Background
The authors write that Blue Carbon, which is carbon captured by living marine organisms, "has recently been highlighted as a new option for climate change mitigation initiatives," noting that "coastal ecosystems have been recognized as significant carbon stocks because of their high burial rates and long-term sequestration of carbon." But they say that "the direct contribution of Blue Carbon to the uptake of atmospheric CO2 through air-sea exchange remains unclear."

What was done
In the hope of clarifying this situation, Tokoro et al. investigated carbon flows, including air-sea CO2 fluxes, changes in dissolved inorganic carbon (DIC), net ecosystem production and carbon burial rates in boreal, temperate and sub-tropical seagrass meadows of Japan from 2010 to 2013, where the air-sea CO2 flux was measured using three different methods: (1) the bulk formula method, (2) the floating chamber method, and (3) the eddy covariance method. This they did over different temporal and spatial scales, after which they quantified the ecosystem function of direct CO2 uptake from the atmosphere by shallow coastal waters with submerged aquatic vegetation.

What was learned
Contrary to the conventional perception that most near-shore ecosystems are sources of atmospheric CO2, the eight Japanese researchers discovered that "submerged autotrophic vegetation in shallow coastal waters can be functionally a sink for atmospheric CO2." In fact, they indicate that "some studies have estimated that the rate of carbon accumulation is higher in coastal sediments (238 Tg C/year) than that in open ocean sediments (6 Tg C/year)," citing Nellemann et al. (2009).

What it means
In the words of Tokoro et al., "this study thus identifies a new ecosystem function of coastal vegetated systems; they are direct sinks of atmospheric CO2."

Reference
Nellemann, C., Corcoran, E., Duarte, C.M., Valdes, L., DeYoung, C., Fonseca, L. and Grimsditch, G. 2009. Blue Carbon: A Rapid Response Assessment. United Nations Environmental Programme, GRID-Arendal, Birkeland Trykkeri AS, Birkeland.