Manatee Grazing Impacts on Seagrasses in Hobe Sound and Jupiter Sound in Southeast Florida during the Winter of 1988-89

摘要

Manatees (Trichechus manatus) are among the few large vertebrate grazers in seagrass systems. Manatee grazing impacts on seagrasses in Hobe Sound and Jupiter Sound on the east coast of Florida were investigated during the winter of 1988-89. Aerial surveys were conducted to document manatee distribution and habitat use patterns in the study area and to locate feeding manatees. Manatee feeding behavior was observed by boat and the effects of manatee grazing on seagrass beds were assessed using paired biomass cores in grazed and ungrazed areas. Manatee grazing significantly reduced both above and below ground biomass of the dominant seagrasses, Halodule wrightii and Syringodium filiforme, at 16 feeding locations (7 in Hobe Sound and 9 in Jupiter Sound). Manatees appeared to prefer Halodule in Hobe Sound, but not in Jupiter Sound. Grazing removed 82 and 85% of the shoots, 79 and 89% of shoot biomass, and 46 and 67% of the root+rhizome biomass in grazed patches of Halodule and Syringodium, respectively. The average depth of identified grazing sites was 0.9m (range = 0.4-1.6m); Halodule and Syringodium are restricted to depths under 2m in Hobe Sound.