Biogeochemical characteristics of suspended particulates atdeep chlorophyll maximum layers in the East China Sea

Continental shelves and marginal seas are key sites of particulate organic matter (POM) production, remineralization and sequestration, playing an important role in the global carbon cycle. Elemental and stable isotopic compositions of organic carbon and nitrogen are frequently used for characterizing organic matter and distinguishing their sources in suspended particulates and surface sediments in the marginal seas. Here we investigate suspended particulate matters (SPM) collected from the deep chlorophyll maximum (DCM) layer in the continental shelf of the East China Sea for particulate organic carbon and nitrogen (POC and PN) contents and their isotopic compositions (δ 13 C POC and δ 15 N PN ) to understand biogeochemical characteristics of POM straddling at biotic-dominated DCM depths. When combined with hydrographic parameters, such as temperature, salinity and turbidity, and chlorophyll a (Chl a ), these elemental and isotopic results revealed that POM in the DCM layers was largely from the newly-produced, in situ phytoplankton-dominated OM and have wider δ 13 C POC and δ 15 N PN compositions than previously thought. As supported by the POC to Chl a ratio, a large variation of δ 13 C POC was resulted from the changes in primary productivity and phytoplankton species, whereas the nutrient status and δ 15 N of dissolved nitrate were the main controlling factors of δ 15 N PN variability in the DCM layers. Consistently, the spatial distribution of δ 15 N PN showed a similarity with the current pattern in the East China Sea, with 15 N-enriched freshwater in the coastal region and Kuroshio Water in the northeast of Taiwan Island, but nutrient-depleted Taiwan Warm Current Water in the mid-shelf; as the latter seems to have promoted the N 2 -fixation, resulting in the depleted δ 15 N PN in the mid-shelf. Furthermore, SPM investigated here seems not to be influenced by the terrestrial organic matter supplied by the Yangtze River (Changjiang) in summer 2013, a finding that is contrary to a number of previous studiesu0027 conclusion. Nonetheless, given the complications associated with stable isotopes of organic matter, additional parameters such as radiocarbon and biomarkers are crucial to revalidate whether or not SPM in the DCM depths is influenced by terrestrial organic compounds in the river-dominated East China Sea.