Variation in food sources of the macrobenthos along a land–sea transect: a stable isotope study

Abstract

To evaluate the relative importance for the macrobenthos of possible food sources, including riverine particulate organic matter, reeds, benthic microalgae, seaweeds, coastal phyto- plankton and aquaculture-derived matter, over 194 macrofaunal species were collected from 9 inter- tidal and subtidal stations along the axis of Gokasho Bay, Japan, and their isotopic compositions ana- lyzed. The δ 13 C values for terrestrial organic matter were very different from those of the consumers (>5‰), suggesting a negligible trophic role for this source. Differences in δ 15 N between seaweeds and primary consumers were small, suggesting a minor contribution of seaweeds. The δ 13 C values of the majority of consumers fell between coastal phytoplankton and benthic microalgae, allowing the calculation of the relative contribution of these microalgae to the diets of consumers. The estimated contribution ratios and the between-site comparison of the δ 13 C value for the same species along the bay axis suggest a shift of the main food source from benthic microalgae on the mudflat to coastal phytoplankton at the subtidal seaward stations. Several consumers collected from the fish-farm area had enriched δ 15 N and depleted δ 13 C values relative to samples of the same species that occurred out- side of the farm area. The δ 13 C values of the consumers at the fish farm were significantly more depleted than those at the other stations. These findings suggest that the consumers inhabiting the farming area incorporate 15 N-enriched fish meal and 13 C-reduced cereals that are major constituents