Distributions and Relationships of CO2, O2, and Dimethylsulfide in the Changjiang (Yangtze) Estuary and Its Adjacent Waters in Summer

The distributions and relationships of O₂, CO₂, and dimethylsulfide (DMS) in the Changjiang (Yangtze) Estuary and its adjacent waters were investigated in June 2014. In surface water, mean O₂ saturation level, partial pressure of CO₂ (pCO₂), and DMS concentrations (and ranges) were 110% (89%–167%), 374 μatm (91–640 μatm), and 8.53 nmol L^?1 (1.10–27.50 nmol L^?1), respectively. The sea-to-air fluxes (and ranges) of DMS and CO₂ were 8.24 μmol m^?2 d^?1 (0.26–62.77 μmol m^?2 d^?1), and ?4.7 mmol m^?2 d^?1 (?110.8-31.7 mmol m^?2 d^?1), respectively. Dissolved O₂ was oversaturated, DMS concentrations were relatively high, and this region served as a sink of atmospheric CO₂. The pCO₂ was significantly and negatively correlated with the O₂ saturation level, while the DMS concentration showed different positive relationships with the O₂ saturation level in different water masses. In vertical profiles, a hypoxic zone existed below 20 m at a longitude of 123°E. The stratification of temperature and salinity caused by the Taiwan Warm Current suppressed seawater exchange between upper and lower layers, resulting in the formation of a hypoxic zone. Oxidative decomposition of organic detritus carried by the Changjiang River Diluted Water (CRDW) consumed abundant O₂ and produced additional CO₂. The DMS concentrations decreased because of low phytoplankton biomass in the hypoxic zone. Strong correlations appeared between the O₂ saturation level, pCO₂ and DMS concentrations in vertical profiles. Our results strongly suggested that CRDW played an important role in the distributions and relationships of O₂, CO₂, and DMS.