Contributions of riverborne inorganic and organic matters to the benthic food web in the East China Sea as inferred from stable isotope ratios

Abstract. Coastal areas adjoining rivers are nourished by both the riverborne nutrients and organic matters. Annually, the East China Sea (ECS) receives large quantities of particulate organic carbon transported from the Changjiang (Yangtze River), as well as nutrients, which have brought about high primary production in the ECS. This study evaluated the respective contributions of terrigenous organic matters (allochthonous food source) and nutrient-induced marine production (autochthonous food source) to the ECS benthic ecosystem by analyzing the stable isotope compositions for zooplankton, benthic crustacea and demersal fish. Zooplankton exhibited consistently higher δ¹³C values (−21.31‰ ~ −19.22‰) in the inner shelf than in the outer shelf. The δ¹³C signals of fish (−19.64‰ ~ −13.46‰) and crustacea (−18.87‰ ~ −15.00‰) showed strong reliance on the marine production across the ECS continental shelf, regardless of distance from the shore. Moreover, the benthic crustacea and fish exhibited significantly higher δ¹³C values in the highly productive inshore sites and the δ¹³C values decreased seawards, implying a higher intrusion of atmospheric CO₂ and lower photosynthetic fractionation due to algal blooming in the inner shelf. The δ¹³C values of fish also showed significant positive correlations with the concentration of surface chlorophyll a and nitrogen. Riverborne nutrients closely linked marine benthic consumers to the terrestrial watershed and tightly coupled the pelagic and benthic ecosystems in the ECS. The stable isotope compositions of benthic consumers can act as an indicator for pelagic trophic status. The future research combining analyses of stable isotope and community structure may improve assessment on the balance between contribution and risk of phytoplankton blooms.