LAUSR

Abstract The isoscape of particulate organic matters (POM) in the euphotic zone at the shelf of the East China Sea (ECS) is affected by hydrological interactions and biological activities. Based on the stable carbon and nitrogen isotope values (δ13C and δ15N) and C:N ratio of POM (0.7 to 50 μm) in the euphotic zone of the ECS from 2008 to 2018 with a total of 113 samples, we inferred their resources and then investigated what hydrographical and biological conditions determined these spatio-temporal variations through synoptic and intensive seasonal monitoring. The isotopic signatures (δ13C: −26.9 to −12.4 ‰, δ15N: −1.0 to 10.8 ‰) and C:N ratios (4.4 to 9.0) of euphotic POM varied within the range of values reported for marine phytoplankton, suggesting limited contribution of terrigenous POM to the shelf of the ECS. The δ13C values of POM increased with the increase of chlorophyll-a (chl-a) concentration at the depth of chl-a maximum and the δ15N values decreased near the Changjiang River plume and inner shelf with freshwater input and coastal upwelling in summer. This indicated that phytoplankton production was enhanced by high nutrient supply and warmer temperature. Besides, the δ15N values were higher under lower inorganic nitrogen nutrient concentrations, and were negatively correlated with POM C:N ratio (i.e., δ15N value was less negative with higher nitrogen content in POM). Such correlations may suggest the enhanced trophic interactions in microbial food chains in oligotrophic environments, where phagotrophic protists with high δ15N values could be included into the size fraction (0.7-50 μm) of POM. Based on our study, the spatial and temporal δ13C and δ15N isoscape of euphotic POM serves as an ecological indicator to infer primary production and plankton food web properties of the East China Sea.