Eva Hillman, Victor H. Rivera-Monroy and Megan La Peyre
The ability of natural ecosystems to sequester significant amounts of organic carbon provides a good example of an ecosystem service that can be used in climate mitigation programs on local and regional scales. These mitigation programs may reduce the potential impact of increasing carbon dioxide concentrations in the atmosphere that are directly and indirectly driving climate change.
The capture of carbon dioxide by photosynthesis, followed by the conversion to organic carbon by plants and subsequently held in above-ground (for example in leaves, branches, stems) and below-ground biomass (such as roots) is called “blue carbon” in coastal and marine environments given their distinct conditions called “blue water.” These coastal environments potentially play a significant role in organic carbon storage because the pervasive lack of oxygen in sediments limits, for example, the decomposition of plant organic material. This low oxygen results in accumulated plant organic matter that can last from hundreds to thousands of years, depending on the level and frequency of disturbances such as hurricanes and human activities affecting these natural environments.
While organic carbon sequestration has been widely assessed in terrestrial ecosystems, limited information is available about the magnitude of ecosystem activity in many coastal and marine environments. In particular, while blue carbon in seagrass, mangrove and saltmarshes has been quantified in numerous locations over the past 15 years, little data from submerged aquatic vegetation communities exist from fresh to saline coastal environments (Figure 1).
LSU AgCenter scientists assessed the blue carbon storage capacity of submerged aquatic vegetation habitats in southeast Louisiana. Approximately 618 square miles of submerged aquatic vegetation habitat exist across this productive region, potentially representing a large, but as yet unquantified pool of organic carbon. Although the Mississippi River Delta has extensive submerged aquatic vegetation, its extent had not been determined until recently, especially across large basins located in the key areas such as Pontchartrain, Barataria and Terrebonne watersheds (Figure 2).
The researchers’ preliminary assessment revealed that blue carbon within submerged aquatic vegetation habitats can store more than 89 tons of organic carbon per acre just in the top 36 inches of sediment. Further, this organic carbon storage capacity is spatially allocated depending on the salinity gradients (fresh, intermediate, brackish, saline) and location (interior deltaic, barrier island). In particular, submerged aquatic vegetation in the Chandeleur Islands was found to sequester less than one-third of the amount of organic carbon stored at interior deltaic sites (Figure 3). Differences between the interior deltaic and barrier island sites largely reflect distinct sediment characteristics, local hydrodynamics and sediment deposition.
Specifically, sediment from the Chandeleur Islands sites has more mineral (less organic) sediment and is more affected by high-energy waves and storms in contrast to more protected interior deltaic sites. The dominant hydrodynamic environmental conditions in barriers islands could affect turnover rates and organic carbon release, resulting in less organic carbon stocks. In contrast, the high organic matter and less dynamic setting of the interior sites present conditions ideal for long-term storage, barring significant natural or human disturbance to the sediments.
Submerged aquatic vegetation habitat throughout the Mississippi River Delta represents up to 35 million tons of organic carbon storage. This is equal to or higher than other terrestrial and marine environments and contributes to improving global estimates of blue carbon in coastal environments. This storage represents a significant and as yet unaccounted for pool of stored blue carbon, particularly if these values extend beyond this region to other extensive deltaic and estuarine habitats across the region and globe. As scientists and managers work to integrate blue carbon estimates into management programs, incorporating estimates of organic carbon stocks in submerged aquatic vegetation will provide more accurate estimates of blue carbon.
Eva Hillmann is a Ph.D. candidate in the School of Renewable Natural Resources; Victor H. Rivera-Monroy is an associate professor in the Department of Oceanography and Coastal Sciences; and Megan La Peyre is with the U.S. Geological Survey Fish and Wildlife Cooperative Research Unit and an adjunct professor in the School of Renewable Natural Resources.
(This article appears in the winter 2018 issue of Louisiana Agriculture.)
Figure 1. Submerged aquatic vegetation within southeast Louisiana. Photo by Kristin DeMarco
Figure 2. Map of southeast Louisiana with salinity zones identified for comparison and sampling for sediment organic carbon in submerged aquatic vegetation habitats.
Figure 3. Estimated organic carbon sequestered in soils for interior deltaic submerged aquatic vegetation (SAV) and barrier island settings in southeast Louisiana compared to published mean estimates for seagrass, salt marsh, mangroves, tropical forests and temperate forests.