Rapid Drop in Marsh Methane Emissions Explored by Cardon, Colleagues with New Grant

Solar panels in the Plum Island Sound cattail marsh power equipment for monitoring methane emissions in late September. Credit: Zoe Cardon

It’s not often that an entire ecosystem switches gears rapidly, changing its function naturally and nearly overnight. But the brackish cattail marsh along the Parker River at Plum Island Sound did exactly that last year, during the depth of summer drought. Emission of the greenhouse gas methane from the marsh to the atmosphere dropped from high to very low over the course of just a few hours, and it was not at all clear why.

MBL Senior Scientist Zoe Cardon and colleagues Inke Forbrich (University of Toledo), Anne Giblin (MBL), Jen Bowen (Northeastern University), and Teri O’Meara and Ben Sulman (Oak Ridge National Laboratory, ORNL) were recently awarded a large grant from the Department of Energy’s Environmental System Science (ESS) program to dig into this ecosystem-scale mystery.  

Coastal marshes are vibrant ecosystems with productivity rivaling that of tropical rainforests, but they are under threat from sea-level rise and increasingly intense and sporadic storms. Marshes protect our coastlines, process pollutants flowing from land to sea, and are important nurseries for fish and habitats for migratory birds. Marshes are also home to largely invisible yet powerful microbes, some of which support the plants themselves, and others that recycle nutrients, contribute to carbon storage in sediments, and produce and consume greenhouse gases.

These various ecosystem functions are at the core of highly valued ecosystem services ranging from coastal infrastructure protection to maintenance of fisheries.

But surprises like the major overnight drop in marsh methane emission detected last year by Inke Forbrich’s specialized equipment highlight that there are basic aspects of critical marsh functions that we still do not understand. 

scientist in marsh
Zoe Cardon monitors oxygen concentrations affecting the fate of methane in the cattail marsh at Plum Island Ecosystems LTER, June 2023. Credit: Suzanne Thomas

The key may lie in the fact that such coastal marshes are flooded regularly by large tides propagating upriver from Plum Island Sound. The Plum Island Ecosystems Long Term Ecological Research (PIE-LTER) program has monitored these tides and their salinities for years, and last year, as the drought progressed, river water became surprisingly saline. After being flooded by just one particularly saline tide in July, methane emission to the atmosphere from the cattail marsh stopped in its tracks.

The million-dollar question funded by DOE is… Why? Cardon and colleagues have several hypotheses, one focused on changing microbial activities and one focused on salinity stress among the cattail plants. DOE funding over the next three years will support field and lab work, integrated with modeling at ORNL, with the goal of understanding the causes and consequences of this dramatic system tipping point in our coastal marsh.