TROPHIC ROLES OF PARTICLE FEEDERS AND DETRITUS IN A MANGROVE ISLAND PROP ROOT ECOSYSTEM

Swarms of Dioithona oculata, shoals of Mysidium columbiae, and schools of planktivorous fish were observed among mangrove prop roots on a la goonal island. Detritus covering epiphytic and benthic macroalgae and invertebrates was a probable food source for pelagic mysids, and copep ods, as well as benthic oysters and sponges. Open water blooms of dino flagellate Amphidinium klebsii were probably only accessible to the sw arming dioithonans which spend the night away from the mangrove prop r oots. Dominant species were analyzed for stable carbon isotopic compos ition (expressed as deltaC-13 parts per thousand) to elucidate the ori gin and fate of detritus. Detrital components (-23.6 to -19.1 parts pe r 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 thousan d) and A. klebsii (-25.8 to -21.2 parts per thousand), although other sources which are end-members could theoretically contribute since det rital isotopic ratios are half way between end-members. These other so urces 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 oyst ers, sponges, mysids, and dioithonans were usually enriched with C-13( -21.0 to -16.2 parts per thousand) compared to their probable food sou rces, detritus and A. klebsii. At Anchovy Bay, adult mysids and plankt ivorous fish were more C-13 enriched (1-3 parts per thousand) than the ir 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-1 3 enriched (2-7 parts per thousand) than in the Lair Channel where tur tle grass beds were sparse.