"Migration" of blowouts in seagrass beds at Barbados and Carriacou, West Indies, and its ecological and geological implications

Abstract

Blowouts are grass-free depressions within seagrass beds at Barbados and Carriacou and reported in the literature to be common elsewhere in the Caribbean region. They are typically crescent-shaped in plan view with the convex side seaward, and are characteristic of elevated seagrass beds in regions of moderate to strong wave action. The seaward edge is steep and exposes rhizomes of Thalassia while the leeward edge slopes gently upward onto the seagrass plateau and is usually colonized by Syringodium. The general morphology of the blowouts, the zonation of organisms across them, and the existence at some blowouts of a lag deposit of cobble-sized material at the scarp base continuous with a rubble layer below the seagrass carpet suggested the blowouts "migrate" seaward. Measurements of erosion at the scarp and of advance of Syringodium onto the blowout floor over a period of one year confirmed this. It is estimated that in the region of blowouts anyone point will be recurrently eroded and restabilized at intervals of the order of 5-15 years. Such processes limit successional development of the seagrass beds, disrupt sedimentary structures, and may result in deposits much coarser than those characteristic of the sandy seagrass carpet.