Intertidal bare mudflats subsidize subtidal production through outwelling of benthic microalgae

An investigation of food web structures in the northwestern soft-bottom area, including the intertidal and subtidal zones, of inner Ariake Bay, suggests that resuspended BM are substantial for offshore secondary production, and the main source of BM would be outwelling from the intertidal mudflat. Carbon and nitrogen stable isotope ratios were measured for macrobenthos samples from the intertidal mudflats, subtidal fringe and offshore subtidal zones, together with their potential food sources. Carbon and nitrogen signatures indicated that most macrobenthos assimilated offshore particulate organic matters (POM) (δ13C: −23.9 ± 0.1‰; δ15N: 8.2 ± 0.1‰) and benthic microalgae (BM) (δ13C: −16.8 ± 1.5‰; δ15N: 8.0 ± 0.4‰) among other potential sources. The relative BM contribution to filter feeders significantly decreased seaward from about 50% in the mudflats to 25% in the offshore zone, and the 99% confidence intervals of the contribution did not include zero even in the offshore zone.