Considering carbon in our Nation's tidal freshwater wetlands

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This article is part of the Spring 2019 issue of the Earth Science Matters Newsletter.

Tides can reach far inland along rivers throughout the southeastern United States and elsewhere in the world. Where a combination of tides and flows from upstream cause water levels to rise higher than the river banks, tidal wetlands form, often in freshwater environments. These upstream freshwater wetlands grade into low salinity marshes closer to the ocean, where considerable wetland area can form. Carbon stored in tidal freshwater forested wetlands, tidal freshwater marshes, and even low-salinity marshes are not commonly included within the burgeoning field of blue carbon science. “Blue Carbon” typically refers to ocean-sourced carbon stores but this concept has recently started to include tidally-influenced wetlands; e.g., mangroves, salt marshes, and sea grasses. The US has considerable tidal freshwater wetland area for inclusion as well.

transition from tidal freshwater forested wetland and low-salinity marsh

Ecotone between a tidal freshwater forested wetland and a low-salinity marsh along Turkey Creek, which feeds into the Sampit River near Georgetown, South Carolina, USA. Tidally influenced wetlands along the upper estuary of many US rivers store, convey, and bury large amounts of carbon annually.

(Public domain.)

In a recent paper, USGS scientists and researchers at Clemson University reported results from more than a decade of research along two Atlantic coastal tidal rivers (the Waccamaw and Savannah Rivers) to quantify the carbon standing stocks (total amount of carbon stored in the ecosystem). Among the study sites are tidal freshwater forested wetlands, transitional forest/marsh, and marshes that range from freshwater to low salinity. Using long-term study plots, the scientists quantified the total carbon entering the sites through primary productivity and sedimentation as well as all of the carbon exiting the sites through decomposition, greenhouse gas losses, and lateral fluxes of dissolved carbon. Finally, long-term soil carbon burial was described over different Holocene time periods, ranging from 144 to 4346 years before present. Thus, full carbon budgets were developed for each site under study.

Carbon standing stocks ranged from 322-1264 Mg C/hectare, with higher values (>1100 Mg C/hectare) surpassed only by the carbon storage potential of a few tropical mangrove forests. Accordingly, annual mass balance of carbon (all carbon entering versus all carbon exiting the ecosystem) was high for all sites, indicating that these ecosystems are responsible for taking up 340-900 g C/ m 2 /year, with approximately 7-337 g C/ m 2 /year buried annually in the soil, and 267-849 g C/m2 /year exported laterally from the sites into the river and ocean to feed important biological and chemical transformations. The percentage of the carbon mass balance appearing as lateral carbon flux increased as wetland habitats transgressed from upstream forests to coastal marshes that are dominated by herbaceous vegetation. Still unknown is the fate of lateral carbon export from these wetlands; is the carbon eventually lost to greenhouse gas emissions from aquatic respiration or is that carbon buried long-term in the riverine environment and ocean? New studies are revealing that a high percentage of lateral carbon can remain persistent in aquatic environments or sediments for decades to centuries.

diagram showing carbon movement through coastal wetlands

Diagram showing the transition from a) tidal freshwater forested wetland (TFFW) to b) tidal fresh water marsh to c) low-salinity marsh to d) saltmarsh with increasing salinity and the movement of carbon in, out, and through this system. Modified from Figure 1 in (Krauss et al., 2018)

(Credit: Ken Krauss, USGS. Public domain.)

Understanding the role and fate of carbon in these wetland environments allows scientists and managers to understand ecosystem resilience and persistence. It also informs policymakers about the energy stores and protective values of coastal wetlands managed in public trust. This research on the Waccamaw and Savannah Rivers demonstrates how alterations in land use (e.g., river dredging), management activity, and natural change affect Atlantic Coastal Plain rivers and provide a model to apply to other National Wildlife Refuges, Parks, and public lands on the east coast.

The paper, “The role of the upper tidal estuary in wetland blue carbon storage and flux”, was published in Global Biogeochemial Cycles and is available here:  https://doi.org/10.1029/2018GB005897

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