Distribution Features Of Chemical Parameters And Associated Implications In The Western Tropical Indian Ocean

Due to the geographic features of the three sides of the land,and driven by the monsoon,the circulation system of the Indian Ocean is obviously different from that of the Pacific and Atlantic Oceans,showing significant seasonal variations.The western tropical Indian Ocean（WTIO）is bordered by the Arabian Sea to the north and the African continent to the west,and the south-central part is influenced by the South Equatorial Current and the Seychelles-Chagos thermocline ridge（SCTR）,and its physical-biogeochemical processes are more complex.Based on the hydrological,chemical,and biological data obtained in the western tropical Indian Ocean from December 2021 to January 2022,this research analyzed the hydrological characteristics and the spatial distribution patterns and transport of dissolved oxygen（DO）,nutrients and other chemical parameters during the northeast monsoon（NEM）,focusing on the DO distribution,hypoxia characteristics and their significance in the WTIO during the NEM period.The spatial distribution patterns and transport of nutrient elements such as DO,and hypoxia and their significance in the WTIO during NEM were studied.The following main conclusions were drawn:（1）The distribution characteristics of DO and nutrients in the WTIO during the winter monsoon had good correspondence with the current and water mass patterns.Temperature had a good correlation with DO and nutrients.The Arabian Sea High Sea Water,the South Equatorial Counter Current and the South Equatorial Current occupied the surface waters in the northern,central and southern parts of the survey area respectively.The combined effect of the three forms a spatial pattern of high temperature,low salinity and low oxygen in the southern part of the survey area.The existence of upwelling in the southern part of the investigation sea led to the nature of low temperature,low salinity,low oxygen and high nutrient salinity of its water body.The deep seawater was mainly controlled by the Northern Indian Ocean intermediate water（NIIW）and the Antarctic intermediate water（AAIW）,which led to the hypoxia distinction above and below.（2）There was a top-down double hypoxic zone in the WTIO,which was influenced by the NIIW and the AAIW.The oxygen-rich AAIW impeded the southward intrusion of the NIIW in the process of leaping northward and hindered the southward expansion of the hypoxic zone.The hypoxic zone in the WTIO during the NEM could indicate upwelling and acidifying condition.The upper water of the SCTR were low temperature and low oxygen,and their leapfrog position was obviously higher than the surrounding water mass,which reflected the indicator role of DO on upwelling.there were two hypoxic zones in the WTIO during the NEM,the upper and lower hypoxic zones,and their low oxygen water mass correspond to the distribution of low pH water mass.At the same time,the DO of the WTIO water column had a good correlation with pH and DIC,and the 750m low oxygen zone corresponded to the low pH and high DIC water column,which indicated the acidification condition to some extent.（3）Upwelling had an important role in regulating nutrient transport and ecology in the WTIO.Upwelling had an important influence on the vertical transport of nutrients and their fluxes.The upwelling transported a large amount of nutrients to the upper water column and raised the N/P ratio in the SCTR,forming a high-value zone in the southern part of the survey area.The low temperature seawater brought by the upwelling was not conducive to the growth of phytoplankton,resulting in the formation of chlorophyll a（Chl a）high-value area only in the northern part of the survey area.Meanwhile,the annual flux of nutrients transported by upwelling was estimated to reach mmol magnitude,but it still could not improve the N deficit of water mass in the SCTR.Upwelling had an important impact on the ecology of the WTIO.Upwelling also delivered large amounts of acidified water to the upper water column,leading to significant acidification in the upper water column of the SCTR.The upwelling had a significant regulatory effect on the vertical distribution of pCO₂ partial pressure,mainly affecting the pCO₂of water mass below the thermohaline.The mean CO₂flux to the atmosphere is 2.16 mmol/m²/d.High value of>5mmol/m²/d were present in the Seychelles-Chagos upwelling region.The upwelling had a significant lifting effect on the high pCO₂water mass,while the stronger photosynthesis near the thermocline in this region could weaken the effect of the high pCO₂water mass carried by the upwelling on the upper pCO₂to some extent.