Low marshes emit more CO2 as temperatures rise

Scientists have long known that salt marshes are a major carbon sink and can be of critical help in carbon sequestration efforts. However, they are also highly dynamic ecosystems that constantly change with seasons and tides. By focusing on the Sage Lot Pond on Cape Cod, a new study led by the Marine Biological Laboratory (MBL) Ecosystems Center has now investigated how seasonal cycles and the ocean dynamics affect the amount of carbon stored in New England marshes.

“In our recent study, we asked: how does marsh respiration (the conversion of organic carbon to carbon dioxide) vary as a function of temperature, both of air and sediment? This can help us understand how climate change, particularly warming, alters respiration rates in a marsh,” said study lead author Joanna Carrey, an associate professor of Environmental Science at Babson College, who conducted this research during a postdoctoral fellowship at MBL. 

The scientists examined gas exchange from the Sage Lot Pond marsh system over 16 months between 2014 and 2016, by installing static gas chambers in six plots, ranging from high elevation marsh (which is flooded by tides just a few days per month) to low elevation marsh (flooded twice daily). The chambers were used to record the amount of carbon dioxide released in the atmosphere each second for four minutes straight.

The analysis revealed that each degree of warming correlated with an exponential increase in carbon dioxide emissions. “But what was surprising is that we found higher temperature sensitivity in the low marsh habitat. That means for each degree of warming, there was significantly more carbon dioxide released from the marsh at lower elevation relative to the high-marsh habitat,” Carey explained.

The fact that low marshes stored more carbon had seemed a beneficial thing for natural carbon sequestration, particularly since sea-level rise is resulting in the conversion of high-marsh to low-marsh environments. However, this study shows that, as the world continues to warm, these benefits may be diminished, since CO2 is also released there at a significantly higher rate with increasing temperatures.

In addition, the researchers found that the Sage Lot marsh environment loses much more respired carbon to the ocean via ebbing tides than it does directly through vertical fluxes to the air. Thus, although some of the respired carbon will be returned to the atmosphere as a CO2 flux from the sea, a part of it most likely remains dissolved in the ocean for an extended period – maybe even thousands of years – thereby enhancing the carbon sequestration value of salt marshes.

In future research, the scientists aim to study also nitrogen loads and emissions by using the same methods, as well as to examine how their findings might extend to other marshes.

The study is published in the journal Journal of Geophysical Research: Biogeosciences.

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By Andrei Ionescu, Earth.com Staff Writer

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