Alternate Title

Assessment of Seagrass Plant Demography Within and Among Beds of Turtle Grass (Thalassia testudinum), Manatee Grass (Syringodium filiforme), and Shoal Grass (Halodule wrightii) From the Northern Gulf of Mexico


Variability in the demographic patterns of seagrasses may affect a variety of physical and biological processes in aquatic communities. We investigated seasonal seagrass demography, sediment size structure, and sediment organic carbon from two sites (north and south) in St. Joseph Bay, Florida, for Thalassia testudinum, Syringodium filiforme, and Halodule wrightii. Geologically, sands dominated both sites, but the south site had significantly greater proportions of fine and very fine sands, whereas the north site was dominated by coarse and medium sands. Within each site, demographic parameters were assessed for edge (<1 m) and interior portions (>10 m) of grass beds. The objective was to quantify within-habitat demographic characteristics and assess whether differences in plant biomass and shoot density existed between edge and interior portions. Results for T. testudinum showed significantly greater shoot density (P < 0.05) and plant biomass (P < 0.001) from interior portions of the bed than from the edge for the north site, but there were no significant differences for the south site. Syringodium filiforme showed significantly greater shoot density from bed interiors than from edges for both the north and south sites (P < 0.0001, P < 0.001, respectively) as well as plant biomass (P < 0.0001, P < 0.01, respectively). Halodule wrightii showed a similar response, with greater shoot densities from the north interior (P < 0.008) and significantly greater plant biomass from interior portions of beds for both the north and south sites (P < 0.003, P < 0.01, respectively). Within-bay comparisons showed that plant biomass and shoot density were generally greater from the north site than from the south site for T. testudinum and H. wrightii, but the reverse was true for S. filiforme. Although sands dominated both sites, the south site showed greater percentages of finer sediments, and S. filiforme biomass has been shown to be inversely related to sediment size. Although we were unable to resolve within-habitat differences in sediment size structure, which may suggest sediment baffling and water velocity changes, we surmise that physical and sedimentary differences exist between edge and interior portions of seagrass beds. These differences may have dramatic effects on processes such as larval settlement, predator-prey interactions, and per unit area primary production. As such, further investigations into within-habitat variability and edge effects in plant demography and associated biological and physical processes are needed.