Isotopic evidence for phytoplankton as a major food source for macrobenthos on an intertidal sandflat in Ariake Sound, Japan

To delineate a soft-shore trophic structure, macrobenthos was collected from an intertidal sandflat at the mouth of the Shirakawa River in the central part of Ariake Sound, Kyushu, Japan, on 3 occasions in 2003 (January, March, and June to July during the rainy season), and the isotopic compositions of animals and their potential food sources were analyzed. Food sources of animals were assessed based on the diet-tissue isotopic fractionation (delta C-13 = 0.6 to 2.0 parts per thousand, delta N-15 = 3.4 to 3.9 parts per thousand) of 3 main constituent species (the bivalves Mactra veneriformis and Ruditapes philippinarum and the ghost shrimp Nihonotrypaea japonica), which had been determined by an earlier laboratory experiment. The results revealed that (1) consumers were classified into 3 groups; a group consisting of 51 species including 4 main species (the above-mentioned species and the mud shrimp Upogebia major) that derived nutrition mainly from coastal phytoplankton, one consisting of 7 species that incorporated more C-13-enriched material (e.g. benthic microalgae) and a group consisting of 4 species that incorporated more C-13-reduced material (e.g. riverine organic matter); (2) the phytoplankton-based trophic structure was composed of 3 trophic levels; (3) M veneriformis juveniles collected during the rainy season appeared to incorporate riverine organic matter; and (4) reeds from the flood plain, detrital terrestrial plant material, sewage effluents and seaweeds were not food sources for most consumers. We concluded that the high phytoplankton biomass and scarcity of benthic microalgae on this sandflat account for its phytoplankton-based trophic structure.