Northeastern University

Blue is the new green

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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.


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