Sea Level Rise is a Double-Edged Sword for Coastal Carbon Sequestration

We demonstrate that sea level rise enhances carbon preservation in soils, but changes the location of carbon storage- from relatively stable forests to more vulnerable marshes.
Published in Earth & Environment
Sea Level Rise is a Double-Edged Sword for Coastal Carbon Sequestration
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Rapid climate change and sea level rise directly impacts the world’s coastlines, affecting the processes that govern their formation and persistence. Coastal ecosystems provide many benefits, ranging from wildlife habitat and flooding protection to enhanced carbon storage. While the coastal zone is a relatively short distance, it can span wildly different ecosystems largely dependent on elevation relative to sea level. Historically, these different environments (bay, marsh, coastal forest) have been studied independently from each other, with intense focus on their future morphodynamic change or carbon processes. Here we coupled geomorphic processes and carbon dynamics in a numerical model that spans the a bay-marsh-forest transect to understand the future of the entire coastal zone.

Alex Smith walking through marshland
Alex Smith walking through the marsh at Goodwin Island during field sampling. Photo by Kendall Valentine.

Our team has done a lot of work across this bay-marsh-forest landscape. While this coupled ecosystem is relatively common along the Atlantic coast of the United States, we spend much of our time on the Virginia coast at sites at the Virginia Coastal Reserve Long-Term Ecologic Research site (VCR LTER). While our model can represent many different coastal morphologies, our time in the field in Virginia helped in developing the model and understanding the driving processes in this system. Here, you can walk the rapid transition from an upland forest to salt marsh in only a few minutes. What’s even more incredible about this landscape is that it is far from static – these ecotones are rapidly shifting in response to rising sea levels.

Marshes evolve both vertically and laterally. At the bay-marsh edge, wind waves impact the marsh leading to erosion. Vertical evolution of the marsh is driven by deposition of sediment at high tide, vegetation productivity leading to organic deposition, and decomposition. These processes are sensitive to the marsh elevation relative to the sea level. At the marsh-forest boundary, elevation drives the dynamics. The marsh transitions to forest as the land no longer is regularly flooded by seawater. With sea level rise, forests become stressed from flooding and salinity, leading to forest die-off and migration of marsh vegetation into this region. We aimed to capture these processes and tie them to how carbon is moved across the landscape and alters the landscape dynamics.

Model schematic
Cartoon of the bay-marsh-forest transect and the geomorphic and carbon processes that drive landscape evolution.

We had noticed in the field that while marshes are very important for coastal morphology and carbon sequestration, they do not exist in isolation. The coastal landscape is a mosaic of ecosystems that shift over time, and the ‘individual’ ecosystems – bay, marsh, coastal forest -   are woven together. You often see pieces of marsh slumping into the open bay, marsh vegetation encroaching in coastal forests, and bay sediments accumulating on the marsh surface. These exchanges are well known and well documented in marsh and coastal literature. Even so, when it comes down to modeling the carbon in these systems, the coastal ecosystems are represented individually. Here, we took a different approach and coupled both carbon processes and coastal geomorphic processes along a bay-marsh-coastal forest landscape to explore how the entire coast will respond to a changing climate.

The key takeaway is that the coastal system is resilient to climate change and can continue to store increasing amounts of carbon as sea level rises. This manifests as marshes expanding their extent into lands that were previously coastal forest and increased vertical accretion rates, as well as increases in bay carbon storage with the increasing accommodation space from sea level rise. However, if sea level rise rates are too great (> 10 mm/yr in our scenario), the marsh is unable to keep pace and the entire marsh system collapses resulting in lower coastal carbon storage.

Our findings have direct implications for blue carbon projects globally. Our model uniquely tracks both allochthonous (produced somewhere else and transported) and autochthonous (produced in situ) carbon across the ecosystems as part of understanding connectivity, allowing us to separate these different components within marshes. Corroborating claims from field experiments, we find that allochthonous carbon – or carbon not produced in situ - is a substantial component of total marsh carbon. This is important because blue carbon policy only counts autochthonous, or locally-produced carbon – in offsetting programs. Allochthonous carbon does not remove atmospheric carbon, thus limiting the marsh-climate feedback. Here we show that allochthonous carbon could be up to 50% of the total marsh soil carbon.

An interesting caveat of this work is that not all carbon is the same. We mean this in several different ways. Perhaps the most interesting is the difference in forest and marsh carbon. Earlier works from the Coastal Geomorphology and Ecology lab at VIMS found that forests store more carbon in tree biomass, while marshes store carbon in soils (Smith and Kirwan, 2021). Our work demonstrates that as sea level rises, the coastal landscape switches where most of its carbon is stored – from forest trees to marsh soils. While this is a compensatory mechanism that buffers changes to the coastal carbon stock, it creates a subtle change. Carbon in forests is more stable both chemically and physically, with more recalcitrant carbon compounds compared to marshes and physically removed from oceanic forcings. Marsh soil carbon, on the other hand, is, on average, more labile compared to forest biomass and is more vulnerable to loss from the ocean via erosion and storm activity. So, although coastal carbon remains high under increasing rates of sea level rise, perturbations in the system (i.e. storms) could have larger consequences for carbon storage in the coastal zone than before.

Poster Image is of Goodwin Island, taken by David Walters.

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