<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
All indications suggest that most North American coastal and estuarine environments, from Canada to Mexico, are changing rapidly as a result of global- and local-scale changes induced by climate alteration and human activities. The sustainability and quality of estuarine and intertidal wetland habitats, including the magnitude and direction of carbon fluxes, are uncertain, especially due to limited monitoring time series relevant to changing extents and conditions of these habitats. Simulation models have illustrated the long-term sensitivity of coastal carbon fluxes to land-use and management practices while decadal and interannual variations of carbon export are attributable primarily to climate variability and extreme flooding events (Ren et al., 2015; Tian et al., 2015, 2016). Further, tidal wetland sustainability is strongly influenced by human modifications that generally reduce resilience (e.g., groundwater withdrawal, lack of sediment, nutrient loading, and ditching; Kirwan and Megonigal 2013).
Climatic changes affect entire watersheds, so the integration of small changes to terrestrial carbon cycling leads to a significant impact on the quantity, quality, and seasonality of riverine inputs to coastal zones (Bergamaschi et al., 2012; Tian et al., 2016). Within wetlands, accelerating sea level rise and increasing temperature yield a range of responses from enhanced wetland flushing, salinity intrusion, and productivity to enhanced respiration, tidal carbon export, and CH4 emissions, which have all been postulated. Increased rates of sea level rise may enhance sedimentation and carbon burial rates up to a threshold of marsh resilience, above which erosion processes will dominate (Morris et al., 2016). This effect of accelerated sea level rise on morphology also affects carbon fluxes in shallow estuaries, whereby the loss of barrier islands to erosion will increase tidal mixing.
Estuaries show significant regional drivers of carbon cycling, such as the dominance of land-use change in Atlantic coast (Shih et al., 2010) and GMx (Stets and Striegl 2012) watersheds. In Pacific coast estuaries, ocean drivers (i.e., upwelling patterns) and rainfall variability are dominant controls on carbon fate and CO2 degassing from Alaska to Mexico. In Arctic regions, along both Pacific and Atlantic coastlines, ice-cover melt and permafrost thaw appear to be critical drivers of wetland extent and estuarine mixing. Tidal wetland carbon dynamics, however, show more local variability than regional variability, with multivariate drivers of extent and carbon fluxes, such as sediment supply (Day et al., 2013), nutrient supply (Swarzenski et al., 2008), tidal restrictions (Kroeger et al., 2017), and subsurface water or hydrocarbon withdrawal (Kolker et al., 2011). These coastal drivers illustrate the complexity of projecting carbon fluxes and their potential to alter fundamental habitat quality. For example, estuarine acidification is observed along all coastlines with potential stress to shell fisheries (Ekstrom et al., 2015), often with changes in riverine input, circulation, and local biological dynamics more significant than direct atmospherically driven ocean acidification (Salisbury et al., 2008).
Thus, expected changes in climate and land use for the remainder of this century likely will have a major impact on carbon delivery to and processing in tidal wetlands and estuaries. While terrestrial carbon loads likely will continue to drive ecosystem heterotrophy, extreme flooding events might shunt material directly to the continental shelf, thus decreasing processing, transformation, and burial in the estuary and tidal wetlands. Overall, estuarine area likely will increase relative to that of tidal wetlands (Fagherazzi et al., 2013; Mariotti and Fagherazzi 2013; Mariotti et al., 2010), and estuarine production will become more based on phytoplankton relative to benthic algae and macrophytes (Hopkinson et al., 2012). While this trajectory may be reversible (see Cloern et al., 2016), by the end of this century tidal wetland and estuary net CO2 uptake and storage as organic carbon quite likely will be significantly reduced throughout the United States due to passive and active loss of tidally influenced lands.
15.5.1 Observational Approaches
Coastal observations of carbon stocks and fluxes cross many spatial and temporal scales because of their intersection in multiple contexts: past or future, land or ocean, and managed or unmanaged. A variety of observational approaches has been applied to study tidal wetland habitats and carbon fluxes and exchanges with the atmosphere and adjacent estuarine and ocean waters. Currently lacking is a standardized, consistent methodology on carbon-relevant wetland mapping, wetland carbon flux monitoring, and repeated assessment. Wetland mapping, inventories, and sampling efforts include the National Wetlands Inventory (USFWS NWI 2017), a national effort to map and classify the wetland resources in the United States (data updated at a rate of 2% per year), using aerial photography and high spatial resolution remote-sensing color infrared imagery. Light detection and ranging, or LIDAR, imagery has been applied to develop high-resolution digital elevation models for wetlands and incorporate those maps into coastal resilience (NOAA 2015) and response mapping (USGS 2018). Satellite optical (e.g., Landsat; see Appendix C: Selected Carbon Cycle Research Observations and Measurement Programs) and synthetic aperture radar (SAR) imagery has been used for decades in mapping wetland structure and biomass, with tidal hydrologies potentially interpretable through repeat measures. High-resolution satellite ocean color observations can be used to examine wetland impacts on estuarine carbon dynamics and stocks, which, combined with hydrodynamic models, may provide information on lateral fluxes and wetland contributions to estuarine and coastal carbon budgets, especially in the actively restoring Mississippi-Atchafalaya River Delta. However, existing remote-sensing algorithms could be improved, adding the capability for representing and quantifying carbon-related properties in highly turbid estuarine and nearshore waters (Son et al., 2014). Various ground-based approaches have been applied to validate mapped carbon stocks and inventories. Deep soil cores provide quantification of carbon stocks and, when dated, can provide long-term rates of net carbon accumulation or loss (Callaway et al., 2012). Exchanges of CO2 and CH4 between wetlands and the atmosphere have been measured historically using static (closed) chamber systems, but, increasingly, continuous eddy covariance approaches are being deployed (Forbrich and Giblin 2015; Knox et al., 2018). Continuous gas flux measurements (i.e., NEE) over a range of temporal scales (hours to days to seasons to years) can be very effective at quantifying photosynthesis and respiration in tidal wetlands. An example of observational NEE data from estuarine ecosystems is illustrated in Figure 15.5a. Similarly, in Figure 15.5b observational NEE from a tidal wetland ecosystem is shown. Estuarine NEE is typically quantified using measurements of the gradient in partial pressure across the air-water interface in combination with a model of the gas transfer velocity; more direct approaches are needed to reduce uncertainty (e.g., McGillis et al., 2001; Orton et al., 2010). Deployment of automated water quality sondes and optical sensors within channels of tidal wetlands provides a method for continuous bidirectional measurements of physicochemical and optical parameters that can be used as proxies for hydrological carbon concentrations and flux (Wang et al., 2016). These findings emphasize the importance of time-series measurements to provide in situ measurements of variability across timescales.
Figure 15.5: Example Observational Net Ecosystem Exchange (NEE) Data from (a) an Estuarine Ecosystem and (b) a Tidal Wetland Ecosystem
15.5.2 Modeling Approaches
While there have been numerous applications of three-dimensional estuarine biogeochemical models (Azevedo et al., 2014; Feng et al., 2015; Ganju et al., 2012; Irby et al., 2016; Kenov Ascione et al., 2014), none specifically allow integration with hydrological exchange of tidal wetlands. With unstructured meshes that provide topological flexibility, the Finite Volume Community Ocean Model (FVCOM; Chen et al., 2003) and the Semi-implicit Cross-scale Hydroscience Integrated System Model (SCHISM; Ye et al., 2016, 2018) have been successfully applied to wetland-estuarine environments. Currently, there are no biogeochemical models that include accurate parameterizations for the sources and sinks that drive variability in carbon fluxes, amount, and quality at the wetland-estuary interface (e.g., allochthonous sources, photochemical transformation, and viral lysis). Further, coupled biogeochemical-geomorphic models are necessary for full tidal wetland carbon accounting and projection with accelerated sea level rise, but they have yet to be validated successfully (Kirwan et al., 2010). Efforts to couple tidal wetland lateral exchanges with estuarine dynamics are ongoing.
Empirical approaches to modeling include synthetic cross-site comparisons and relationships. The National Wetlands Condition Assessment (U.S. EPA 2016) illustrates homeostasis among tidal wetland soil carbon densities spatially and downcore (Nahlik and Fennessy 2016). National Aeronautics and Space Administration (NASA) synthesis efforts, which include the Wetland-Estuary Transports and Carbon Budgets (WETCARB; NASA 2017b) project and the Blue Carbon Monitoring System (Blue CMS; NASA 2017a) project, have integrated literature-derived field data and national datasets (e.g., USFWS and U.S. Department of Agriculture) and identified key differences and similarities among tidal wetland and estuarine processes for CONUS. These approaches provide boundary conditions for new observations and identify critical knowledge gaps.
Key areas to aid further research and development are:
Mapping approaches that characterize key drivers of tidal carbon accounting (organic carbon burial and CH4 production), such as multiple salinity classes, relative elevations, and tidal boundaries;
Unbiased, landscape-level sampling protocol to quantify sediment carbon stock change in tidal wetlands (similar to U.S. Forest Service Forest Inventory Analysis approaches for carbon accounting);
Remote-sensing capability suitable for highly turbid estuarine waters;
Networks for continuous measurements of wetland-atmosphere exchanges (CO2 and CH4 emissions) and wetland-ocean exchanges (dissolved and particulate carbon fluxes) and better constraint and linkage of these important fluxes;
New biogeochemical models that account for critical processes at the wetland-estuary interface, both ocean drivers (sea level rise) as well as watershed influences (land use); and
Estuarine gas flux monitoring, including CO2 and CH4, especially in large, undersampled, episodic or rapidly changing environments, such as high latitudes (Arctic).
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