Blue Carbon

While you may not have heard the term “blue carbon” before, you are likely familiar with it if you have ever visited the South Carolina coastal region. Blue Carbon is the carbon that is stored in coastal and marine ecosystems. Worldwide, this includes habitats such as mangroves, tidal marshes, and seagrasses. Specifically in South Carolina, most of the blue carbon storage occurs in our coastal tidal marshes, which are dominated by the salt-tolerant plant Sporobolous alterniflorus (commonly referred to by the previous scientific name Spartina alterniflora). There are an estimated 350,000 acres of saltmarsh ecosystems along the SC coast (Sanger & Parker, 2016).

South Carolina’s coastal marshes and wetlands (Purcell et al. 2020).
Map/Image credit: Andrew Purcell, ©2019, Clemson University

Carbon sequestration in salt marsh ecosystems occurs in both the plants and in the soil. Plants remove carbon dioxide from the atmosphere through the process of photosynthesis. During this reaction, carbon from CO₂ is converted into sugars to provide an energy source for the plant; excess sugar is stored in the plant tissue as living carbon. When salt marsh plants die back in the winter, those that aren’t carried away by the water decompose within the marsh. Because saltmarsh habitats are regularly flooded by the rise and fall of the tides, decomposition is slowed due to limited oxygen in the saturated soils. This cycle maintains a robust marsh platform (primary surface of the marsh) composed of river-borne sediments and organic material decomposing into “pluff mud.” In this way, vertical accretion or the upward growth by deposition of sediment and plant material enables a healthy marsh to keep pace with sea level rise, raising its elevation to stay above rising water levels.

Blue Carbon is stored in both the plants and soil in this marsh on Edisto Island, SC.
Amy Scaroni, ©2021. Clemson University

As long as the marsh remains healthy, the soil carbon can be stored over the long term, possibly thousands of years. However, coastal marshes are increasingly being threatened by the dual pressures of climate change and coastal development. Sea level rise can drown coastal marshes if the marsh cannot raise its elevation at an equal rate to the rising water. In addition to migrating upward, coastal marshes can also migrate landward, converting former uplands to wetlands as the rising waters inundate previously dry areas. However, this marsh migration can be obstructed by coastal development such as bulkheads, roads, and other infrastructure. If marshes can’t vertically accrete or migrate landward in response to sea level rise, the marsh can drown. Without live plant roots stabilizing the soil, the marsh platform can begin to erode, resulting in the release of the stored carbon. As a result, ecosystems that have long served as sinks for carbon can suddenly become sources of carbon.

Houses can restrict the ability of salt marsh to migrate landward in response to sea level rise.
Amy Scaroni, ©2021. Clemson University

Roads can also restrict the ability of salt marsh to migrate landward in response to sea level rise.
Amy Scaroni, ©2021. Clemson University

While coastal ecosystems provide a variety of benefits, their ability to capture and store carbon is a key ecosystem service that helps to regulate the amount of carbon in the atmosphere. Salt marshes can sequester approximately 1,940 pounds of carbon per acre per year (McLeod et al). With an estimated 350,000 acres of salt marsh across the coast of South Carolina, this translates to an estimated 6.8 million pounds of carbon sequestered per year by the state’s salt marshes.

A tidal creek cuts through the salt marsh on James Island, SC.
Amy Scaroni, ©2021. Clemson University

Conserving coastal marshes will protect the ability of these ecosystems to play their part in regulating carbon. The complex root system of a healthy marsh will hold the pluff mud and associated soil carbon in place better than an unhealthy marsh; learn to recognize signs of stress and causes of salt marsh decline at HGIC 1891, Life Along the Salt Marsh: Troubleshooting Salt Marsh Decline. Reducing the use of bulkheads and other hard structures in favor of living shorelines is one way to preserve and restore the existing marsh. Learn more about living shorelines at https://www.clemson.edu/extension/living-shorelines/ Conserving undeveloped land corridors inland from existing salt marsh will also help to ensure that the marsh has somewhere to go as sea levels rise. Maintaining natural area buffers between marshes and coastal development can also help to reduce impacts (such as pollution from stormwater runoff) to existing marshes, ensuring that they remain healthy and can adapt to changing conditions. Learn more about vegetated buffers at HGIC 1856, Life Along the Salt Marsh: Protecting Tidal Creeks with Vegetative Buffers.

References:

1. E. McLeod et al (2011) A Blueprint for Blue Carbon: Toward an Improved Understanding of the Role of Vegetated Coastal Habitats in Sequestering CO₂. Frontiers in Ecology and the Environment 9 (10): 552-60.
2. Purcell AD, Khanal P, Straka T, Willis DB. Valuing Ecosystem Services of Coastal Marshes and Wetlands. Clemson (SC): Clemson Cooperative Extension, Land-Grant Press by Clemson Extension; 2020 Jan. LGP 1032. https://doi.org/10.34068/report4.
3. Sanger, Denise, and Catherine Parker. Guide to the Salt Marshes and Tidal Creeks of the Southeastern United States. South Carolina Department of Natural Resources, 2016.

Originally published 12/21