Large areas of seasonal hypoxia have been detected off the Changjiang River Estuary over the last decade. Although seasonal bottom hypoxia was transient due to complex dynamical processes, it occurred with high frequency in certain regions. Summer hypoxia hot spots off the estuary were first identified using a comprehensive, high‐resolution ecosystem model and 17 years of observations collected from historical literature and research cruises. Comparisons between long‐term observations, a 4‐year averaged simulation, and a 1‐year simulation implied that the geographical locations of summer hypoxia hot spots were generally consistent. This ecosystem model suggested that the Changjiang River plume predominantly spread either over the Yangtze Bank or the Submarine Canyon and the Mud Belt depending on the direction of the wind; the temperature, salinity, and residence time of bottom water mass off the estuary depended highly on the properties of the Kuroshio subsurface intrusion. The dual effects of stratification and prolonged bottom water residence time created favorable condition for hypoxia to develop. The analysis in this study identified wind and the Kuroshio subsurface intrusion as the factors that facilitated the migration of seasonal hypoxia and the establishment of summer hypoxia hot spots off the estuary. In spite of the persistent hypoxia hot spots, the timing of individual hypoxia can be variable because of distinct dynamical processes over a wide range of time scales. Tidal advection was quantified as a secondary factor that influences the spatial extent and migration of bottom hypoxia.

In the face of ongoing global warming and glacier retreat, the composition and flux of organic matter in glacier–fjord systems are key variables for updating the carbon cycle and budget, whereas the role of Arctic valley glaciers seems unimportant when compared with the huge Greenland Ice Sheet. Our field observations of the glacier-fed Bayelva River, Svalbard, and the adjacent Kongsfjorden allowed us to determine the compositions of particulate organic matter from glacier to fjord and also to estimate the flux of organic carbon, both for the river and for Svalbard in general. Particulate organic carbon (POC) and dissolved organic carbon (DOC) in the Bayelva River averaged 56 and 73 µM, respectively, in August, 2012. Amino acids (AAs) and phytoplankton carbon accounted for ∼ 10 % of the bulk POC in the Bayelva River, while AAs represented > 90 % of particulate nitrogen (PN) in fjord surface water, suggesting the strong in situ assimilation of organic matter. Bacteria accounted for 13 and 19 % of the POC in the Bayelva River and the Kongsfjorden, respectively, while values for PN were much higher (i.e., 36 % in Kongsfjorden). The total discharge from the Bayelva River in 2012 was 29 × 10⁶ m³. Furthermore, we calculated the annual POC, DOC, and PN fluxes for the river as 20 ± 1.6 tons, 25 ± 5.6 tons, and 4.7 ± 0.75 tons, respectively. Using the POC content and DOC concentration data, we then estimated the annual POC and DOC fluxes for Svalbard glaciers. Although the estimated POC (0.056 ± 0.02 × 10⁶ tons year⁻¹) and DOC (0.02 ± 0.01 × 10⁶ tons year⁻¹) fluxes of Svalbard glaciers are small in amount, its discharge-weighted flux of DOC was over twice higher than other pan-Arctic glacier systems, suggesting its important role as a terrestrial DOC source.

Observed oxygen concentrations collected at discrete locations during research cruises were conventionally used to estimate spatial extents of bottom low‐oxygen/hypoxia. Yet observed oxygen concentrations were often not quantitatively representative of spatial patterns in instantaneous oxygen concentrations in coastal oceans, especially when the bottom hypoxia was transient. Over the Changjiang River estuary and its adjacent sea, research cruises could easily be longer than the time scale of variability of bottom hypoxia extent. The Changjiang River plume is extremely mobile due to changes in wind magnitude and direction, and the redistribution of this freshwater cap strongly regulates vertical stratification on which bottom hypoxia formation depends. A high‐resolution ecosystem model was developed, which successfully reproduced observed temperature, salinity, and bottom oxygen concentration. This model suggested fast response of bottom oxygen to vertical stratification evolution (generally ∼6–50 hr) and a transient spatial extent of summer bottom hypoxia off the Changjiang River estuary. Comparisons between observed and modeled oxygen concentrations implied that the hypoxic area calculated from dissolved oxygen at discrete locations often had possible errors and the estimated magnitude of hypoxic area which depended on the chronological order of observations. Therefore, it is risky to estimate the spatial extent of hypoxic area based on observations exclusively, and the relevant quantification of annual hypoxia area trend is also questionable. Integration of quasi‐simultaneous observations is required to advance the understanding of oxygen dynamics, to minimize observational uncertainties. The development of skillful ecosystem models that profit from ample observations and have the power to reproduce dissolved oxygen is indispensable.