Tracking the invisible: carbon dioxide flux in an ecotonal wetland

Blog By: Amanda Richey
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The salty is getting saltier. All around the world, changes in freshwater flow combined with intruding saltwater is salinizing coastal freshwater and brackish wetlands. Salinization is stressing these transitional habitats, which leads to vegetation loss and with it, the carbon stored in stems, leaves, and root systems. Additionally, lush, freshwater plant communities are being replaced by more salt tolerant species. The variation in coastal wetland response to salinization due to water level changes makes the fate of these systems uncertain, as well as the fate of the carbon they store. Understanding carbon dynamics in coastal vegetated ecosystems is essential because these ecosystems are globally important carbon sinks, and they are at risk of being lost.  

Saltmarsh, mangrove forests, and seagrass meadows are all known as “blue carbon” ecosystems because they are efficient at capturing atmospheric CO2, converting it to organic carbon, and storing it over long time periods in sediment. However, carbon is only captured, converted, and stored if these ecosystems can keep “their feet above water.” This means outpacing sea level rise and salinization stress through increased plant growth and sediment development, which leads to elevation gain. Plant sediment development is especially crucial for ecosystems in the Everglades, where there aren’t large sources of external sediment that can contribute to elevation. 

In the Malone Disturbance Ecology Lab, we investigate how ecosystems respond to global changes, like shifting disturbance regimes and climate change. In ecotone coastal areas where freshwater meets saltwater, we find plants living at the edge of their environmental tolerances. Any small change in environmental conditions like salinity or nutrient levels can make stressful conditions too extreme for the current plant species, can increase plant mortality, and shift plant species distributions. These dynamic conditions make ecotones good indicators of hydrologic or salinity fluctuations. 

My master’s thesis focused on an ecotone sandwiched between freshwater marl prairies and mangrove scrubs on the southeast coast of South Florida within the Everglades (Figure 1). This ecotonal region in the Southeastern Saline Everglades has not always looked like this. Only a few years ago this site was full of sawgrass, the dominant freshwater plant species in the Everglades. But now, large pockets of the ecosystem sit barren and saltwater mangroves are beginning to establish (check out the blog post “The "Bad Guy" in the Everglades” from Zhouren Yu to see the change that has taken place!). With such a drastic change in plant community, I had a great opportunity to evaluate carbon dynamics in a novel ecosystem.

Figure 1: The ecotone on the southeast coast under different spatial scales (Figure 1a and 1b) and a ground view of the eddy covariance tower field site in early 2022 (Figure 1c).  

Visual evidence shows that parts of the southeast Everglades ecotone are changing from freshwater or low-salinity marsh to a mangrove scrub, but we cannot see how this transition influences the ecosystem’s ability to take in and store CO2. How will the ecosystem act when in a transitional state containing some mangroves, some marsh species, and barren pockets? And what does ecosystem transition mean for the overall Everglades’ blue carbon value in the future? 

To answer the first question, we used data from an eddy covariance tower in the Southeastern Saline Everglades, which measures the movement of energy, water, and carbon (CO2 and CH4) between the atmosphere and the terrestrial ecosystem. Towers measure the concentration of trace gases, like CO2, in the air and the direction and velocity of the wind. With these measurements, we can calculate carbon flux, or how much carbon is being taken up by plants photosynthesizing and released by plants and microbes respiring (Figure 2). When we pair carbon flux data with other environmental data, like prolonged inundation or short-term high salinity events, we can investigate how an ecosystem functions under fluctuating conditions. 

Figure 2: Dr. Steve Oberbauer downloads continuous carbon flux data from the eddy covariance tower in the ecotone tower site in early 2022. Photo courtesy of L. Wood.  

Using the eddy flux data, I found that inundation differentially affected the processes that make up the carbon flux in the ecotone; ecosystem respiration (carbon release) on one side of the equation and photosynthesis (carbon uptake) on the other side. When water levels increased, respiration decreased, which we expected as plant roots and microbes lose access to oxygen and become less efficient at breaking down material for energy. Because carbon release decreased at higher water levels, the ecosystem will hold onto the carbon it already contains in soil pools. On the other side of the equation, carbon uptake (photosynthesis) remained consistent, even as water levels increased. So, under higher water levels, the balance between ecosystem carbon uptake and carbon release shifted to higher relative storage when the water was high. This indicates that even if the site looks stressed in some areas, it still maintains its ability to take up CO2.  

 

As low-salinity coastal wetlands continue to experience salinization, it is important to understand how these ecosystems respond and develop with changing conditions. Tracking carbon fluxes allows us to see how these ecosystems function in the short term and gain insight on their capacity to store carbon over longer periods. This study showed that even as coastal wetlands experience change and stressful conditions, they can still do what they do best, which is taking up carbon dioxide. Time will tell what this means for the Everglades’ landscape scale carbon sequestration capacity, especially under increasing sea level rise.