Spatial dietary shift of macrozoobenthos in a brackish lagoon revealed by carbon and nitrogen stable isotope ratios

The food sources of 14 benthic consumer species (polychaetes, bivalves, amphipods and decapods) were investigated at 2 locations in a brackish lagoon (Gamo Lagoon, Japan) using carbon and nitrogen stable isotope ratios. Consumer diets were elucidated from the 13 C values of food sources, including riverine and terrestrial materials (< -25.6 ), autochthonous particulate organic matter (POM, -23.6 ), the macroalga Gracilaria vermiculophylla (-20.4 ), marine POM (-19.9 ), benthic and epiphytic diatoms (-16.5 to -15 ) and the macroalga Enteromorpha prolifera (-13.1 ). G. vermiculophylla was distinguished from marine POM by a high 15 N value. Consumers showed dietary shifts on a small scale (~200 m) corresponding to changes in the food supply. At the lagoon mouth (Site A) benthic diatoms were the major diet for deposit feeders with enriched 13 C and/or 15 N values. Suspension feeders also assimilated a certain amount of benthic diatoms as well as marine POM due to current induced resuspension at this site. In contrast, autochthonous POM (phytoplankton) was an important diet component in the inner lagoon (Site B-bare) where chlorophyll a concentration was high (110 to 113 g l -1 ). In macroalgal patches (Site B-algae), epiphytic diatoms were a major dietary component of consumers. Based on the 13 C range of consumers (-21.5 to -13.5 ), the benthic food web was primarily based on estuarine and marine microalgal production, whereas riverine and terrestrial materials were of minor importance.