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Ecosystem Level Methane Fluxes from Tidal Freshwater and Brackish Marshes of the Mississippi River Delta: Implications for Coastal Wetland Carbon Projects

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Ecosystem Level Methane Fluxes from Tidal Freshwater and Brackish Marshes of the Mississippi River Delta: Implications for Coastal Wetland Carbon Projects Wetlands DOI 10.1007/s13157-016-0746-7 ORIGINAL RESEARCH Ecosystem Level Methane Fluxes from Tidal Freshwater and Brackish Marshes of the Mississippi River Delta: Implications for Coastal Wetland Carbon Projects Guerry O. Holm Jr.1 & Brian C. Perez1 & David E. McWhorter 1 & Ken W. Krauss2 & Darren J. Johnson3 & Richard C. Raynie4 & Charles J. Killebrew4 Received: 10 July 2015 /Accepted: 28 January 2016 # Society of Wetland Scientists 2016 Abstract Sulfate from seawater inhibits methane produc- emissions (4.1 tCO2e/ha/yr). Carbon sequestration rates tion in tidal wetlands, and by extension, salinity has would need case-by-case assessment, but the EC meth- been used as a general predictor of methane emissions. ane emissions estimates in this study conformed well to With the need to reduce methane flux uncertainties from an existing salinity-methane model that should serve as tidal wetlands, eddy covariance (EC) techniques provide a basis for establishing emission factors for wetland an integrated methane budget. The goals of this study carbon offset projects. were to: 1) establish methane emissions from natural, freshwater and brackish wetlands in Louisiana based Keywords Methane . Tidal wetland . Carbon sequestration . on EC; and 2) determine if EC estimates conform to a Eddy covariance methane-salinity relationship derived from temperate tid- al wetlands with chamber sampling. Annual estimates of 2 methane emissions from this study were 62.3 g CH4/m / Introduction 2 yr and 13.8 g CH4/m /yr for the freshwater and brack- ish (8–10 psu) sites, respectively. If it is assumed that Methane currently has a global warming potential 30 long-term, annual soil carbon sequestration rates of nat- times that of carbon dioxide (Myhre et al. 2013), and 2 ural marshes are ~200 g C/m /yr (7.3 tCO2e/ha/yr), wetlands are the dominant natural source of methane healthy brackish marshes could be expected to act as a comprising an estimated one-third of the global emis- net radiative sink, equivalent to less than one-half the sions (Bridgham et al. 2013). For tidal wetlands, there soil carbon accumulation rate after subtracting methane is a need to refine the understanding of the environmen- tal variables that constrain methane emissions to im- prove regional-ecoystem emissions predictions and to Electronic supplementary material The online version of this article reduce uncertainty for blue-carbon wetland restoration (doi:10.1007/s13157-016-0746-7) contains supplementary material, which is available to authorized users. projects. Tidal wetlands typically have high rates of carbon 2 * Guerry O. Holm, Jr. sequestration, burying approximately 200 g C/m /yr, [email protected] which is equivalent to 7.3 tCO2e/ha/yr (Mcleod et al. 2011). Herbaceous tidal marshes of the northern Gulf of Mexico and Louisiana generally have similar baseline 1 CH2M, 700 Main Street, Suite 400, Baton Rouge, LA 70802, USA burial capacity regardless of salinity regime (Nyman 2 U.S. Geological Survey, USGS Wetland and Aquatic Research et al. 2006; Piazza et al. 2011; Hansen and Nestlerode Center, 700 Cajundome Blvd, Lafayette, LA 70506, USA 2014). In Louisiana alone, there are 1.47 million ha of 3 Cherokee Nations Technical Solutions, Wetland and Aquatic freshwater to saline herbaceous coastal wetlands (Sasser Research Center, 700 Cajundome Blvd, Lafayette, LA 70506, USA et al. 1996); these coastal wetlands are subject to recent 4 Louisiana Coastal Protection and Restoration Authority, P.O. Box loss rates of 4290 ha/yr (Couvillion et al. 2011). While 44027, Baton Rouge, LA 70804-4027, USA tidal herbaceous and forested wetlands have high Wetlands capacities for carbon accumulation, from a global atmospheric lifetimes, it is currently understood that ni- warming perspective, it is recognized that climate bene- trous oxide and methane have single-pulse Global fits from wetland restoration generally increase, or be- Warming Potentials of 265 and 30 over a 100 yr time come more predictable, as salinity increases and meth- horizon (Myhre et al. 2013). It is also becoming clearer ane emissions decrease (Poffenbarger et al. 2011). Thus, that methane flux from restored freshwater wetlands low salinity freshwater wetlands or those that have re- may have a warming effect on climate over decadal duced availability of terminal electron acceptors that periods, but across century scales wetlands can be ex- suppress methanogenesis, could emit enough methane pected to act as net radiative sinks (Whiting and to offset annual rates of carbon accumulation Chanton 2001;Mitschetal.2013;Neubauer2014; (Poffenbarger et al. 2011). Bridgham et al. 2014). Thus, the current guidance for Aside from the differences among wetland types with voluntary wetland carbon offset projects (ACR and respect to annual rates of carbon sequestration and VCS) is that GHG emissions will be monitored or esti- greenhouse gas (GHG) emissions, the continued loss mated for periods of 20–100 years to calculate the net of deltaic wetlands in the Mississippi River Delta carbon benefits of a project versus the baseline Plain (MRDP) may be resulting in accelerated emissions condition. from organic-rich soils that took centuries to form The classic field studies from Virginia and Louisiana tidal (DeLaune and White 2011). The wetland loss, while wetlands (DeLaune et al. 1983, Bartlett et al. 1985, 1987) partially attributable to natural delta decay, has been formed the basis of our understanding on the importance of linked to systematic hydrologic alteration of interior salinity and methane interactions across a fresh to saline gra- wetlands (i.e. oil and gas canals; Bass and Turner dient (0.4 to 26 psu). This inverse log-linear relationship was 1997) and the reduction in sediment delivery from the further improved by the meta-analysis of Poffenbarger et al. river to the wetlands (Blum and Roberts 2009). Current (2011) who compiled data from 31 tidal marshes covering a wetland restoration measures are being planned and im- broad geographic extent. Their updated relationship was plemented to reduce the wetland loss rate, commonly interpreted in the context of methane emissions relative to including wetland creation with dredged sediment, hy- the potential wetland carbon accumulation rate. Based on this drologic restoration, shoreline protection, and analysis, the researchers concluded, in general, that wetlands Mississippi River sediment diversions (CPRA 2012). with salinity above 18 psu could reliably act as a net radiative These measures, to varying degrees, may have positive sink, because methane emissions are inhibited. They identi- climate regulation benefits in the form of increasing fied the need to refine estimates of methane emissions from sequestration capacity and avoiding the future emissions wetlands within the salinity range of 5–15 psu, which are of eroded soil carbon. under-represented and exhibit high variability in methane emis- There has been a renewed interest in wetland carbon cy- sions. Recognizing that a large portion Louisiana tidal wetlands cling research, as different groups are advancing the monitor- fall below 18 psu salinity, the main impetus for this study was to ing and verification methods required to support wetland res- refine methane emission estimates from these deltaic systems toration as eligible carbon offset projects. The voluntary mar- using the Eddy Covariance (EC) technique (Baldocchi 2014), kets, through the Verified Carbon Standard (VCS 2014)and which has been applied in other wetland systems (Rinne et al. American Carbon Registry (ACR 2012), have approved wet- 2007; Teh et al. 2011; Hatala et al. 2012). This study was de- land offset methodologies that apply to wetland creation and signed to: hydrologic restoration project types. Currently, the onus is on the project developer to provide a high level of confidence to a 1) Develop annual estimates of ecosystem-scale meth- verifier on the legitimacy of the carbon offset. This ane emissions from natural, tidal freshwater and process requires intensive monitoring on a project-by- brackish (mesohaline) wetlands in Louisiana; project basis, which can reduce the return on investment 2) Evaluate how annual methane emissions estimates to the point that developing a carbon offset project is from eddy covariance compare to those from a geo- not attractive to investors, thus, limiting the resources graphically broader chamber-based salinity-methane needed for additional restoration. To improve the viabil- relationship to assess the use of salinity as a predic- ity of a project and advance needed restoration, devel- tor of methane emissions across a salinity gradient; opers require region-specific research and tools to build 3) Compare the influence of low and moderate fresh- more robust estimates for carbon sequestration rates and water discharges from a riverine source on methane GHG emission factors. emissions from a freshwater wetland; and, A primary uncertainty is the potential for wetlands to 4) Understand the interactions of water level, temperature, emit nitrous oxide and methane under different baseline and salinity on methane emissions to refine predictive (un-restored) and restoration scenarios. Given the relationships. Wetlands Methods to manage salinity in the Barataria Basin through a controlled introduction of freshwater from the river (Day et al. 2014). Study Sites The marsh is typical of freshwater deltaic plain wetlands (Swarzenski et al. 1991;Sasseretal.1996)inthatit This study was conducted on two herbaceous tidal wetland has
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