Seagrass Meadows — Underwater Carbon Sinks More Efficient Than Forests

The Greatest Threat to Seagrass is the Very Phenomenon It Counteracts, Global Warming

Adapted from the upcoming book, Ocean Country

Humor me: take the deepest breath you can, inhaling deep into your abdomen. And hold it as long as possible. When you finally can’t hold it any longer, exhale and take a new breath. Last October I witnessed something that dramatically changed that experience: breath. Even I, the ever-so-keen conservationist, found my eyes glaze over when I read about how the oceans support the air we breathe, discussions of things like photosynthesis and carbon-sinks. But then I saw something that drove it home in such an immediate and tactile way I would never forget it.

Photo by Liz CunninghamPoisidonia oceanica, a seagrass variety found only in the Mediterranean Sea, is over 100,000 years
old. Human activity and climate change have depleted this underwater meadow. It now covers only
3 percent of its original area.

So, when you take a breath, nearly 70 percent of the oxygen in that one breath comes from plants and algae in the ocean, scientists estimate. But I had known that for a while. And numbers are numbers and names, names. And the names are hard to wrap your head around – cyanobacteria, prochlorococcus, phytoplankton. The dots still didn’t connect with my own experience.

What drove it home for me was a quiet moment underwater in the seagrass meadows of Port-Cros National Park, a marine protected area in the Western Mediterranean. Often referred to as the “lungs of the Mediterranean,” the seagrass meadows there play a key role in the health of that sea.

Poisidonia oceanica, as they are called scientifically, are extremely effective carbon sinks – plants that absorb carbon dioxide and output oxygen at a very high rate. “A square meter of Posidonia,” Nicolas Gerardin, the Port-Cros Park Operations Director told me, “ produces twice the volume of oxygen in 24 hours than a square meter of tropical forest.” On top of it, they act as nurseries for juvenile fish. “We call it a keystone species,” says Geradin, “which means that the whole marine ecosystem balance relies on this species.”

For my research I needed to find a place in the Mediterranean where seagrass meadows were exceptionally healthy and Port-Cros Park is one of them. But I had no idea of the richness of life dwelling within those tall strands of grass. My first morning diving in the meadows, I sank to a patch of sand and stilled myself in the water. In the glistening, turquoise-azure water, the angular fall light undulated through the surge like threads of silk. After a few minutes of stillness in the sparkling waters, life resumed its normal pace.

Tiny baby fish, no larger than an inch, hesitantly came out from their hiding places in the sea grass, which to their scale was like a redwood forest. Some lingered in front of me as if they were observing the light reflecting in my mask. The sheer quantity of very young fish was astounding, at moments as dense as light snow – damselfish, breams, groupers ... the list goes on. Over 70 percent of all fish in Med take shelter at some point in their life cycle in the Posidonia. I was, literally, in the cradle of the sea.

A baby flounder, two or three inches long, darted in front of me. A slender, inch-long gold and white fish hovered just at the edge of the seagrass — a very young barracuda. As soon as it was more than a foot long, it would depart for deeper waters and join other barracuda in massive schools to breed and feed. But for the moment, it took shelter in the safety of the tall slender reeds of the seagrass meadows. I ran my hands through the porous dark green strands. They were sturdy, strong. I remembered exploring the meadows on my grandmother’s farm as a child, how it teemed with minute life – crickets, caterpillars, butterflies, salamanders, frogs and toads. I had reveled in exploring that life, the “extraordinary ordinary.” I had no idea that beneath the surface of the sea a similar life-rich, grassy terrain existed.

But most remarkable of all was something, if I had not been looking for it I might not have seen it, like some hapless, dehydrated traveler who walks right by a well, because they don’t recognize it. It was this: tiny bubbles rising from the sea grass. “Unbelievable,” I thought to myself. I was seeing something that I had known, but seeing it and feeling it, in this place brought a whole new meaning to it. The bubbles were oxygen.

I know, grasses don’t have lungs. But we have something critically in common with them: gas exchange. We take in oxygen and breathe out carbon dioxide; that’s our end of the equation. The grass absorbs carbon dioxide and water and sunlight and by photosynthesis, voila! Oxygen! So there it was, an ultra-fine fizz streaming upward through the water from the sea grass, their “out-breath.” And I could hear my own thick noisy “in-and-out breath” through my scuba gear. Imagine you could see oxygen streaming out of the plant in your office. Would that change how you view it?

Now all the scientific facts fit together like puzzle pieces, the big picture finally assembled. I had thought of “breath” as inhale and exhale, like an accordion with two folds. But now I realized there was another fold in that accordion, a key and in fact, rather large-scale one: the biosphere. I turned slowly in the water with my fins, making a full circle. For 360 degrees I saw nothing but rolling hills of sea grass meadows. I remembered again how life overflowed from the meadows of my grandmother’s farm. For once in my life, I really could imagine that “third fold” of breath in the accordion, or really rather, almost innumerable folds:  miles and miles and miles of seagrass meadows, from the Mediterranean to Mexico to New Guinea to Zanzibar. And mangroves and plankton and algae — tiny microscopic and near-microscopic creatures floating in the ocean currents — all pumping out oxygen throughout the seas of the world. The aquatic counterparts to terrestrial forests and grasslands, and yes, even a geranium on a fire escape, a vast mosaic of organic life forms making each breath we take possible. Breath connects us to the world as a whole and the life of the seas is key to it. And I could see it right in front of me.

Now, I know, you might think like some poet wandering in a cricket-filled summer evening on Cape Cod having drunk too much wine, I might just be waxing poetic. But science provides ample support for this point of view.

For starts, even though they occupy less than 0.2 percent of the world’s oceans,seagrass meadows are visible from space. Especially the one that spans 770 miles along the southwest coast of Australia. That’s the distance between New York City and Florida. Another just below the Everglades is 5,380 square miles, that’s bigger than the state of Connecticut (which is 4,842 square miles). And the carbon uptake, oxygen output thing? One acre of sea grass meadow absorbs 7,401 pounds of carbon a year. Yes, over 7,000 pounds. And that’s the equivalent of the carbon emissions of one car traveling 3,860 miles.

Seagrass meadows, like mangroves and plankton and algae are like the Teddy Roosevelts of marine life – they speak softly, but carry a big stick. These delicate, reedy strands of grass hidden beneath the water’s surface are a significant factor in counteracting global warming. But there’s a hitch to that big stick: it will only work if we preserve them. It is an uphill battle, especially because there is so little public awareness them. In some areas of the Mediterranean alone, scientists have estimated as much as a 30 percent loss in the last 50 years.

Globally, about 54 percent of seagrass meadows have lost parts of their distribution, according to Seagrass-Watch, the Australia-based seagrass conservation group.. The documented losses of seagrass meadows since 1980 are equivalent of two football fields per hour. Some losses are natural, due to storms and underwater herbivores, however most losses are due to human activities, suchas pollution from oil spills and by boat propellers and cargo that can rake through seagrass meadows and cut through roots.These activities cloud the waters and reduce available light that seagrass meadows need to thrive.

Efforts to protect seagrass include prohibiting anchoring in them, which uproots and flattens the grass, reducing pesticide and other forms of chemical runoff, limiting destruction by coastal development, and restricting the use of dragnets and other damaging types of fishing gear.

But the greatest threat to the meadows is the very phenomenon which they counteract, global warming. The specific type of sea grass in the Mediterranean, Posidonia, may not be able to survive as the water temperature become warmer.

Several days later I did my last dive in the Posidonia. Kicking back to the boat, I turned around for one last look. I felt such specific gladness, knowing that these “lungs of the Mediterranean” were so rich with life because of efforts to protect them. Life in these sea meadows thrive because they are in a marine protected area. While they are still terribly at risk around the world, many people are at work to preserve them. Port-Cros’s Operations Director, Nicolas Gerardin, is at work to implement the expansion of the Port-Cros protected area, which was approved in 2006. And he works with The Network of Managers of Marine Protected Areas in the Mediterranean, which facilitates exchanges between Mediterranean protected areas to improve their efforts to protect sea life.

Before my visit, seagrass meadows were an interesting, but a bit arcane facet of ocean conservation. Now I see them as a magnificent and fascinating terrain. No dramatic drop-offs in ocean abysses or large pelagic creatures such as whales and sharks, but like my grandmother’s meadow the seagrass meadows have a peacefulness, a quietude to them, filled as they are with the “extraordinary ordinary” of life.

Below is a short video version of this essay, using footage shot by Cunningham in the Mediterranean and featuring the bluegrass music of Grant Hunnicutt.

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