Swarms of Dioithona oculata, shoals of Mysidium columbiae, and schools of planktivorous fish were observed among mangrove prop roots on a lagoonal island. Detritus covering epiphytic and benthic macroalgae and invertebrates was a probable food source for pelagic mysids, and copepods, as well as benthic oysters and sponges. Open water blooms of dinoflagellate Amphidinium klebsii were probably only accessible to the swarming dioithonans which spend the night away from the mangrove prop roots. Dominant species were analyzed for stable carbon isotopic composition (expressed as deltaC-13 parts per thousand) to elucidate the origin and fate of detritus. Detrital components (-23.6 to -19.1 parts per thousand), which included floating detritus, marine snow, prop root detritus, and detritus below prop roots, probably originated from the dominant subtidal macroalgal species (-24.2 to -14.6 parts per thousand) and A. klebsii (-25.8 to -21.2 parts per thousand), although other sources which are end-members could theoretically contribute since detrital isotopic ratios are half way between end-members. These other sources include the intertidal red algal Bostrychia spp. (-30.9 to -29.6 parts per thousand), red mangrove Rhizophora mangle leaves (-28.2 to -27.0 parts per thousand), and turtle grass Thalassia testudinum (-12.3 to -11.1 parts per thousand). Particle feeders such as mangrove oysters, sponges, mysids, and dioithonans were usually enriched with C-13(-21.0 to -16.2 parts per thousand) compared to their probable food sources, detritus and A. klebsii. At Anchovy Bay, adult mysids and planktivorous fish were more C-13 enriched (1-3 parts per thousand) than their probable prey, the dioithonans. Turtle grass may have a minor role in food webs, since dioithonans and mysids from Outer Twin and Anchovy Bays where turtle grass beds were abundant were consistently more C-13 enriched (2-7 parts per thousand) than in the Lair Channel where turtle grass beds were sparse.
