those for aerobic respiration. While gradients in organism size will affect overall patterns along latitudinal gradients [54], most metabolisms in vegetated wetlands are either autotrophic (little gradient in size of wetland plants) or microbial (no latitudinal gradients in size). In a model that predicted C turnover in three important pools (autotrophs, decomposers, labile soil C from a variety of terrestrial and aquatic systems), it was shown that rates controlled by respiration (such as decomposition) would increase 16 over a 0–308C temperature range due to activation energies of ATP synthesis reactions, while rates controlled by photosynthesis (such as NPP) would increase only 4 over the same temperature range because of the activation energy associated with Rubisco carboxylation reaction [54]. Warming experiments with freshwater mesocosms confirmed theoretical predictions derived from the MTE and suggest that NEP will be reduced 13% under the A1B IPCC 2007 warming scenario [57]. These results are important because they suggest that because of the difference between the temperature effect on primary production and respiration, global warming will quantitatively decrease whole ecosystem CO2 flux, NEP, labile C storage in sediments and by inference C burial. The theoretical basis for these suggestions is robust and well grounded with empirical observations that hold across a variety of terrestrial and aquatic systems. We lack models to predict the carbon sink capacity of vegetated coastal ecosystems [61], thus these theoretical predictions of the differential effect of temperature on P and R and hence NEP are fundamental to understanding how the importance of these systems as a future global C sink may change. Current Opinion in Environmental Sustainability 2012, 4:186–194
Scaling to the globe and considering current and future distribution of temperature over latitudinal gradients, model predictions show that a 18C increase in temperature over the growing season will have a 4 larger effect in high latitude wetlands (T near 08C) than in tropical mangroves and seagrasses (T near 258C) [54,56]. Thus we can expect C burial and sequestration to decrease as a result of global warming in all vegetated coastal wetland systems, but more so in boreal and arctic regions than in tropical systems.
Land use change and population growth There is a direct positive relation between population growth and conversion of forest land to urban or agricultural land and nutrient loading of coastal waters [62,63]. Sediment erosion and export to the coastal zone is directly related to agricultural land area [62]. With the world’s population continuing to rapidly grow, we can expect increased demand for agricultural land and productivity. The implications of these trends for coastal wetland C sequestration are unclear as damming of rivers for flood control and water supply could negate the expected increases in sediment erosion from expanded agricultural lands that could help offset increasing rates of SLR. The increased nutrient runoff from food production and human waste is likely to further coastal eutrophication, which severely limits water transparency and growth and distribution of seagrass beds. It is difficult to imagine population growth and the resultant land use change not severely impacting overall C sequestration by coastal wetland vegetation.
Summary While there are great uncertainties in the absolute magnitude of the present day C sequestration rate of coastal vegetated wetlands (largely due to accuracy of area estimates and inadequate sampling of C burial within and across systems of the world), it is clear that C sequestration in these systems is great, rivals that of C burial associated with sediment deposition in the coastal zone [3], and is a significant and important term in the global C budget [5]. However, it is equally clear that the magnitude of this sink has decreased substantially over the past century [7] and with climate change and global population growth is likely to decrease even further. Declines in sink strength are primarily the result of intertidal wetlands inability to keep up with increasing rates of SLR, temperature increase negatively affecting NEP, and coastal armoring. For seagrass beds estuarine eutrophication decreases light availability and areal expanse of seagrass beds declines. Considerable additional research is critically needed to refine and better understand the relationships between wetland persistence and global change. Most research on intertidal wetland persistence has been conducted in wetlands of North America and Europe. The extent to www.sciencedirect.com
Carbon sequestration in coastal vegetated wetlands Hopkinson, Cai and Hu 193
which models of marsh persistence apply to tropical mangrove systems remains to be determined. Finally global C models need to explicitly recognize the importance of C sequestration and that the trend in decreasing rates of sequestration that began over 100 yr ago will only accelerate in the future. This will represent an additional flux of C to the atmosphere.
Acknowledgements This work was supported by NSF grants BCS-0709685, OCE-0423565 and OCE-1058747 to CSH, NASA grant NNX10AU06G to W-JC and CSH, and NSF grant OCE-0752110 to W-JC.
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