Seasonal hypoxia and its potential forming mechanisms in the Mirs Bay, the northern South China Sea

• There is seasonal hypoxia occurred in the Mirs Bay.
• The mechanisms explain the depletion of dissolved oxygen.
• The low-oxygen upwelled water intrudes into the Mirs Bay in summer.
• High primary productivity results in particulate organic matter to sink to the bottom.

Hypoxia often occurs in the coastal waters of China. In the present study, data obtained from monthly cruises in the period 1998–2010 were used for analyzing temporal and spatial distributions of hypoxic bottom-water (BW) in the Mirs Bay, the northern South China Sea (NSCS). Also, potential forming mechanisms of the hypoxia were studied and depletion of the dissolved oxygen (DO) in the BW was quantitatively determined. The results showed that the BW hypoxia occurred each year in 10 years and mainly appeared from July to September in the bay within the 13-year study period. During summertime, strong density stratification formed due to both the T increase and S decrease in the surface-water in the bay. The coastal continental shelf BW had continuously intruded into and stably resided in the bay for months under influence of the NSCS southwestern monsoon. Quick biogeochemical cycles of nitrogen and phosphorus supported high phytoplankton productivity in the bay and then resulted in lots of particulate organic matter (POM) continuously to sink to the bottom. Degradation of the detritus POM, in which most was the autochthonous, resulted in DO depletion in the BW, which was estimated at about 3.6 mg L−1 in the inner Tolo Harbour and 2.9 mg L−1 in the main zone of the bay while the hypoxia events occurred.