Wednesday, April 28, 2004

Turning the tide on global warming (salt marshes & carbon)

Geography professor Gail Chmura has made a career of playing in mud. Chmura studies salt marshes, the transitional zone between land and ocean. Salt marshes filter pollutants, buffer coasts from flooding and provide habitats for waterfowl and fish. Chmura's latest research suggests that salt marshes also function as a carbon sink -- an area that removes carbon dioxide from the atmosphere, thereby slowing global warming.

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Professor Gail Chmura, Department of Geography
Claudio Calligaris

"The global significance of salt marshes is often overlooked because they comprise just a fraction of the Earth's surface, and relative to other ecosystems like tropical mangrove swamps, they host few species," explained Chmura. These temperate wetlands flood with salt water at high tide but are dry at low tide. Rising sea levels for over two centuries have created bigger tides that flood further inland, expanding salt marshes. As their area increases, so does their environmental importance.

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You can see the thickness of the salt marsh soil in this photo of an exposed marsh, submerged at high tide. Chmura placed her knapsack there for scale. There are no rocks in the marshes, the ones here were once part of a road bed. Gail Chmura

Two distinct characteristics make salt marshes efficient carbon sinks. First, salt marshes accumulate soil at a remarkable rate. "They can grow in depth by up to three millimeters a year because the tides constantly deposit sediments in the marsh," said Chmura. "Forest soil might not accumulate that depth for a century." Plants and microscopic organisms living in salt marshes can get buried under newly deposited sediments. The carbon contained within these organisms gets trapped in the salt marsh and is prevented from escaping into the atmosphere, where it might act as a greenhouse gas and contribute to global warming.

A second feature that makes salt marshes efficient carbon sinks is that bacteria living in salt marshes do not emit methane. Bacteria are nature's recyclers -- they decompose organic material and release the contents into the environment. In certain conditions bacteria produce methane as a decomposition product; released into the atmosphere, this carbon-based gas might contribute to global warming. Bacteria living in salt marshes do not produce methane, perhaps because the sulfide present in salt marshes is toxic to methane producers. Alternatively, bacteria that subsist on sulfide might outcompete methane producers within the marsh.

Much of Chmura's research is based at the Huntsman Science Centre, a McGill-affiliated field station at the Bay of Fundy on Canada's East coast. Famous for its world-record high tides, the Bay of Fundy also contains salt marshes where Chmura measures sedimentation rates. "We bury metal markers on the marsh and cover the marsh surface with a layer of white clay. We come back after a year or two to measure how much sediment has accumulated" she explained. "The fun part is locating the markers we left behind. We get to walk around the marsh with a metal detector because they are completely covered in mud." Once located, the depth of accumulated soil at each marker site is documented and samples are obtained using a coring device that preserves mud's vertical profile. Back at Chmura's McGill lab, samples are tested for carbon content and the rate of carbon accumulation is calculated.

An added angle to Chmura's research is that humans have affected salt marshes to varying degrees over time. An estimated 85 percent of the salt marshes in the Bay of Fundy have been altered by humans. Starting with the Acadians in the 17th century, salt marshes were dammed and dyked to create agricultural land. Over time, many of these structures fell into disrepair, allowing marshes to flood again, though not always to the same extent as the original marsh. Chmura explained that human alteration of Atlantic salt marshes has created a giant natural experiment. "The question is to determine how each salt marsh, given its unique history and its current use, is able to accumulate carbon and perform other ecosystem functions like providing habitat for fishes and birds." In order to compare the various marshes, Chmura must piece together each marsh's muddy history. She uses old documents, aerial photographs and abandoned structures on salt marshes to unravel the human impact at each site.

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Low tide at Wood Point Marsh, New Brunswick. Here McGill gratuate student Marie Graf researches grass production and mud deposition. Gail Chmura

Chmura's work suggests that natural and human modified salt marshes do not perform identical ecosystem services. "Unfortunately, farmed salt marshes no longer act as carbon sinks," she explained. "They are not nearly as valuable as carbon stores because they do not flood, and so they cannot accumulate sediments brought in by the tides." Chmura estimates that if all of the Bay of Fundy's original salt marshes were allowed to flood tidally, or were "restored," the increased carbon stored would be the equivalent to around five percent of Canada's targeted reduction of greenhouse gas emissions pledged under the Kyoto Protocol; however, dyked salt marshes serve as farmland, residential areas and even crucial infrastructure like the Trans-Canada Highway. Chmura recognizes that reverting them all back to flooding salt marshes is unrealistic. Still, she hopes that future coastal developments will consider the important ecological services that salt marshes perform.

McGill's SPARK program (Students Promoting Awareness of Research Knowledge) is funded by NSERC and run by the Faculty of Education, VP Research Office and the University Relations Office. See for more information and articles.