Blue is the new green

Salt marsh at high tide. Photo via Thinkstock.

Salt marsh at high tide. Photo via Thinkstock.

When I was a kid, back when the hole in the ozone layer was the big topic of envi­ron­mental dis­cus­sion, I was obsessed with saving the rain forests. I’d heard (prob­ably watching Fern Gully) that these majestic places, full of life and wonder, were the anti­dote to all the bad stuff we’d done to our planet: they sucked up our carbon dioxide and spit out clean oxygen at a rate faster than–I thought–any other type of habitat on the planet.

But yes­terday, reading a research article from marine and envi­ron­mental sci­ences pro­fes­sors David Kimbro and Ran­dall Hughes, I learned that another habitat is actu­ally 55 times better at sucking up carbon dioxide than my beloved rain forests. Salt marshes, while occu­pying only 0.1 to two per­cent of the global land area as trop­ical rain forests, trap sub­stan­tially more CO2 and store it for mil­lennia, rather than just decades. All this time I’d been trying to “go green,” when per­haps thinking blue could be even more important.

In the last few years, Kimbro told me, “blue carbon” has become a hot item for inquiry. Researchers have begun exam­ining salt marshes and sea grass beds, which cover hectares of the ocean floor, as so-​​called “carbon sinks.” Below them, thou­sands of years worth of carbon has been buried in the sed­i­ment. Above ground, the plants are busy cap­turing gaseous carbon through pho­to­syn­thesis and fil­tering out par­tic­u­late carbon in the sed­i­ment bed. On the one hand we could be thinking of ways to lever­aged these blue carbon sinks for envi­ron­mental clean up and reme­di­a­tion efforts. On the other, we could be con­tributing even more to the green­house gas problem when we dis­rupt these habi­tats, poten­tially releasing stored carbon as CO2.

A lot of work lately has focused on that second bit, looking at vast areas of marsh­land and how their dis­rup­tion through human forces may impact the envi­ron­ment. “Big scale is the focus,” said Kimbro. “We wanted to show that it’s impor­tant on the small scale also.” They wanted to look at how local, nat­ural dis­tur­bances play into a salt marsh’s capa­bil­i­ties as a carbon sink, an impor­tant vari­able when fig­uring out how much value the habitat pro­vides to humans, and thus how much we should weigh it when cal­cu­lating the cost of land use devel­op­ment projects.

Kimbro and Hughes, along with Peter Macreadie, a col­league from the Uni­ver­sity of Tech­nology in Sydney, Aus­tralia, inves­ti­gated the impact of small dis­tur­bances on a salt marsh’s ability to sequester carbon. Specif­i­cally, they looked at some­thing called “wrack accu­mu­la­tion,” which hap­pens when sea grass gets uprooted from its own habitat and swept on top of a salt marsh by the sea. If it sits there long enough, it can cause the marsh grass to die off, which, the researchers hypoth­e­sized, could cause big changes in the carbon pro­file of the soil below.

What those changes might look like was another ques­tion all of its own. Per­haps the carbon trapped in the sea grass would get buried under the dead salt marsh, increasing the carbon con­cen­tra­tion in these areas. Or per­haps the salt marsh destruc­tion would release pre­vi­ously sequestered carbon and get into the envi­ron­ment, either as par­tic­u­late matter or as CO2.

They found the latter to be the case, at least in the par­tic­ular loca­tion they studied — a soccer field sized area of salt marsh off the  coast of Florida. The team exam­ined nearly 300 patches of salt marsh that had been dis­turbed by sea grass for at least three months prior to the study, as well as a matching, undis­turbed plot nearby for each. They took a 15 cm soil core from each bed and tested the amount of carbon in it, both that trapped in the soil sed­i­ment and that in the plant bio­mass. They found a stag­gering 30% decrease in carbon con­tent in the dis­turbed plots versus the undis­turbed areas. In the first cen­timeter of the core, the carbon was mostly lost from plant bio­mass, whereas below that it was lost from the sed­i­ment carbon.

Now Kimbro wants to do sim­ilar studies in other areas, such as salt marshes in the north­east, which are also sus­cep­tible to rack accu­mu­la­tion. “That’s how you get a better handle on the story,” said Kimbro. “If I then go and study it in more places, and each place dif­fers in some little bit, and by seeing how the story changes across all those sites you really get to know the system better.” For example, he may find that sea grass actu­ally does con­tribute to the carbon con­tent in some areas but not others.

The research was recently released in the open-​​access journal PLOS ONE and they hope it will spark more inves­ti­ga­tions in this area by other labs. For one thing, they’d like to figure out where exactly the carbon goes once it’s been released by the dis­tur­bance. It could be get­ting released as CO2, which would not be good, but it could also be get­ting released as par­tic­u­late that even­tu­ally gets buried else­where in the sea. The area of research is becoming increas­ingly impor­tant Kimbro said, because these kinds of local dis­tur­bances will only increase with increasing sea level, allowing the dis­tur­bance to reach more areas. If we’re to really do any­thing with or about these carbon sinks, Kimbro said, we need to under­stand how they behave. That’s what he and his col­leagues intend to do.