The problem has become urgent. Posidonia only develops in the Mediterranean between the surface and 40 metres in depth. It is therefore a coastal plant and this is the area worst affected by human activity in our seas and oceans. Several causes of the decline of this plant can be listed. For example, the development of ports and marinas. An increase in the nutrients that are consumed by epiphytic algae that develop more easily. These multiply, cover the posidonia and deprive them of the light required for photosynthesis. Another destructive factor is the increase in the turbidity of the water. This prevents the light from passing through it and limits the depth at which posidonia can develop. Very often, they do not venture beyond 15 to 25 meters below the surface. This is a pity from an ecological point of view. “These are veritable forests under the sea”, explains Sylvie Gobert. “They enable other plants and animals to live and lead to increased biodiversity. But they also have a slowing effect on erosion and stabilize sediment, their long leaves slow down currents and protect beaches. They produce oxygen and constitute an extremely important carbon sink”. (See also Posidonia under surveillance and The vigils of the coastal environment)

By way of comparison, a forest stores 8 grams of carbon per square meter per year. For a similar surface and over the same period, a posidonia meadow can absorb 200 to 300 grams. Its storage capacity is 20 to 30 times greater than that of a forest. “These ecosystems have surfaces that are not comparable to European, African or Amazonian forests”, explains Alberto Borges, senior research associate at the FRS-FNRS and head of the Chemical Oceanography Unit of ULg. He goes on, “But they also constitute non-negligible carbon sinks. We can also see the problem from the opposite viewpoint. If these systems disappear, not only will we lose a sink of CO₂, but the stored carbon will be remobilized and rereleased into the atmosphere”.

Sediment as a means of storage

The mechanism that transforms these meadows into CO₂ sinks also transforms their disappearance into a veritable environmental sword of Damocles. “The posidonia leaves can reach one meter in height”, explains Willy Champenois, who is a chemist and a doctoral student of oceanography. “Their large size and the way there are organised into clumps has the effect of slowing down the currents. These same currents are laden with sediment. When they slow down, the sediment falls and covers the seabed. If the posidonia had no way to counteract this drawback, they would end up being completely buried”.  These leaves grow from a network of rhizomes, horizontal underground rods. In order to prevent themselves from being buried, these rods can also grow vertically. Thanks to this mechanism, the bottom of the meadow is raised by 1 milimeter per year on average. “In one thousand years the meadow will have risen by one meter. Within this meter of sediment known as the mat, the old rhizomes, roots and other forms of waste are captured at an almost fossilized state. They do not decompose, and the organic carbon of which they are partially composed, remains trapped”.  

If the plant dies and decays it can no longer act as a protective barrier to the seabed and does not therefore slow the current down. The hydrodynamics will progressively erode the surface of the mat and release the organic matter that was trapped there. This matter will then decompose. The carbon which was stocked there up to that point will then come into contact with an aerobic environment and released in the form of CO₂. “We recorded a mat depth of six to seven meters in places. This represents carbon-stocking of six or seven thousand years which goes back to a period just after the last ice age. This will give some idea as to the concentration of carbon that would be released into the atmosphere if these meadows were to disappear completely”. “And if the vertical growth of the posidonia communities is one milimeter per year, continues Alberto Borges, “their horizontal growth over a similar period is one or two centimeters. They take a considerable amount of time to spread out. And when, for example, the anchor of a sailboat is raised tearing off a square meter of seagrass, one century of growth has been destroyed”.