- Lead Authors:
- Lisamarie Windham-Myers, U.S. Geological Survey
- Wei-Jun Cai, University of Delaware
- Contributing Authors:
- Simone R. Alin, NOAA Pacific Marine Environmental Laboratory
- Andreas Andersson, Scripps Institution of Oceanography
- Joseph Crosswell, Commonwealth Scientific and Industrial Research Organization
- Kenneth H. Dunton, University of Texas, Austin
- Jose Martin Hernandez-Ayon, Autonomous University of Baja California
- Maria Herrmann, The Pennsylvania State University
- Audra L. Hinson, Texas A&M University
- Charles S. Hopkinson, University of Georgia
- Jennifer Howard, Conservation International
- Xinping Hu, Texas A&M University, Corpus Christi
- Sara H. Knox, U.S. Geological Survey
- Kevin Kroeger, U.S. Geological Survey
- David Lagomasino, University of Maryland
- Patrick Megonigal, Smithsonian Environmental Research Center
- Raymond G. Najjar, The Pennsylvania State University
- May-Linn Paulsen, Scripps Institution of Oceanography
- Dorothy Peteet, NASA Goddard Institute for Space Studies
- Emily Pidgeon, Conservation International
- Karina V. R. Schäfer, Rutgers University
- Maria Tzortziou, City University of New York
- Zhaohui Aleck Wang, Woods Hole Oceanographic Institution
- Elizabeth B. Watson, Drexel University
<b>Windham-Myers</b>, L., W.-J. <b>Cai</b>, S. R. Alin, A. Andersson, J. Crosswell, K. H. Dunton, J. M. Hernandez-Ayon, M. Herrmann, A. L. Hinson, C. S. Hopkinson, J. Howard, X. Hu, S. H. Knox, K. Kroeger, D. Lagomasino, P. Megonigal, R. G. Najjar, M.-L. Paulsen, D. Peteet, E. Pidgeon, K. V. R. Schäfer, M. Tzortziou, Z. A. Wang, and E. B. Watson, 2018: Chapter 15: Tidal wetlands and estuaries. In Second State of the Carbon Cycle Report (SOCCR2): A Sustained Assessment Report [Cavallaro, N., G. Shrestha, R. Birdsey, M. A. Mayes, R. G. Najjar, S. C. Reed, P. Romero-Lankao, and Z. Zhu (eds.)]. U.S. Global Change Research Program, Washington, DC, USA, pp. 596-648, https://doi.org/10.7930/SOCCR2.2018.Ch15.
Tidal Wetlands and Estuaries
SUPPORTING EVIDENCE
KEY FINDINGS
Key Finding 1
The top 1 m of tidal wetland soils and estuarine sediments of North America contains 1,886 ± 1,046 teragrams of carbon (Tg C) (high confidence, very likely).
Description of evidence base
Several sources were available to verify the extent of intertidal wetland and subtidal habitats in North America for Key Finding 1. First, the U.S Fish and Wildlife Service National Wetlands Inventory (USFWS NWI 2017) is a conservative but definitive source due to inclusion of tidal modifiers to clarify hydrology. Second, a synthesis of Mexican, Canadian, and U.S. saline coastal habitats was provided by the Commission for Environmental Cooperation (CEC 2016). For carbon density in intertidal wetland environments, a synthesis of datasets from tidal wetland habitats reviewed (Chmura et al., 2003; Ouyang and Lee 2014; Holmquist et al., 2018a) found a very narrow distribution measured in kilograms (kg; 27.0 ± 13.0 kg C per m3) in wetland carbon stocks across North American tidal wetlands, regardless of salinity or vegetation type, as did a national dataset review (28.0 ± 7.8; Nahlik and Fennessy 2016). A global synthesis (Sanderman et al., 2018) provided data to synthesize a new estimate for Mexico’s mangroves (31.8 ± 1.3 kg C per m3). For carbon stocks in seagrass environments, synthetic data from literature reviews reporting bulk density and organic carbon along 1-m profiles were used for coast-specific estimates: 2.0 ± 1.3 for the Atlantic Coast, 3.1 ± 2.4 for the Gulf of Mexico coast, 1.4 ± 1.2 for the Pacific Coast, and 2.0 for boreal and Arctic regions. For carbon density in estuarine open-water sediments, coastal regions played no clear role and geomorphic settings were not available (Smith et al., 2015), so a mean of 1.0 kg per m3 was chosen, using a literature-based average for total organic carbon (TOC) content (0.4% organic carbon; range 0.17% to 2%; Premuzic et al., 1982; Kennedy et al., 2010) coupled with a literature average of percentage of dry bulk densities (2.6 g C per cm3; Muller and Suess 1979).
Major uncertainties
Uncertainties vary for each subhabitat, and these data likely represent an underestimate of total stocks, which may be many meters deep. For tidal wetland soils to 1 m in depth, the primary uncertainty is in underestimates of mapped boundaries, with, for example, no accounting of freshwater tidal systems in either Mexico or Canada, and likely undercounting of freshwater tidal wetlands in the United States. For seagrass, the spatial data are conservative estimates of located and documented habitat, although seagrass populations can shift boundaries rapidly and potentially there are far more currently unmapped seagrass beds in North America. For estuarine spatial data, the boundaries are constrained by bathymetry maps, which generally are more uncertain in higher latitudes. In contrast, carbon densities have narrow ranges in tidal wetland and estuarine soils but a skewed representation in seagrass soils, a difference which may be due to limited sampling in northern latitudes.
Assessment of confidence based on evidence and agreement, including short description of nature of evidence and level of agreement
There is theoretical and empirical convergence on tidal marsh carbon densities but a likely bias to underrepresenting tidal freshwater habitats. Further, seagrass carbon densities show a wider range and an apparent latitudinal gradient of decreasing carbon density from tropical to temperate environments. Geomorphic variability (e.g., shallow waters versus fjords) in estuarine sediments may reduce uncertainty in stock assessments, but map layers are not available for North America.
Estimated likelihood of impact or consequence, including short description of basis of estimate
The likely impact of information is high because it has not been synthesized previously at the continental scale.
Summary sentence or paragraph that integrates the above information
For Key Finding 1, although sediment carbon densities in tidal wetlands are high with a narrow range and carbon densities in subtidal habitats are substantially lower with a wider range, there are still underrepresented samples from high-latitude regions, especially tidal forested wetlands and subtidal seagrasses. Further, the data reported thus far are limited to documented tidal habitats, although there is an appreciation that large areas are likely missing for freshwater tidal marsh and for seagrass extent.
Key Finding 2
Soil carbon accumulation rate (i.e., sediment burial) in North American tidal wetlands is currently 9 ± 5 Tg C per year (high confidence, likely), and estuarine carbon burial is 5 ± 3 Tg C per year (low confidence, likely).
Description of evidence base
Carbon burial, which accounts for all carbon accumulated in coastal sediments over an annual time period, has been documented for Key Finding 2, with geological approaches in multiple studies. Accumulation of carbon stock over a period of time using a marker horizon is relevant to specific periods of time by the method used (e.g., recent years, marker horizons, and radioisotope tracers of different decay rates). The data reported here refer to isotopes of cesium (137Cs) and lead (210Pb) dates alone, thus representing long-term average annual accretion rates for the past 50 years (since 1963). Rates of burial (Ouyang and Lee 2014; n = 125 samples) provide a range for comparison with other reviews that do account for mangrove subhabitats. No significant differences in carbon burial are detected for habitat types by salinity or vegetation type when comparing with Chmura et al. (2003) or with Breithaupt et al. (2014). Estuarine carbon burial is estimated for CONUS using the model of Herrmann et al. (2015) and scaled to all of North America using estimates of estuarine area.
Major uncertainties
Carbon burial rate is a bulk measure of multiple processes, both old and new carbon inputs as well as both autochthonous and allochthonous sources. As such, carbon burial through those processes has varied drivers, with different dominating processes across the landscape. Overestimation is possible when accretion of mineral sediment brings lower carbon densities than equilibrium conditions. Underestimates are possible when accretion is reported at historic rates and not adjusted for current rates of sea level rise. Mapped areas are a likely underestimate because they do not include freshwater tidal marshes in Canada or Alaska. Further, high uncertainties are associated with wide ranges of rates through different dating approaches. Estuarine carbon burial rate uncertainties stem from errors in the model of Herrmann et al. (2015) and, more importantly, the scaling of CONUS results to all of North America. Particularly problematic is the lack of rigorous mapping of estuarine extent outside of CONUS.
Assessment of confidence based on evidence and agreement, including short description of nature of evidence and level of agreement
Because mapping limitations and 50-year averages of tidal wetland carbon accumulation are inferred rather than being the current rates under accelerated sea level rise, these estimates likely are lower than the actual rates of burial. Thus, while these data represent measured rates, this analysis relies on a fairly small range of locations and a small subset of available published data. Estuarine burial rates are not confident because Canada and Mexico have limited data applicable to the modeling strategy of Herrmann et al. (2015).
Estimated likelihood of impact or consequence, including short description of basis of estimate
The likely impact of the information on tidal wetland and estuarine burial is high, as it has not yet been synthesized at the continental scale.
Summary sentence or paragraph that integrates the above information
For Key Finding 2, burial of carbon sourced from within wetlands and from terrestrial sources is similar among regions and wetland types, driven primarily by accretion rates, which are tied to geomorphic feedbacks with sea level rise. Burial of carbon in estuaries is linked most closely to residence time and total nitrogen input.
Key Finding 3
The lateral flux of carbon from tidal wetlands to estuaries is 16 ± 10 Tg C per year for North America (low confidence, likely).
Description of evidence base
In Key Finding 3, 16 studies were conducted to quantify the lateral flux of organic carbon (12 studies) and inorganic carbon (4 studies) from tidal wetlands to estuaries at individual locations. The organic carbon flux studies are summarized in Herrmann et al. (2015) and the inorganic carbon flux studies are summarized in Najjar et al. (2018). These studies were scaled to all of North America using estimates of tidal wetland area.
Major uncertainties
The major uncertainty in this Key Finding is the limited spatial and temporal extents of the 16 individual flux measurements. Tidal wetlands are highly heterogeneous and vary in their processing of carbon on a wide variety of timescales. Hence, tidal wetlands are likely to have been undersampled in terms of lateral exchanges. However, tidal wetlands consistently export carbon and the range of estimates is less than an order of magnitude.
Assessment of confidence based on evidence and agreement, including short description of nature of evidence and level of agreement
The low confidence is due to the limited number of measurements and time periods. There is appreciation, however, that at a continental scale, there is a strong likelihood that tidal wetlands export carbon to estuaries, although the magnitude of the flux is highly uncertain.
Estimated likelihood of impact or consequence, including short description of basis of estimate
This flux represents 60% (best estimate) of the net uptake of atmospheric carbon by tidal wetlands. Per knowledge gained, this is the first such estimate for North America.
Summary sentence or paragraph that integrates the above information
For Key Finding 3, there is enough information to make a first-order estimate of the flux of carbon from tidal wetlands to estuaries for North America as a whole, and there is high confidence in the order of magnitude of the flux. The high heterogeneity of tidal wetland systems and limited field data prevent a more accurate estimate of the flux.
Key Finding 4
In North America, tidal wetlands remove 27 ± 13 Tg C per year from the atmosphere, estuaries outgas 10 ± 10 Tg C per year to the atmosphere, and the net uptake by the combined wetland-estuary system is 17 ± 16 Tg C per year (low confidence, likely).
Description of evidence base
The uptake of atmospheric carbon dioxide (CO2) by tidal wetlands is assessed for Key Finding 4 by net ecosystem exchange (NEE) estimates from eddy covariance measurements. It is similar to an alternative estimate of uptake that assumes uptake as the sum of burial (8 Tg C) and lateral export (16 Tg C). Burial and lateral exports are discussed in the supporting evidence for Key Findings 2 and 3. Estuarine outgassing is based on studies of individual estuary summaries (Chen et al., 2013) and estuarine areas (Laruelle et al., 2013). The flux of the combined system is a simple sum of the fluxes from tidal wetlands and estuaries and compounded error.
Major uncertainties
The major uncertainties in this Key Finding are the limited spatial and temporal extents of tidal wetland atmospheric flux measurements, burial, lateral flux, and estuarine outgassing measurements. Estuarine outgassing uncertainties also stem from the low spatial resolution of the datasets used to estimate areas.
Assessment of confidence based on evidence and agreement, including short description of nature of evidence and level of agreement
There is low confidence on this calculation at the scale of North America. The low confidence is due to the residual between competing fluxes; on the one hand, there is strong likelihood that tidal wetlands take up CO2 from the atmosphere and estuaries outgas CO2 to the atmosphere and, on the other hand, that there is large uncertainty in the magnitude of each, assessments which stem from the high spatial and temporal variability of these systems and the limited field data. The fate of carbon released from tidal wetland degradation remains unknown.
Estimated likelihood of impact or consequence, including short description of basis of estimate
These are not major fluxes in the carbon budget of North America, but they are regionally important. Accounting for current knowledge, such estimates are the first for North America.
Summary sentence or paragraph that integrates the above information
For Key Finding 4, there is enough information to make first-order estimates of the exchange of atmospheric CO2 with tidal wetlands and estuaries for North America as a whole. The high heterogeneity of these systems and limited field data prevent a more accurate estimate of the flux.
Key Finding 5
Research and modeling needs are greatest for understanding responses to accelerated sea level rise; mapping tidal wetland and estuarine extent; and quantifying carbon dioxide and methane exchange with the atmosphere, especially in large, undersampled, and rapidly changing regions (high confidence, likely).
Description of evidence base
Tidal wetland and estuarine area are first-order drivers of the spatially integrated flux (e.g., in units of Tg C per year) of all carbon fluxes in these ecosystems. The lack of an accurate quantification of tidal wetland and estuarine area, particularly in Canada and Mexico, is thus a major gap in understanding the role of tidal wetlands and estuaries in the carbon cycling of North America. Carbon cycle research is largely motivated by the impact of greenhouse gases on climate and how climate change affects fluxes of these gases to the atmosphere from terrestrial and aquatic systems. However, the database of tidal wetland and estuarine CO2 and CH4 exchanges with the atmosphere is severely limited. In particular, direct estimates of these fluxes are rare. Furthermore, some of the most poorly sampled regions are those that are changing the most rapidly (e.g., the Arctic).
Major uncertainties
There are few uncertainties in Key Finding 5 because there is a clear lack of data on extent and atmospheric exchange.
Assessment of confidence based on evidence and agreement, including short description of nature of evidence and level of agreement
Confidence is high in Key Finding 5 because systematic studies (with error estimates) of tidal wetlands and estuaries are extremely limited. Very few direct estimates of exchanges of atmospheric CO2 and CH4 with tidal wetlands and estuaries exist. While research needs are present in other aspects of the tidal wetland and estuarine carbon cycling, these needs are unlikely to be more pressing than the needs for quantifying area and gas exchange with the atmosphere.
Estimated likelihood of impact or consequence, including short description of basis of estimate
Key Finding 5 is not an estimate but a recommendation. It could impact future research on tidal wetland and estuarine carbon cycling in North America.
Summary sentence or paragraph that integrates the above information
Key Finding 5 synthesizes the existing research on tidal wetland and estuarine carbon cycling in North America, providing a future direction for research in this area.
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