Hypoxia, defined as dissolved oxygen (DO) in water < 2 mg L-1, has occurred worldwide in estuarine and coastal environments during the past five decades. Estuarine and coastal hypoxia not only alters regional biogeochemical processes and also affects biodiversity and fisheries. It is well known that climate warming and eutrophication are increasingly important to hypoxia occurrence in terrestrial aquatic environments, the mechanisms underlying estuarine and coastal hypoxia, however, are still poorly constrained, mostly due to the limited long-term observation and interpretation.
In a new study published in the journal JGR Biogeosciences, authors compared DO concentrations and the driving factors of hypoxia between northwestern and southern Hong Kong and Mirs Bay.
According to the study, deoxygenation was weak in the bottom layer in northwestern Hong Kong, although the DO was consistently undersaturated, whereas a rapid decrease in the annual minimum DO was observed in the bottom layer in southern Hong Kong and Mirs Bay. “The seasonal DO depletion and/or hypoxia in the bottom water was accompanied by supersaturated DO and high Chl-a in surface water in southern Hong Kong, indicating local excessive productivity and triggering oxygen depletion” comments Dr. Wwi Qian from the Key Laboratory for Humid Subtropical Eco-geographical Processes of the Ministry of Education, Fujian Normal University, China.
The predeoxygenation of bottom water and long water residence time have contributed to the deoxygenation in Mirs Bay. Water stratification could exacerbate hypoxia by preventing oxygen replenishment from the surface to the bottom layers. The upwelling water from the South China Sea and/or Kuroshio contributed inappreciably to the significant deoxygenation in southern Hong Kong and Mirs Bay.
“These results suggest that enhanced productivity and oxygen consumption, combined with stratification and currents, are increasingly driving hypoxia in the Pearl River Estuary and adjacent areas” concludes Prof. Josep Penuelas from CREAF-CSIC Barcelona
Reference: Qian, W., Zhang, S., Tong, C., Sardans, J., Peñuelas, J., Li, X. 2022. Long-term Patterns of Dissolved Oxygen Dynamics in the Pearl River Estuary. JGR Biogeosciences 127(7), e2022JG006967, doi: 10.1029/2022JG006967.