Mat of Oceanic Posidonia
The vertical growth of the rhizomes leads to the formation of a structure called Mat, consisting of a tangle of dead rhizomes and roots among which the sediment remains trapped. Only the top part of these structures is formed by live plants.
The formation of the Mat depends mostly on sedimentation rates. A high sedimentation rate can lead to excessive silting of the rhizomes and therefore to their suffocation; On the contrary, too slow a sedimentation can lead to the lifting of the rhizomes and the regression of the prairie. Because the rate of decay of the rhizomes is very slow they can remain within the wild even for millennia.
The growth of the Matta has been estimated by many scholars in about 1 m per century.
In This movie we show you a mat dead Posidonia. The plant of Posidonia, due to pollution, to climate change or to reasons that we do not know, is dead leaving only the rhizomes.
You can see very well how the wave motion has undermined part of the substratum and this phenomenon contributes to the erosion of the beaches. The Posidonia prairie is in fact a natural barrier to coastal erosion.