Biogeochemical characteristics of suspended particulate matter in deep chlorophyll maximum layers in the southern East China Sea

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.

Impact of Hydrological Conditions on the Biogeochemical Dynamics of Suspended Particulate Organic Matter in the Upper Mixed Layer of the Southern East China Sea

“…The POC and PN concentrations are significantly correlated, and the zero intercept on the PN axis indicates that the nitrogen is in organic form (Hedges et al, ). All autumn data points plot below the Redfield C/N ratio and are comparable to marine source‐dominated POM from DCM layers in the same area (Figure a; Liu et al, ), implying the predominant contribution of marine organic matter in this study.…”

“…The relationship between POC and chlorophyll a has been used for understanding the role of phytoplankton in POM production (Furuya, ; Hung et al, ). It is expected that in situ phytoplankton productivity is mainly responsible for the variation of POC when the concentrations of POC and chlorophyll fluorescence are positively correlated, which was also observed in the DCM layer of the southern ECS in summer 2013 (Liu et al, ). However, this relationship is insignificant in this study ( R 2 = 0.02; Figure c), suggesting that the in situ phytoplankton biomass is not the only or main source of POM in the upper mixed layer.…”

“…Recently, the C/N of marine plankton and POM has been subjected to revision with trends of lower and higher than Redfield ratio (6.63; Redfield, ; Geider & La Roche, ; Martiny et al, ). In order to evaluate the influence of inorganic nitrogen incorporation on PN and the C/N ratio, the relationship between POC (μmol/L) and PN (μmol/L) was plotted (Figure a), along with data obtained from DCM layers in summer 2013 (Liu et al, ) for comparison. The POC and PN concentrations are significantly correlated, and the zero intercept on the PN axis indicates that the nitrogen is in organic form (Hedges et al, ).…”

“…However, the defining of end‐member isotope values is an unsettled issue (Kandasamy et al, ; Miller et al, ). Earlier studies have indicated that the δ 13 C of planktonic OM in the coastal region widely varied from the inner to the outer shelf, with less negative δ 13 C and high δ 15 N in the inner shelf (Liu et al, ; Miller & Page, ; Oczkowski et al, ). This suggests the difficulty in addressing carbon sources in the marine environment based on δ 13 C values alone.…”

“…Previous work on the carbon and nitrogen dynamics in the deep chlorophyll maximum (DCM) layers from the southern ECS concluded a weak contribution of the Yangtze‐transported POM to the DCM layers in summer 2013, when the shelf water column was stratified (Liu et al, ). However, the POM distribution and dynamics during autumn/winter when the shelf water column is well‐mixed remain poorly understood.…”