Seasonal Variation Of Hypoxic Zone In The Central Eastern Indian Ocean

Dissolved oxygen(DO)is one of the important physicochemical parameters in the ocean,and it is also an important material for maintaining marine life activities.The study of the spatial distribution characteristics of dissolved oxygen in seawater and its influencing factors has a good indication of the study of the ocean circulation and water mass movement,the circulation of marine biogenic materials,and the primary production process.The Indian Ocean is an important part of the Maritime Silk Road.The environment and resources of the Indian Ocean are of great significance to the implementation of the Maritime Silk Road strategy.The presence of hypoxic zones and the intensification of the degree of hypoxia would seriously threaten the ecological environment and fishery resources in the area.Therefore,it is necessary to conduct in-depth research on the distribution of the hypoxic zone in the Indian Ocean.In this study,field survey data for the four seasons from the central East Indian Ocean were used to analyze the seasonal variation of the boundary of the hypoxic zones in the central East Indian Ocean,combining with the observations of hydrological and marine chemistry,the intrinsic link between the hypoxic region and the Bay of Bengal was explored.It is expected to provide scientific basis and basic data for the further study of biogeochemical characteristics in the East Indian Ocean.The main findings are as follows:(1)There is a significant stratification in the distribution of DO in the water column of the central East Indian Ocean: the oxygen uniform layer-oxycline-minimum layer-slowly changing layer.The thickness of the uniform layer gradually decreases from the equator to the north and south,and the seasonal variation of the thickness of the uniform layer is winter> summer and autumn> spring.The DO value in the oxycline decreases rapidly with depth,and forms an oxygen minimum layer(OMZ).The variation law of the intensity of the oxygen layer with the season is basically the same as that of the thermocline;summer> winter> autumn> spring.(2)The oxygen content in the oxygen minimal layer in the central part of the East Indian Ocean reaches the Hypoxia standard(DO<2.00 mg/L).The hypoxic areas extend from north to south with a double water tongue.The double water tongues are located in different depth levels,within 100-200 m and 600-800 m depth respectively.(3)The range of the hypoxic zone(DO<2.00 mg/L)changed significantly with the season: from spring to winter,the southward expansion range of the hypoxic region first decreased and then increased(the south boundary of upper hypoxia: 1.2°S in the spring,0.8°N in the summer,0.7°N in the fall,and 0.1°S in the winter;the south boundary of deep hypoxia : 7.2°S in the spring,3.0°S in the summer,3.6°S in the fall,and 4.1°S in the winter),which is just opposite to the variation laws we thought in past that the range of hypoxic zones in the East Indian Ocean became larger in the spring and summer and smaller in autumn and winter.(4)The analysis shows that the hypoxic zone in the central East Indian Ocean is derived from the southward extension of the hypoxic zone in the Bay of Bengal.The equatorial subsurface water is the main driving factor of the seasonal variation of the extended range of the hypoxic zone,and the monsoon circulation also has an important influence on the extended range.Seawater stratification becomes an important external factor to promote the development of the hypoxic zone;At the same time,oxygen consumption caused by sedimentation of upper biodetritus is the internal cause of exacerbation of hypoxia.