Stable isotope ratios reveal food source of benthic fish and crustaceans along a gradient of trophic status in the East China Sea

The East China Sea (ECS) receives large quantities of particulate organic matter (POM) and inorganic nutrients transported from the Changjiang (Yangtze River), which have produced high productivity in the northwestern ECS. This study evaluated potential contributions of terrigenous POM (allochthonous food source) and nutrient-induced marine production (autochthonous source) to the ECS benthic ecosystem by analyzing stable isotopic compositions of phytoplankton, zooplankton, benthic crustaceans and fish. Benthic consumers exhibited δ13C values similar to those of their autochthonous food sources (i.e., phytoplankton and zooplankton), revealing their major reliance on marine production. In contrast, the δ13C values of benthic fish (−19.6‰ to −13.5‰) and crustaceans (−18.9‰ to −15.0‰) were much higher than that of terrigenous POM (−25.7‰), which generally accounted for less than 20% of the most fish diet. Phytoplankton and zooplankton generally exhibited higher δ13C values at eutrophic and highly productive inshore sites than at oligotrophic offshore sites. This enrichment of inshore δ13C values was mainly attributed to lower photosynthetic fractionation during algal blooms, an effect that was further enhanced during flood period of the Changjiang. The δ13C values of demersal fish assemblages were also significantly higher at inshore sites and decreased seaward. However, fish δ15N values and their estimated trophic levels showed relatively small spatial variation. The disproportionate variations in δ13C and δ15N values suggested that the enriched C isotopic signatures derived from an elevated δ13C baseline of the inshore food web instead of trophic enrichment of the isotopic ratios. The significantly positive correlations between concentrations of chlorophyll a and nutrients versus fish δ13C provided further evidence for the use of pelagic algal bloom materials by inshore consumers. The isotopic and oceanographic survey data suggested that inorganic nutrients discharged from the Changjiang River nourish benthic consumers in the ECS and play an important role in linking marine benthic ecosystems to local pelagic primary production as well as to the adjacent terrestrial watershed.