Dual isotope assessment of trophic dynamics of an intertidal infaunal community with seasonal shifts in food sources

Stable isotopes are widely used to better understand dietary contributions and trophic interactions in benthic ecosystems. However, the traditional δ¹³C–δ¹⁵N bi-plots can only depict food-web structure for a single time period or space location. Various statistical approaches have now been developed to identify food-web changes in temporal or spatial scale. In the present study, three community-level statistical methods, including circular statistics, Layman et al.’s metrics and Bayesian Ellipses, were applied to compare natural abundance stable carbon and nitrogen isotope data from primary producers, meiofauna (nematodes and copepods), and macrofauna (polychaetes) in an intertidal area between summer and winter. Bayesian mixing model was also used to determine the food source contributions of consumers. In summer, data suggested that benthic microalgae were a dominant food source for most consumers, except for nematode Spilophorella sp. which mainly relied on seagrass detritus. While in winter, there was a significant increase in reliance on seagrass-based carbon for the consumers. The trophic positions of polychaete Eunice sp. and nematode Ditlevsenella sp. also differed between summer and winter, suggesting that they could change their feeding habits and behaviour according to time and availability of food. Using community-based statistics to quantitatively analyze stable isotope data can thus elucidate changes in food-web structure. In addition, considerable variations in food source utilization and trophic level for the same genus or species may imply that the feeding ecology of benthic infauna should be interpreted with caution and cannot be relied on buccal morphology alone.