UNDERSEA WORLD: Marine plants are a vital resource for productive fisheries as well as playing a role in storing carbon dioxide, the leading greenhouse gas causing climate change. Image: Flickr/smilla4
BAR HARBOR, Maine -- Jane Disney steered her kayak along an edge of Frenchman Bay a mile or so north of here. "Look, there's a great blue heron," she said, gesturing toward the shoreline, where the elegant bird perched on a rock.
It was a postcard scene in a setting that is increasingly feeling man's imprint, especially underwater. For generations, Mainers have scoured the depths of this bay for pleasure and profit. Abundant populations of fish have long since disappeared due to overfishing. But the bay continues to supply ample mussels, clams and lobsters.
Commercial fishing continues to help drive this area's economy, but it has also left parts of the aquatic ecosystem that supports healthy sea life and stores carbon dioxide either battered or gone.
Disney, a scientist at the Mount Desert Island Biological Laboratory, estimates that around 80 percent of Frenchman Bay's underwater eelgrass has disappeared since 1996, primarily from activities such as raking for clams or mussels. In some areas, the sea grass, which used to cover the muddy or sandy subtidal areas near shore, is missing.
Individual rakes, she said, do little damage. But the collective devastation of thousands of fishing expeditions over the years has taken a heavy toll.
Eelgrass (Zostera marina) does a number of valuable things. It pulls nitrogen and phosphorus from the water, helping to counteract the damage caused by fertilizer-infused soil runoff that can spawn deadly algae blooms.
Eelgrass also provides a habitat for all sorts of marine animals -- not least of which are lobsters and mussels. And the structure of the grass helps stabilize sediment on the bay floor, keeping the water clear and creating a nursery for eggs and juvenile animals to thrive.
Volunteers replant eelgrass
But eelgrass and other coastal and subsea ecosystems like sea grass, salt flats and mangroves also have vast potential as a tool to help mitigate the impacts of global warming. Just like forests and other terrestrial biomass, the undersea plants store carbon dioxide, which is why they're sometimes called "blue carbon sinks."
Conversely, the destruction of marine ecosystems emits carbon dioxide, exacerbating global warming (ClimateWire, Sept. 7).
The U.N. Environment Programme (UNEP) estimates that 93 percent of the Earth's carbon dioxide passes through the oceans. Ocean grasses, the agency says, account for 50 to 71 percent of the carbon storage capacity of ocean sediments, which is equal to half the emissions from the entire global transportation sector.
UNEP says halting the destruction and degradation of blue carbon sinks would equal at least 10 percent of the reductions needed to keep atmospheric concentrations of carbon dioxide below 450 parts per million.
Standing in bright yellow boots on the shore of Hadley Point in Frenchman Bay, Disney said 140 acres of deteriorated or destroyed eelgrass habitat is targeted for restoration.
Below the water's surface, Disney and groups of volunteers have placed flat metal grids, about 2 feet square, on the bay surface. About a thousand eelgrass plants per year are introduced into the subtidal zone through this method. The most recent grids that Disney and the volunteers have submerged into the cold late-summer water are made of wood and biodegrade, making labor less intensive.
A decade of decline
"Some said that restoration would cost $250,000 per acre," she said, "We've spent, maybe, tens of thousands."
The historical dynamics of the eelgrass habitat in Frenchman Bay are difficult to pin down because not until 1996 was any thorough mapping conducted. Anecdotal information from people living in the area suggests the grasses were abundant until 1996, the same year the first survey of eelgrass was conducted. Between 1996 and 2008, though, 70 percent of the eelgrass habitat disappeared.