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.

Climatic warming has lengthened the photosynthetically active season in recent decades, thus affecting the functioning and biogeochemistry of ecosystems, the global carbon cycle, and climate. The temperature response of carbon uptake phenology varies spatially and temporally, even within species, and the daily total intensity of radiation may play a role.

In a new study, published in the journal Global Change Biology, authors empirically modelled the thresholds of temperature and radiation under which daily carbon uptake is constrained in the temperate and cold regions of the Northern Hemisphere, which include temperate forests, boreal forests, alpine, and tundra biomes.

According to the study, radiation will constrain the trend towards longer growing seasons with future warming, but differently during the start and end of season and depending on the biome type and region. The study revealed that radiation is a major factor limiting photosynthetic activity that constrains the phenology response to temperature during the end-of-season. The beginning of carbon uptake, on the other hand, is highly sensitive to temperature but not constrained by radiation at the hemispheric scale. Dr. Adrià Descals from CREAF-CSIC says “Our results show that the photosynthetically active season in evergreen needleleaved forests begins shortly after conditions for growth become favorable and ends when these conditions get worse.” He also says, “It is important to take into account radiation, temperature, and their covariance when modeling the photosynthetically active season in evergreen needleleaved forests and their responses to climatic warming.” 

The study shows that the senescence stage has a low temperature dependency due to the constraints of radiation, and this might be a reason for the lower magnitude in the end-of-season delay than the start-of-season advance.

“This study thus revealed that while at the end-of-season the phenology response to warming is constrained at the hemispheric scale, at the start-of-season the advance of spring onset may continue, even if it is at a slower pace”, concludes Prof. Josep Penuelas from CREAF-CSIC Barcelona

Reference: Descals, A., Verger, A., Yin, G., Filella, I., Fu, Y.H., Piao, S., Janssens, I.A., Peñuelas, J. 2022. Radiation‐constrained boundaries cause nonuniform responses of the carbon uptake phenology to climatic warming in the Northern Hemisphere. Global Change Biology, doi: 10.1111/gcb.16502, in press.