Southward spreading of the Changjiang Diluted Water in the La Niña spring of 2008

Air-sea CO2 fluxes and cross-shelf exchange of inorganic carbon in the East China Sea from a coupled physical-biogeochemical model

The East China Sea (ECS) has been reported to be a significant sink of atmospheric CO2, but less is known about horizontal transport of dissolved inorganic carbon (DIC) across the shelf. A coupled physical-biogeochemical model has been implemented for the ECS to simulate the inorganic carbon system and estimate CO2 fluxes and cross-shelf DIC transport in the ECS. A 6-year model hindcast (2013-2018) was performed and assessed. Multiple existing datasets from in-situ observations are used to constrain and validate the model. The model reproduces the spatial and temporal patterns of nitrogen, chlorophyll and CO2 parameters in general agreement with observations. Modeling estimation reveals that the ECS takes up CO2 at an annual mean rate of about 8.20 ± 3.13 mmol m-2 d-1, and experiences substantial seasonal variability. The total annual CO2 uptake in the ECS is about 21.55 Tg C yr-1. Modeling estimation suggests that the biological processes contribute to about 15 % of the shelf CO2 uptake in the ECS, leaving ~80 % of the shelf uptake contributed by other physical-chemical processes, e.g., physical pump and/or solubility pump. The horizontal fluxes of DIC between the ECS and the adjacent ocean are more than two orders of magnitude larger than the air-sea CO2 flux on the ECS and result in a net DIC export of about ~33.8 ± 14.87 Tg C yr-1 from the shelf area. Modeling results suggest that this conveyance of DIC to the open ocean is equivalent to about 70 % of the inorganic carbon inflow from riverine and atmospheric pathways in the annual scale.

Nitrate dynamics and source apportionment on the East China Sea shelf revealed by nitrate stable isotopes and a Bayesian mixing model

The excess input of nitrate is one of the primary factors triggering nearshore eutrophication. To estimate the source apportionment of nitrate on the East China Sea (ECS) shelf, the nitrogen and oxygen stable isotopes in nitrate (δ¹⁵N-NO₃^- and δ¹⁸O-NO₃^-) collected in winter and late spring 2016 were analyzed alongside essential physical, chemical and biological parameters. The temporal and spatial distributions and characteristic values of nitrate-bearing water masses were presented. Accordingly, the end-member mixing model and Rayleigh model were applied to systematically analyze biogeochemical processes. The biogeochemical processes of nitrate were weak in winter, except in the southern ECS, where assimilation and nitrification probably occurred. In contrast, the biogeochemical processes were intensive in spring. The stable isotopic fractionations of N and O were unified in the whole area, and the ratio between δ¹⁸O-NO₃^- and δ¹⁵N-NO₃^- was 1.81 ± 0.04, which indicated significant assimilation accompanying nitrification in spring. Furthermore, a Bayesian stable isotope mixing model was used to reveal the source contributions of nitrate on the ECS shelf for the first time, demonstrating that the Changjiang Diluted Water and Kuroshio Subsurface Water were always sustained and provided steady nitrate sources for the whole ECS. The nitrate inputs from the Yellow Sea to the northern ECS increased from approximately 30 % in spring to nearly 70 % in winter, while that from the Taiwan Strait Warm Water to the southern ECS decreased from approximately 40 % in spring to zero in winter. Moreover, although the nitrate contributions from nitrification were significantly weak in the middle and northern ECS during winter, they were important over the entire ECS during spring. This study qualitatively and quantitatively improves the understanding of seasonal nitrate control from various sources, and these findings are important for nutrient management and policy making to mitigate nearshore eutrophication.

Spatiotemporal patterns and influencing factors of dissolved heavy metals off the Yangtze River Estuary, East China Sea

Dissolved heavy metal pollution in the ocean is becoming an environmental concern. Their distribution patterns are complex and influenced by multiple factors in the coastal ocean. Therefore, more investigations are needed to understand their behavior in the seawater. This study systematically investigated the distribution of Cu, Pb, Cd, As, Zn and seawater properties in the surface and bottom water off the Yangtze River Estuary, East China Sea in spring, summer and autumn, 2019. The results showed significant spatiotemporal distribution that three-zone-pattern of estuary, nearshore, and offshore can be divided. While sources, hydrodynamics, biological uptake and sediment resuspension affected the overall distribution, dissolved oxygen and pH dominantly influenced the estuary and offshore respectively, with more complex factors in the nearshore. Low ecological risks were assessed during the study, but global warming, ocean acidification and hypoxia are essential concerns to understand the biogeochemistry of dissolved heavy metals in the ocean.