Temporal evolution of sand corridors in a Posidonia oceanica seascape: a 15-years study

The spatial dynamics of the Posidonia oceanica meadows is a process extending over centuries. This article shows evidence of the natural dynamics of the P. oceanica "shifting intermattes" or "sand corridors" (hereafter SCs) - unvegetated patches within a dense meadow. We have studied the features and temporal evolution (2001-2015) of five SCs in the Calvi Bay (Corsica) at a 15 m depth and followed the characteristics of the P. oceanica meadow lining the edge of patches. All SCs show a similar morphology. The eroded side has a vertical edge where roots, rhizomes, and sediments are visible, whereas, on the opposite colonized side, the sand is at the same level as the continuous meadow. The vertical edge reaches a maximum height of 160 cm and is eroded by orbital bottom currents with a maximum speed of 12 cm. s(-1)- the erosion speed ranging from 0.6 to 15 cm.a(-1). SCs progress toward the coastline with a mean speed of 10 cm.a(-1). The rate of colonization by the P. oceanica shoots ranges from 1.5 to 21 cm.a(-1). We have calculated that the studied SCs will reach the coastline within 500 to 600 years. We finally discuss the implication of such dynamics in the framework of the assessment of meadow colonization and the seascape dynamics.