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

Abstract

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 δ 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 δ 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 δ 13 C and/or δ 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 (phytoplank- ton) was an important diet component in the inner lagoon (Site B-bare) where chlorophyll a concen- tration was high (110 to 113 µg l -1 ). In macroalgal patches (Site B-algae), epiphytic diatoms were a major dietary component of consumers. Based on the δ 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.