Transport pathways of microphytobenthos-originating organic carbon in the food web of an exposed hard bottom shore in the Seto Inland Sea, Japan

Benthic microalgal production has been clarified to be comparable to water column production in coastal shallow water ecosystems. The present study examined carbon transport pathways from microphytobenthos to predators in a food web of an exposed hard bottom shore in the Seto Inland Sea of Japan in winter, noting the intermediary role of suspension feeders in the carbon transport. Natural abundances of carbon and nitrogen stable isotopes were examined for epilithic organic matter (EOM), suspended particulate organic matter (SPOM), a macroalga, molluscs, crustaceans and fish. All the heterotrophs, including herbivores, suspension feeders and carnivores, showed high δ13C signatures of –15.5 to –9.7‰, relative to the δ13C of –20.7 to –19.7‰ for the SPOM collected from the offshore surface water as an indicator of phytoplankton. At the shoreline point, the SPOM showed high δ13C values of –16.0 to –13.0‰, overlapping with the values of –15.4 to –12.6‰ for the EOM indicative of microphytobenthos collected at the low sea level points, and abundantly included microalgae of the same species found in the epilithic matter. It was thus considered that resuspension of13C-enriched microphytobenthos increased the δ13C of SPOM in the surf zone and consequently increased the δ13C of the suspension feeders through their feeding on the suspended matter. The δ15N of carnivores, which overlapped in δ13C with these primary consumers, markedly increased with the trophic position. These results indicate that, not only grazing, but also suspension feeding functioned as transport pathways of the13C-enriched microphytobenthos in the food web.