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 delta C-13 values of food sources, including riverine and terrestrial materials (<-25.6 parts per thousand.), autochthonous particulate organic matter (POM, -23.6 parts per thousand.), the macroalga Gracilaria vermiculophylla (-20.4 parts per thousand), marine POM (-19.9 parts per thousand), benthic and epiphytic diatoms (-16.5 to -15 parts per thousand) and the macroalga Enteromorpha prolifera (-13.1 parts per thousand). G. vermiculophylla was distinguished from marine POM by a high 6 15 N value. Consumers showed dietary shifts on a small scale (similar to 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 delta C-13 and/or delta N-15 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 mu g l(-1)). In macroalgal patches (Site B-algae), epiphytic diatoms were a major dietary component of consumers. Based on the delta C-13 range of consumers (-21.5 to -13.5 parts per thousand.), the benthic food web was primarily based on estuarine and marine microalgal production, whereas riverine and terrestrial materials were of minor importance.