Scientists are testing how fast soil is building up in this marsh on Maryland's Eastern Shore. Recording this data over time in a variety of marshes around Chesapeake Bay may yield new insights about whether the increase in marsh elevations can keep pace with rising sea level in the estuary.
Healthy coastal habitat is not only important for seafood and recreation, it also plays an important role in reducing climate change. Salt marshes, mangroves, and seagrass beds absorb large quantities of the greenhouse gas carbon dioxide from the atmosphere and store it, thus decreasing the effects of global warming. These types of habitat are known as carbon sinks and contain large stores of carbon accumulated over hundreds to thousands of years. Using more scientific lingo, coastal blue carbon is the carbon captured by living coastal and marine organisms and stored in coastal ecosystems. Salt marshes, mangroves, and seagrass beds play two important roles:
MIT Sea Grant coastal ecologist Julie Simpson takes samples from a sediment core taken inside an eelgrass bed. This research is part of an ongoing project in collaboration with several partners to determine how much carbon is stored in eelgrass beds in the North Atlantic region. Julie is shown here in West Falmouth, Massachusetts.
Current studies suggest that mangroves and coastal wetlands annually sequester carbon at a rate ten times greater than mature tropical forests. They also store three to five times more carbon per equivalent area than tropical forests. Most coastal blue carbon is stored in the soil, not in above-ground plant materials as with tropical forests.
Coastal habitats are important for capturing carbon—but their destruction poses a great risk. When these habitats are damaged or destroyed, it is not only their carbon sequestration capacity that is lost. Carbon stored in the habitats is also released, increasing levels of greenhouse gases in the atmosphere. Unfortunately, coastal habitats around the world are being lost at a rapid rate, largely due to coastal development for housing, ports, and commercial facilities.
This diagram illustrates the mechanisms by which carbon moves into and out of coastal wetlands: (1) Carbon dioxide in the atmosphere is taken in by trees and plants during the process of photosynthesis. This is called sequestration. (2) Dead leaves, branches, and roots containing carbon are buried in the soil, which is frequently, if not always, covered with tidal waters. This oxygen-poor environment causes very slow breakdown of the plant materials, resulting in significant carbon storage. (3) A small amount of carbon is lost back to the atmosphere through respiration, while the rest is stored in the leaves, branches, and roots of the plants. This diagram is adapted from a figure in Sutton-Grier et al. 2014 Marine Policy
Coastal wetland ecosystems (salt marshes, mangroves, and seagrass beds) can store large quantities carbon for two main reasons:
Coastal wetlands tend to be very productive ecosystems—meaning that the plants grow a lot each year. As part of the growth process, plants capture carbon dioxide from the air and convert it to plant parts such as leaves, stems, or roots. This process is called “fixation” or “uptake” of carbon dioxide.
Carbon is also lost back to the atmosphere when plants respire (exhale) carbon dioxide, the same way people exhale carbon dioxide. This carbon dioxide is the byproduct of the plants breaking down sugars (i.e. food) and converting it to energy.
Some of the carbon that plants capture gets added to soils either via internal transport in the plant or when plant parts, such as leaves and roots, die and become incorporated into the soil.
Once carbon is in the soil some of it is respired by microbes and returns to the atmosphere as carbon dioxide. However, some of the carbon stays stored in the soils, often for hundreds or even thousands of years, buried deep underground.
One reason coastal wetlands are particularly good at storing carbon is because the soils are largely anaerobic, which means they lack oxygen.
In most coastal wetlands there is usually a thin layer of soil that is oxygenated and above water, but the remainder of the soil is submerged in water. Oxygen diffuses very slowly through water, so saturated (wet) soils in these wetland habitats tend to have little to no oxygen present.
Decomposition of organic plant material is much slower when there is no oxygen present, so the carbon present in this plant material remains intact, rather than being broken down by microbes and respired back to the atmosphere. As a result, wetlands are very good carbon sinks (meaning they store a lot of carbon).
In summary, coastal wetlands are particularly good at storing carbon because the plants annually sequester (capture) a lot of carbon and then these ecosystems store carbon for long periods of time in their soils.
Recent NOAA-supported efforts related to coastal blue carbon include the inclusion of coastal wetlands into the Inventory of U.S. Greenhouse Gas Emissions and Sinks (see Chapter 6). NOAA also sponsored The National Academies of Sciences, Engineering, and Medicine’s project, Developing a Research Agenda for Carbon Dioxide Removal and Reliable Sequestration.
NOAA’s coastal blue carbon activities are a collaborative effort across NOAA, including the National Marine Fisheries Service, National Ocean Service, and Oceanic and Atmospheric Research offices. In addition, NOAA partners with Restore America's Estuaries.