WATER TRAPPING BY SEAGRASSES OCCUPYING BANK HABITATS IN FLORIDA BAY

Seagrasses, largely Thalassia testudinum, occupy habitats atop shallow ( < 0·5 m deep) carbonate mudbanks adjoining basins up to 3 m deep in Florida Bay. We quantified the phenomenon of water trapping whereby, during low tides, the seagrass meadow matrix retains a thin ( < 20 cm) layer of water high on the bank top despite water levels in the adjoining basins being some 25-70 cm lower. The matrix slows water flow off the banks such that changes in the rate at which water recedes through time approximates a sigmoid function of water level. A meadow with a large seagrass standing crop (59·0 g dry mass m-2) held a 17·4-cm layer of water atop the bank, while a meadow of lesser standing crop (less biomass per area, 13·0 g dry mass m-2) that may have been facilitated by a topographical berm held just 3·3 cm of water. Similarly, on the bank slope the higher standing crop meadow held 10·4 cm of water while the bank slope meadow at the site with lesser standing crop held only 1·6 cm of water. Water trapping by seagrass can keep water on the banks for up to 8 h during low tides, preventing desiccation of the bank, and thereby providing permanent habitat for a diverse community of epibenthic fishes and invertebrates. The water trapping phenomenon presumably enhances overall prey abundance and diversity, and regulates the temporal patterns of prey exposure to different types of predation risk, e.g. to wading birds vs. predatory fishes. © 1991.