Macroalgae and seagrass contribution to gastropods in sub-tropical and temperate tidal flats

Macroalgal and seagrass communities are widely distributed in marine and brackish shallow water and have high productivity. The primary production in marine costal areas is often transported to intertidal flats by waves and currents and beached on the flats. Thus, we hypothesized that the macroalgae and seagrasses are food sources for benthic communities on intertidal flats where some gastropod species often dominate. We performed comparisons of food sources among different gastropod species on sub-tropical and temperate tidal flats (26 degrees and 38 degrees N, respectively), and used isotope mixing models using carbon and nitrogen stable isotopes. A mixing model for stable isotopes, IsoSource, revealed that main food sources for three snails were macroalgae (50-56%) and seagrass (39-45%) at the temperate site. The contributions of terrestrial plants, sediment organic matter and benthic microalgae were weaker than those of macroalgae and seagrasses. At the sub-tropical site, snails fed mainly on macroalgae. The differences in food sources between snail species were not remarkable, although the nitrogen values were slightly different. It would thus appear that macroalgae and seagrass play an important role in the food webs not only in their own habitats but also on the adjacent tidal flats.