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Salt marshes, excellent reservoirs of carbon, are living ecosystems with vegetation and microscopic organisms that live, breathe, poop and die in the marsh mud. 

“This is a place where you could get the biggest bang for your buck, if you will, if you’re interested in trying to invest some resources in sequestering carbon using biological systems,” said Serena Moseman-Valtierra, an associate professor of biological science at the University of Rhode Island. 

Yet as temperatures rise, marshes at the lowest elevations may also be significant emitters of carbon. This complicated relationship between temperature and respiration of carbon was measured in a new study from the Marine Biological Laboratory in Woods Hole, Massachusetts. 

With each seasonal increase in temperature recorded over a 16-month period, researchers found more carbon dioxide emitted from the low marsh (areas that experience twice-daily tides) than areas of the marsh at higher elevation (an area infrequently covered by the tides), although these marshes were still net carbon sinks. The results of this study add to scientists’ understanding of how rising temperatures will impact the carbon cycle in tidal wetlands, which are understudied in comparison to forests and terrestrial ecosystems. It also suggests that the carbon sink benefits of marshes may be undermined by warming temperatures.

Many factors influence the rate of carbon storage in marshes: sea level rise, temperature and nitrogen availability in the soil, plus animal behavior and plant growth. These processes control the elevation of salt marshes relative to sea level, said Kevin Kroeger, co-author of the study and a supervisory research chemist at the U.S. Geological Survey’s Woods Hole Coastal and Marine Science Center. And if marshes are not able to keep up elevation commensurate with sea level, then vegetation will die, he said. 

Kroeger, along with fellow senior author Jim Tang, a senior scientist at the marine biological laboratory, wanted to understand the fate of carbon in the salt marsh by measuring respiration rates and response to warming temperatures. Marsh plants and soil bacteria, like humans, emit carbon dioxide, and sometimes methane when they decompose. “It turns out respiration is pretty hard to measure in these ecosystems,” Kroeger said.

For this project, scientists from nonprofit Marine Biological Laboratory, based in Woods Hole, and the USGS’s coastal science center studied Sage Lot Pond Marsh in nearby Falmouth, which is part of Waquoit Bay National Estuarine Research Reserve. They captured emissions of carbon dioxide with clear gas-tight plastic chambers placed over the marsh surface. The chamber was connected to a machine that measures carbon dioxide, methane and temperature every second. They also measured temperature data every five minutes for a year and a half to calculate the annual carbon dioxide flux.

The seasonal breadth of the data was important for measuring large changes in temperature. But it posed some challenges for field work. Joanna Carey, lead author of the study and an associate professor of earth and environmental science at Babson College in Massachusetts, said the gas chamber machines were heavy to lug through the marsh. “Winter 2015 was rough in New England,” said Carey. “And the marsh was totally covered in snow and ice, and I couldn’t wheel the cart out there.” 

Carey measured the vertical flow of carbon from plants and bacteria into the atmosphere. However, data also showed that a majority of respiration sends carbon dioxide laterally out to sea. The roots of marsh plants and bacteria in the soil sometimes respire directly into seawater instead of into the atmosphere, especially in these tidal marshes that are frequently covered with water. 

“Turns out, in these ecosystems we can’t just from above…measure respiration comprehensively,” said Kroeger. “There’s always some unknown portion of it that’s ending up dissolved in the water and then ultimately fluxing out laterally.” 

The ocean can be an important carbon sink. But for how long and how much carbon can be stored is still unknown. The next research step is to figure out the fate of that exported carbon, both in the ocean and the atmosphere, said Carey. 

Another question to answer is whether these results hold up in marshes elsewhere. Every marsh is unique in its tidal regime, elevation and climate. “You always have to be careful extrapolating something from one site to everywhere,” Carey said. 

It could be that the organisms and vegetation specific to the research site respond differently to temperature. The roots of spartina, the dominant low marsh grass in Sage Lot Pond, fix carbon and store it in the soil. Spartina is a pervasive species across the Atlantic, Gulf Coast and even Europe. 

If these data are representative of other spartina species, it could have wide-ranging implications, said Moseman-Valtierra, who was not involved in this study. She is a former postdoctoral research fellow for Kroeger, but now runs her own lab looking at how human actions impact the cycling of nitrogen in salt marsh ecosystems.

Another study published in March 2022 found a somewhat different relationship between temperature and marsh respiration. Researchers in Maryland recorded that by artificially warming a marsh by about 2 degrees Celsius, there was an increase in the carbon accumulation of the low marsh zone, up to a point. 

But past a certain temperature, the low marsh began to lose its beneficial carbon storing properties. Both these studies underscore the living nature of salt marshes, which are as unique and fickle as a single organism. There’s a need for more research to understand how natural options for carbon sequestration, like tidal wetlands, will be impacted with climate change.

This is especially important, because as sea levels rise low marsh habitats are expanding into formerly high marsh areas. In the last 30 years, over 190 acres of high marsh habitat on the Cape Cod National Seashore have been lost to low marsh vegetation due to rising seas. This trend is beneficial for potential carbon sequestration. 

Yet if the relationship between marsh respiration and temperature holds across other low marshes, then it could accelerate climate change by releasing more greenhouse gases that further drive temperatures. “It definitely is a warning sign that this is something that needs to be further explored,” Moseman-Valtierra said.