Trophic Linkages of Intracoastal Waterway Seagrass Beds in Browards County, Florida

Seagrass habitats support marine food webs and provide essential habitat for a variety of species. Seagrasses and associated algae at three locations along the Intracoastal Waterway in Broward County, FL were assessed for their trophic contribution to the marine organisms in the area. Two seagrass species, along with associated algae, invertebrates, and vertebrates, were analyzed for stable carbon and nitrogen isotope ratios to determine trophic relationships. Significant differences were found in δ13C and δ15N between both seagrass species and among the three sites. The δ13C of Johnson's seagrass Halophila johnsonii ranged from -16.28 to -11.27% while shoal grass Halodule wrightii ranged from -15.78 to -13.36%. The δ15N for shoal grass ranged from 4.69 to 7.08% versus 0.80 to 7.86% for Johnson's seagrass. Neither seagrass species was a dominant food source and epiphytes appeared to be the greater trophic contributor. However, the δ13C (-16.28 to -11.27%) and δ15N (0.80 to 7.86%) of both seagrass species did fall in the fractionation range of potential consumers suggesting that seagrass material could be ingested incidentally while grazing on epiphytes and other primary producers. Our results indicate that seagrass in Broward County are valuable both as a direct food source and as substrate for epiphytes.