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Salt-marsh superpower: Blue marshlands store as much carbon as green forests

The battle to save a mother lode of carbon in Cowichan-Koksilah Estuary

On the edge of Cowichan Bay, between mudflats where the ­Cowichan and Koksilah rivers meet the sea, powerful and ­productive marshlands thrive amid a decades-long fight for their survival.

The defenders of these marshlands are hoping that a new and comprehensive study of this estuary area might help in that fight.

Recently released in the Frontiers in Marine Science journal, the study found that the Cowichan-Koksilah Estuary seizes and stores double the carbon dioxide of a 20-year-old Pacific Northwest forest of the same size.

That isn’t surprising, according to Goetz Schuerholz, chair of the Cowichan Estuary Restoration & Conservation Association and a core supervisor on the study. Several other studies have revealed the carbon sequestering power of estuaries around the world.

“But this was rather comprehensive because we looked at all three parts of the estuary,” Schuerholz said. “The sub-tidal, ­inter-tidal and the salt marshes. The salt marshes are the most effective at sequestering [carbon].”

Tristan Douglas, a University of Victoria graduate student and lead author of the study, said so-called blue carbon, or the greenhouse gas stored in marine and coastal ecosystems, is different from that held on the land.

He said these saltwater estuaries, where fresh water meets the ocean, hold as much carbon as forests even though they represent just a small fraction of the area.

“The plants and algae that grow on the sea floor surface and in the water, they’re very efficient at taking carbon dioxide and converting it into organic molecules,” he said. “Trees sequester greenhouse gases but they have a limited lifespan, die, decompose and are converted back into carbon dioxide.”

In estuaries, Douglas said, carbon is quickly converted into plant-based material, buried in the sediment, and becomes oxygen free just a few millimetres under the surface.

“So, it’s very likely that the deposited organic matter won’t get readmitted as carbon dioxide back into the atmosphere.”

The study found estuaries have far more potential at mitigating climate change than forests, especially because of the threat of wildfires, which send plumes of carbon back into the atmosphere.

“If the carbon that’s stored in estuaries is not disturbed, it doesn’t have that same risk of being all of a sudden converted back into mass amounts of carbon dioxide,” Douglas said.

According to the study, waterlogged areas with plants such as salt marsh grasses, sedges, mangrove forests and seagrasses are particularly efficient natural carbon sinks, capturing and storing “up to 70 per cent of the organic carbon resident in marine systems despite only occupying 0.2 per cent of the ocean surface.”

But Douglas said the world has lost about 70 per cent of mangroves and about 30 to 40 per cent of all marshlands and sea grasses in the past 100 years, and will lose another 40 per cent if it’s a “business as usual approach” in the next century.

The study suggests that human activities have reduced the carbon capture and storage capacity of the 466-hectare Cowichan-Koksilah Estuary by about 30 per cent, equivalent to putting 53 gasoline-powered motor vehicles back on the road.

The estuary, which Cowichan Estuary Restoration & Conservation Association refers to as “the cradle of life” on its website, is also home to a huge variety of plants and animals that thrive in the mix of salt and fresh water. Along with crab, shellfish and salmon, birds of prey use the land as a hunting ground. And eelgrass, a critical piece of the carbon sequestration process, grows in abundance.

Traditionally, the land’s bountiful natural resources were also key to the livelihood of Coast Salish people, who continue to be stewards of its conservation. Douglas said it remains an important area for harvesting shellfish, salmon and seaweed, among other things, but since the mid-1800s, the land has mainly been used for agriculture and sawmills.

In the intertidal zone, roughly 129 hectares of eelgrass, which filters and traps sediment and pollutants, has been disturbed by log handling and storage, while about 100 hectares of salt marsh was drained for farming and cattle pasture since being settled by colonists, the study noted.

Many of those salt marsh dikes have been breached through restoration work, but eelgrass fields have disappeared entirely from the north part of the estuary, according to Schuerholz. He says a root cause is log booming and transport.

“At low tide, log booms sitting on the ground in the estuary do a lot of damage. And also, dredging takes place every year in the estuary to transport those logs to a mill,” he said. “This is what we have been trying to do — to convince the [Department of Fisheries and Oceans] and the provincial government to work with us, remove log booming and inter-tidal log storage in favour of deep-water storage.”

Dredging the sediment at the bottom of the estuary causes carbon to be released back into the atmosphere, disturbing habitat and reducing the waterway’s overall carbon sequestration capacity.

Douglas said it’s up to policy-makers to make it worthwhile to protect these areas by adopting more sustainable land-use and development practices.

According to Douglas, the estuaries also act as a buffer by tempering the incoming tides and mitigating storms. But that isn’t possible when upstream dikes prevent the estuary from adapting to rising sea levels.

“It’s pushed beyond its level of natural resiliency,” he said. “They can withstand a lot of gradual change, but things have changed so fast, and they’re put through so much stress that they can’t keep up with these current land practice uses.”

According to the Cowichan Watershed Board, conflict over land use in the estuary during the 1970s and early 1980s prompted the provincial government to complete an environmental management plan for the area and give it legal status in September 1986. That meant no development could occur in the area covered under the plan without the province’s environment minister attesting that the work would have no significant environmental impact.

Along with advocacy and education, efforts to restore the ecosystem continue. Cowichan Estuary Restoration & Conservation Association performs regular tracking and assessment studies and is currently working on a micro-plastics project with the University of British Columbia. Long-term, the organization hopes to see industrial leases ended and the land rezoned for conservation.

Cowichan Tribes and the Cowichan Estuary Nature Centre also carry out eelgrass replanting projects and work to engage the local community in restoration and conservation of the area.

But threats remain, including encroaching residential development, ongoing log booming and activities up-river, including those designed to control river flow and flooding, that have resulted in gravel build-up and flow alterations, changing the area downstream.

“It is one of the most efficient carbon sinks we have on Earth,” said Schuerholz. “But estuaries are the most threatened ecosystems at the same time.”

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