Alberto Borges and Willy Champenois have just published the first results (1) on the activity of posidonia (posidonia oceanica) meadows, known for their ability to trap carbon dioxide. Based on measurements taken between 2006 and 2009 at  the University of Liège’s research station in Calvi (STARESO, Corsica), they have produced analyses, at the end of an unprecedented period of monitoring owing to its duration, frequency and precision, aimed at better anticipating how these marine communities could react to future climate changes.

We know that the posidonia meadows – a variety of marine seagrass that is found along the Mediterranean coast – have been discretely storing vast quantities of carbon dioxide (CO₂) for thousands of years in the aquatic ecosystem (read the article The vigils of the coastal environment). “These seagrass meadows store approximately 140 grams of carbon a year per square meter”, says Alberto V. Borges, head of the University of Liège’s Chemical Oceanography Unit, a laboratory that specialises in the study of carbon dioxide flows between the aquatic systems and the atmosphere. “In comparison, tropical or temperate forests only store 4 to 5 g per m^₂ per year. Even if we consider that the posidonia meadows cover less than 2 % of the total surface of the Mediterranean, and less than 0.3 % of the oceans on a global level, it can nevertheless be said that they have a significant capacity to store carbon, undoubtedly equivalent to that of forests, which are nonetheless far more extensive”.

Degradation

Within the context of global warming, these posidonia meadows are interesting in other ways too: these underwater carbon traps are disappearing at an alarming rate. If we look at all the seagrass meadows worldwide, the losses observed over the last 30 years amount to 30 % of the total surface area occupied by these meadows. “The surface area of the posidonia meadows is currently decreasing by 7 % a year, having accelerated since the 1990s. A rate that is particularly worrying since these seagrass meadows are home to a very rich biodiversity which, in addition, makes them highly sought-after spots for leisure divers”, continues Alberto Borges. The origin of this degradation is human activity: construction, mooring sites, the passage of sailing boats, but also urban waste or the increased development of phytoplanktonic algae (microscopic algae whose development is increased by urban pollution and whose accumulation on the surface blocks the light required, like all plants, to sustain the seagrass meadows).  “In the Mediterranean, these seagrass meadows don’t grow beyond a depth of 40 metres owing to the lack of light. This coastal proximity therefore makes them vulnerable to degradation. Because they grow very slowly – barely 10 cm a year horizontally -, any loss to the meadows has a major impact since it takes them a long time to recolonise". If these remarkable ecosystems were to disappear, they would no longer be able to store carbon and would therefore add to global warming. Hence, there is every reason to protect them as much as the tropical rain forests. From this perspective, we can clearly understand the importance of the studies carried out on these seagrass meadows by Alberto Borges, among others:  studying these seagrass meadows doesn’t simply mean assessing the extent of the losses suffered, but by better understanding the factors that influence the dynamics of carbon storage by these ecosystems, we can pave the way to their conservation, or even their restoration.

(1) Seasonal and interannual variations of community metabolism rates of a Posidonia oceanica seagrass meadow. Champenois, Willy; Borges, Alberto V. in Limnology & Oceanography (2012), 57(1), 347-361