Trophic roles of particle feeders and detritus in a mangrove island prop root ecosystem

Abstract

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 δ 13C‰) to elucidate the origin and fate of detritus. Detrital components (−23.6 to −19.1‰), 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‰) and A. klebsii (−25.8 to −21.2‰), 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‰), red mangrove Rhizophora mangle leaves (−28.2 to −27.0‰), and turtle grass Thalassia testudinum (−12.3 to −11.1‰). Particle feeders such as mangrove oysters, sponges, mysids, and dioithonans were usually enriched with 13C (−21.0 to −16.2‰) compared to their probable food sources, detritus and A. klebsii. At Anchovy Bay, adult mysids and planktivorous fish were more 13C enriched (1–3‰) 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 13C enriched (2–7‰) than in the Lair Channel where turtle grass beds were sparse.