Abstract
Abstract. 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 thus frequently used to characterize and distinguish POM and its sources in suspended particles and surface sediments in the marginal seas. Here we investigated suspended particulate matter (SPM) collected around deep chlorophyll maximum (DCM) layers in the southern East China Sea for particulate organic carbon and nitrogen (POC and PN) contents and their isotopic compositions (δ13CPOC and δ15NPN) to understand provenance and dynamics of POM. Hydrographic parameters (temperature, salinity and turbidity) indicated that the study area was weakly influenced by freshwater derived from the Yangtze River during summer 2013. Elemental and isotopic results showed a large variation in δ13CPOC (−25.8 to −18.2 ‰) and δ15NPN (3.8 to 8.0 ‰), but a narrow molar C ∕ N ratio (4.1–6.3) and low POC ∕ Chl a ratio (< 200 g g−1) in POM, and indicated that the POM in DCM layers was newly produced by phytoplankton. In addition to temperature effects, the range and distribution of δ13CPOC were controlled by variations in primary productivity and phytoplankton species composition; the former explained ∼ 70 % of the variability in δ13CPOC. However, the variation in δ15NPN was controlled by the nutrient status and δ15NNO3- in seawater, as indicated by similar spatial distribution between δ15NPN and the current pattern and water masses in the East China Sea; although interpretations of δ15NPN data should be verified with the nutrient data in future studies. Furthermore, the POM investigated was weakly influenced by the terrestrial OM supplied by the Yangtze River during summer 2013 due to the reduced sediment supply by the Yangtze River and north-eastward transport of riverine particles to the northern East China Sea. We demonstrated that the composition of POM around DCM layers in the southern East China Sea is highly dynamic and largely driven by phytoplankton abundance. Nonetheless, additional radiocarbon and biomarker data are needed to re-evaluate whether or not the POM around the DCM water depths is influenced by terrestrial OM in the river-dominated East China Sea.
Highlights
Stable isotopes of organic carbon and nitrogen (δ13C, δ15N) and molar carbon to nitrogen (C / N) ratios are natural tracers frequently used to identify the source and fate of terrestrial organic matter (OM) in the estuarine and marine environments (Meyers, 1994; Hedges et al, 1997; Goñi et al, 2014; Selvaraj et al, 2015)
We demonstrated that the composition of particulate organic matter (POM) around deep chlorophyll maximum (DCM) layers in the southern East China Sea is highly dynamic and largely driven by phytoplankton abundance
It is known that stable isotopes (δ13C, δ15N) and molar C / N ratios of POM in estuarine and marine areas are representative of primary production-derived OM when POM are mostly derived from phytoplankton biomass (Gearing et al, 1984)
Summary
Stable isotopes of organic carbon and nitrogen (δ13C, δ15N) and molar carbon to nitrogen (C / N) ratios are natural tracers frequently used to identify the source and fate of terrestrial organic matter (OM) in the estuarine and marine environments (Meyers, 1994; Hedges et al, 1997; Goñi et al, 2014; Selvaraj et al, 2015). This approach is based on the significant difference in δ13C, δ15N and C / N ratios between different end-members (e.g. terrestrial and marine), and the assumption that only a physical mixing of OM from compo-. Since phytoplankton are the main primary producer of marine OM, the elemental and isotopic compositions of phytoplankton should be considered while studying the dynamics of POM in the marine water column
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