| In the past few decades,ocean deoxygenation has been frequently observed,especially in coastal waters and tropical oceans.Most climate models predict that oxygen concentration will gradually decrease under global warming.As a critical environmental factor to the stability of the marine ecosystem,dissolved oxygen is an essential substance for most marine organisms.Ocean deoxygenation affects the biogeochemical cycles of elements,such as nitrogen,phosphorus,iron and other elements.Meanwhile,ocean deoxygenation will lead to some changes of most marine organisms in the behavior,reproduction,growth and other aspects.More seriously,it will threaten organisms’ survival and cause death.Eventually,ocean deoxygenation will affect the function and value of the marine ecosystem and have irreversible negative effects on human social and economic activities.Global warming is one of the main reasons for ocean deoxygenation.Rising sea temperature leads to reduction of oxygen solubility,dissolving less oxygen from the atmosphere into the ocean,which would expand the existing low oxygen area in the ocean.Besides,ocean warming could strengthen stratification and weaken ventilation,which would reduce the dissolved oxygen in the interior ocean from the surface ocean.Mineralization of organic material tends to happen,and respiration rate of organisms increased in the water layer with rising temperature and weaken ventilation,which would further exacerbate ocean deoxygenation.Although the global oxygen distribution has been mapped based on the historical data collected from oceanic surveys,it is challenging to conduct in-depth research on the spatial-temporal variation trend from a single survey or voyage.Using multisource dissolved oxygen observed dataset from1960 to 2019,we investigated the spatialtemporal distribution characteristics of the Oxygen Minimum Zone(OMZ)under scenarios of dissolved oxygen(DO)concentration less than 20 μmol/kg(OMZ20)and 60 μmol/kg(OMZ60)separately in this study.Previous studies mostly focus on the Time of Emergence(ToE)of dissolved oxygen change in the thermocline.However,there has been less research on ToE of oxygenation or deoxygenation signal,respectively.The Community Earth System Model Large Ensemble Numerical Simulation(CESM-LENS)dataset was used to predict ToE of deoxygenation signal in the epipelagic(0-200 m),the mesopelagic(200-1000 m),and the bathypelagic(below 1000 m)zones of the global ocean.Finally,the influence of ocean deoxygenation on biodiversity was quantified with the data of marine richness.The research conclusions are as follows:(1)From 1960 to 2019,the global ocean OMZ60 and OMZ20 have a significant expansion trend in the horizontal direction and a slight weakening trend in the vertical direction.Since the weakening trend of OMZ in the vertical direction is greater than the strengthening trend in the horizontal direction,the volume of OMZ60 and OMZ20 have a weakening trend.(2)In the horizontal direction,the area of OMZ60 and OMZ20 in global and regional oceans(the Atlantic,the Indian,and the Pacific)have a similar change trend,and the correlation coefficients are above 0.90(p<0.001).The upper boundary depth,lower boundary depth,and thickness of OMZ60 and OMZ20 have spatial heterogeneity in the vertical direction,and the upper boundary depth gradually tends to average.(3)Under the background of global warming,the oxygen content in the ocean is not satisfying.Under the high greenhouse gas emission-Representative Concentration Pathway 8.5(RCP8.5)scenario,69%of the epipelagic zone,75%of the mesopelagic zone,and 71%of the bathypelagic zone showed the ToE of deoxygenation signals are earlier than 2050,and their ToEs are 2048,2026 and 2033 year,respectively.Under the low greenhouse gas emission-XGHG scenario,the area ratios of deoxygenation signals in the three zones before 2080 are lower,and their ToEs are later.(4)Under the RCP8.5 and XGHG scenarios,widespread deoxygenation signal emerges in the mesopelagic ocean,and its ToE of deoxygenation signal is earlier than the epipelagic and the bathypelagic zones.This may be because oxygen consumption caused by organisms in the mesopelagic zone is the largest.In the regional ocean,the ToE of deoxygenation signals in the mesopelagic and bathypelagic zones are earlier than the epipelagic zone.The coverages of the deoxygenation signal in the mesopelagic and bathypelagic zones are larger than the epipelagic zone.In addition,the global patterns of ToE with different methods show that most of the global ocean will emerge the deoxygenation signal before 2080.(5)The global richness of marine organisms has a weak correlation with minimum oxygen zones(minDO)and ToE of deoxygenation signal,and their correlations are also spatially heterogeneous in regional oceans.This could be because marine biodiversity would be affected by multiple environmental factors.Biological sensitive areas for dissolved oxygen are mainly distributed in the Southern Ocean,the North Atlantic and the coastal areas.This study accurately quantified the temporal and spatial distribution of OMZ and discussed the distribution characteristics of the deoxygenation signal ToE.These would provide basic information for future research on the mechanism of ocean deoxygenation.In addition,this paper conducts a preliminary analysis of the response of marine richness to deoxygenation.Future research can explore changes in marine biodiversity caused by ocean deoxygenation from multiple perspectives. |
