Loss of Ocean Meadows Poses a Double-Edged Climate Threat

​The seagrass genus Posidonia plays a major role in the ecological health of Mediterranean and South Australian waters and coasts. But over the last several decades, scientists have noted a troubling decline of Posidonia meadows, buoyed on by human activity on both the local and planetary scale.

"The major causes of [Posidonia] loss were widespread local disturbances," wrote the authors of a Biological Conservation study published in August 2014.

"[B]ut recently," the team added, "global disturbances, such as climate change and the spread of invasive exotic species, were also seriously threatening Posidonia meadows in the Mediterranean. These findings urgently call for implementation of management measures aiming at mitigating coastal deterioration by combining local and global actions."

Unfortunately, this seagrass isn't just a potential victim of climate change: it may also contribute to it if this downward spiral remains unchecked. These aquatic meadows are very efficient at carbon sequestration, and have been actively locking away carbon from their undersea environments for upwards of 100,000 years. Like all collectors of "Blue Carbon"—the term for carbon captured by aquatic ecosystems—Posidonia plants reduce the amount of greenhouse gases that end up in the atmosphere.

According to a study published today in the Journal of Ecology, the erosion of Posidonia meadows disrupts this cycle of carbon sequestration in two ways. First and most obvious, the reduced real estate for the meadows will lead to less carbon storage on a larger, regional scale.

But, on top of that, the carbon that the seagrass has already squirreled away will be released as meadows are trampled by human activity and climate change, creating an entirely new source of carbon-based gases. It's an aquatic double whammy, and with Mediterranean biodiversity already taking a worrisome nosedive, it's shaping up to have serious consequences.

Fortunately, the main thrust of the paper involved assessing the merits of revegetation, which is the oceanic equivalent of tree-planting. The authors, based out of the Spanish National Research Council, the Universitat Autònoma de Barcelona, and the Oceans Institute of the University of Western Australia, concluded that revegetation efforts prevent unchecked loss of carbon from obliterated Posidonia colonies.

"Seagrass revegetation enhanced […] carbon deposition and burial," wrote the authors in the study's abstract. "Carbon burial rates increased with the age of the restored sites, and 18 years after planting they were similar to that in continuously vegetated meadows."

This finding challenges earlier speculation that revegetation might not be effective in staving off a massive reduction of oceanic carbon sinks, which the authors claim has stalled the implementation of Blue Carbon policies.

While more research into which methods of re-establishing Posidonia meadows is needed, this study makes a clear argument that revegetation itself is an effective environmental preservation technique on both regional and planetary scales.

"Seagrass meadows are sites of high rates of carbon sequestration and they potentially support 'Blue Carbon' strategies to mitigate anthropogenic CO2 emissions," the authors concluded. "Thus, conservation and restoration of seagrass meadows are effective strategies for climate change mitigation."