Trophic importance of epiphytic algae in subtropical seagrass beds: Evidence from multiple stable isotope analyses

Abstract

Multiple stable isotope analyses were employed to examine food web dynamics in a northern Gulf of Mexico seagrass system in which epiphytic algae were the single most important primary productivity component, being responsible for 46 and 60% of total system and benthic primary production, respectively. The seagrass Halodule wrightii Ascherson contributed only 13% to total system primary production on an annual basis. Stable isotope ratios of carbon (δ13C), nitrogen (δ15N), and sulfur (δ34S) were measured for producer and consumer samples collected from May 1989 through November 1992. Epiphytes and leaves of H. wrightii had distinct δ13C values (-17.5 vs -12‰, respectively) as well as distinct δ34S values ‰ (+18 vs +11‰ respectively). δ13C values for the sand microflora, occasional macroalgae, and phytoplankton were -16, -17, and -22‰, respectively; δ15N values were lowest for epiphytes and H. wrightii (+6‰) and highest for phytoplankton (+10‰). Virtually all consumers had δ13C values that fell within a narrow range of -20 to -15‰, which included all δ13C values of epiphytes and the sand microflora but none of those for either H. wrightii or phytoplankton. Values for δ15N for consumers fell within a range of +8 to +16‰, spanning herbivorous species with diets of microalgae to carnivorous species feeding at secondary to tertiary levels in the local food webs. Consumer values for δ34S ranged from +4 to +20‰ (mean = 14.2‰), and indicate a stronger influence of seawater-derived sulfate than sediment-associated sulfides. The stable isotope data, in combination with measured high biomass and primary production rates of the epiphytic algae, strongly suggest that these algae are the primary source of organic matter for higher trophic levels in seagrass beds of Mississippi Sound. The contribution of H. wrightii to the food web appears to be minimal. The overall picture that has emerged based on the present and previous stable isotope studies is one of the major trophic importance of benthic microalgae (i.e. epiphytic and sediment-associated) in seagrass beds.