Long term expansion of a deep Syringodium filiforme meadow in St. Croix, US Virgin Islands: the potential role of hurricanes in the dispersal of seeds

Abstract

Automated image classification techniques were applied to aerial photographs of a deepwater (10–20 m) Syringodium filiforme bed in Buck Island Channel, St. Croix, US Virgin Islands to quantify change in its extent from 1971 to 1999. An increase in seagrass coverage from 1.33 km 2 of sea bottom to 4.34 km 2 was documented. Ninety-two percent of the area already covered in 1971 was still occupied in 1999. In addition, the relative contribution of horizontal expansion of existing beds in 1971 versus that of seed dispersal and growth were estimated. Fifty-four percent of the new seagrass area in 1999 was within the distance of possible horizontal growth of 1971 patches, whereas the rest was outside of this distance suggesting that only dispersal, germination, and subsequent growth of seeds could be responsible for this new colonization. New seagrass patches were not spread randomly throughout the channel; rather they were concentrated near patch reefs but beyond the usual sand halo typical of reef/seagrass interfaces. The current period of increasing meadow extent is coincident with a greater frequency of hurricanes in the region. Since no other causal mechanism could be identified, we suggest that this higher hurricane frequency enhanced seed and seagrass fragment dispersal.