Listening to posidonia seagrasses
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) Sediment as a means of storageThe 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”. |
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© 2007 ULi�ge
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