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A man-made sea port cuts the Cowichan Bay estuary in half, drastically altering the water movement which played a large roll in killing historic seagrass beds, along with logging.Shane Gross/The Globe and Mail

Estuaries edged by tall grasses and wildflowers that are home to birds, crabs, tiny fish and other wildlife are more effective than young coastal forests at capturing and storing carbon dioxide, a new study says.

The Cowichan Estuary on Vancouver Island captures and stores about double the amount of carbon compared with an actively growing 20-year-old Pacific Northwest forest of the same area, said the recently published study in the journal Frontiers in Marine Science.

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

Mr. Douglas said 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, he said.

In estuaries, Mr. Douglas said, carbon is quickly converted into plant-based material, buried in the sediment and becoming 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 said 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.

Waterlogged areas with plants such as salt marsh grasses, sedges, mangrove forests and seagrasses are particularly efficient natural carbon sinks, the study says.

“They capture and store up to 70 per cent of the organic carbon resident in marine systems despite only occupying 0.2 per cent of the ocean surface.”

Mr. 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.

Eelgrass on about 129 hectares of the intertidal zone 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 says.

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

The Cowichan Estuary has traditionally been used by the Coast Salish people and it is 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, he said.

“A lot of the historical accounts of eelgrass describe it covering almost all of the lower intertidal area, but now it’s really relegated to less than a third of where it historically grew.”

These estuaries act as a buffer by tempering the incoming tides and mitigating storms, he said.

A number of these areas have been wiped out to protect coastal communities, he said, leaving them no room to adapt to rising sea levels.

“It’s pushed beyond its level of natural resiliency,” Mr. Douglas 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.”

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