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One of the largest contiguous seagrass ecosystems in the world is located on the shallow continental shelf adjacent to the west coast of Florida, USA and is comprised of seasonally ephemeral Halophila decipiens meadows. Little is known about the demography of the west Florida shelf H. decipiens , which may produce 4.56 × 10 8 g C day −1 or more during the peak growing season. We documented seagrass distribution, biomass, and productivity, and density of sediment seed reserves, seedlings, flowers and fruits on the southeastern portion of the west Florida shelf by sampling along a transect at three stations in 10, 15, and 20 m water depth. Biomass, flower, fruit, seedling, and seed bank densities tended to be highest at stations in 10–15 m water depth and lowest at 20 m. Flowers and fruit were most prevalent during summer cruises (June and August 1999, July 2000). Seedling germination occurred during summer, fall (October 1999), and winter (January 2000) sampling events, with the highest seedling densities present during the winter. Seed bank density remained consistent through time. A Category I hurricane with sustained winds of 120 km h −1 passed over the stations, but only limited impact on H. decipiens biomass was observed. The presence of a persistent seed bank provides for recovery after storm disturbance, annual reestablishment of populations, and continual maintenance of the 20,000 km 2 of deep water seagrass habitat present on the west Florida shelf.

Few studies have addressed the spatial and temporal variability in seagrass seed banks. For small-seeded species in particular, seed banks are hard to sample quantitatively because of the difficulty in separating small seeds from sediment. Halophila decipiens is a highly fecund and cosmopolitan seagrass species, occupying niches which other larger-sized perennial species cannot utilize. Although many studies have suggested H. decipiens meadows are annual and depend on seed banks to re-establish, none have quantitatively examined this necessary life history component. To process the number of samples required to adequately address questions of spatial and temporal variability in H. decipiens seed banks, we developed a density separation technique which removes seeds from sediment and allows easier enumeration. Sediment samples were treated with refrigerated Ludox, a colloidal silica. The colloidal silica supernatant and two surface sediment rinses were examined for the presence of seeds. Our protocol removed between 78 and 100% of seeds from sediment, with a mean removal efficiency of 89%. Use of this method increased our sample processing capability from 4 to 20 samples per day.