The Spatial-temporal Distributions And Key Processes Of Hypoxia Off The Changjiang Estuary And Its Adjacent Waters

In recent decades,under the combined influence of human activities and global climate change,eutrophication in coastal waters has been aggravated and coastal hypoxia?DO?3 mg/L?has occurred frequently around the world.In addition,the frequency,intensity and duration of hypoxia have shown a consistent growing trend.Hypoxia has become one of the typical worldwide coastal ecological disasters.Hypoxia off the Changjiang estuary?CE?and its adjacent waters is prominent and has caused serious environmental disasters.On the basis of high-frequency and large-scale monthly multidisciplinary field investigations?from February 2015 to January 2016?and in situ experiments?in July 2017?carried out off the CE and its adjacent waters,this dissertation has systematically studied the spatial-temporal distributions of hypoxic zone in the target area during annual cycle.Combined with hydrographic and biogeochemical data as well as other simultaneous observation data,the key factors and processes affecting the distribution of hypoxic zone were analyzed.In addition,the“dual-core”structure of hypoxic zone off the CE and its adjacent waters was further revealed,and the differences in key processes affecting the southern and northern hypoxic zones were discussed and verified.Furthermore,the relative contribution of Sediment Oxygen Demand?SOD?to the formation of hypoxia was quantitatively estimated.The major findings were listed as follows:?1?The spatial and temporal variations of the hypoxia of the CE and its adjacent waters The results of monthly multidisciplinary field investigations showed significant variations in the bottom DO distribution off the CE and its adjacent waters during annual cycle,with the DO concentrations in the range of 1.10–10.59 mg/L.In general,the bottom DO content was high and saturated during February and March and the DO concentrations decreased as the distance from the nearshore increased.Since April,the low–DO water mass?DO?6 mg/L?was initially observed in the southern part of the target area,and then the low–DO water mass gradually expanded from the south to the north and covered a larger area.Additionally,the hypoxic zone was formed at CE and Zhejiang coastal area in August and lasted until October.The hypoxic zone exhibited a discontinuous distribution and was most extensive in August which covered an area of 14800 km².From November,the low–DO water mass retreated from the north to the south and finally disappeared.According to the characteristics of DO distribution,the annual cycle was divided into four stages:the early stage,the development stage,the formation stage and the dissipation stage.There was temporal heterogeneity in the key processes affecting DO distribution among different stages.In the early stage?from February to March?,DO distribution was dominated by its solubility due to the strong mixing of the water column.In the development stage?from April to July?,the intrusion of the Nearshore Kuroshio Branch Current?NKBC?provided a relatively low DO background and triggered water column stratification.Subsequently,stratification was enhanced by the increased river runoff and warming of surface waters and expanded northward,which promoted the low–DO water mass to extend northward.In the formation stage?from August to October?,the presence of strong stratification limited the DO supply by constraining vertical mixing.Additionally,the intense organic matter decomposition at local area facilitated the formation and discontinuous distribution of hypoxia.Finally in the dissipation stage?from November to next January?,the increased vertical mixing facilitated the dissipation of hypoxia from the north to the south.The development,formation and dissipation of hypoxia off the CE and its adjacent waters were the results of the synergistic effects of various influencing processes.Overall,the evolution of DO distribution pattern was dominated by the variations in water column stratification throughout the whole stages.?2?The spatial heterogeneity of hypoxia off the CE and its adjacent waters Hypoxic zone off the CE and its adjacent waters exhibited a discontinuous distribution and had a“south–to–north dual–core”structure,and this feature was most evident in August.In August,the northern hypoxic zone which covered a larger area?9900 km²?was located at CE?30.73–32.30°N,122.96–124.60°E?and extended northeastward to the southern Yellow Sea.The hypoxic zone in the southern region covered an area of4900 km²,and it was located in the near-shore area of the Zhejiang coast?28.43–29.40°N,121.97–122.63°E?.In this dissertation,the“south–to–north dual–core”structure of hypoxic zone was further revealed,and the differences in key processes between the southern and northern regions were discussed.On one hand,NKBC had a significant impact on the southern region,while in the northern region,the impact of CDW was more pronounced.Under the influence of ocean circulation,there were differences in the structure and intensity of stratification between the southern and northern regions,i.e.,the northern region exhibited a stronger pycnocline intensity and the restriction on the DO vertical transport was stronger.Additionally,different biochemical characteristics between the southern and northern regions were observed,i.e.,higher organic matter contents were detected in the southern region.Also,the measured AOU,NO₃^-+NO₂^-,PO₄^3-and N/P ratio in the near-bottom waters of southern region seemed to be well coupled following the classic Redfield ratio than the northern region.?3?Effects of SOD on the spatial heterogeneity of hypoxic zone off the CE and its adjacent waters To verify the differences in the biochemical processes between the southern and northern regions,in situ experiments were carried out in July 2017 to determine the phytoplankton sinking rates in the target area and SOD rates in the northern hypoxic region?at station 3100–2?,in the southern hypoxic region?at station DH4–0A and Station JQ1–1?and in the non-hypoxic region?at station JQ6–1?.The phytoplankton sinking rates were in the range of 0.75–3.34 m/day and exhibited spatial variations.Influenced by biomass and community structure of phytoplankton,the sinking rates in the southern hypoxic regions were significantly higher than that in the northern hypoxic and non-hypoxic regions.The SOD rates were in the range of 7.30–19.00 mmol/?m²·day?and also exhibited spatial variations:SOD rates in the southern region>SOD rate in the northern region>SOD rate in the non-hypoxic region.Thus,these findings further demonstrated that the organic matter decomposition of surface sediment in the southern region was more intense.In addition,the DO net loss under the pycnocline and the relative contribution of SOD from July to August were estimated:SOD accounted for 51%of DO net loss at station 3100–2 in the northern hypoxic region,49%and 56%of DO net loss at stations DH4–0A and JQ1–1 in the southern hypoxic region,and 43%of DO net loss at station JQ6–1 in the non-hypoxic region,respectively.The innovations of this dissertation are as follows:Based on the monthly investigations firstly conducted off the CE and its adjacent waters,the detailed spatial-temporal variations of the hypoxic zone which experienced the process of“expanding from the south the north,and then dissipating from the north to the south”were further depicted and proved,and the key processes dominating the DO distribution in different stages were analyzed.On another hand,differences in the evolution processes and formation mechanisms of the“south-to-north dual-core”structure were explained,and the contribution of SOD to the formation of hypoxia was quantitatively estimated.In conclusion,this dissertation further revealed the spatial-temporal distributions of hypoxia off the CE and its adjacent waters,and improved the understanding of synergistic effect and variations of key processes influencing hypoxia in the target area.The major findings of this dissertation facilitate a comprehensive understanding on the formation mechanisms of hypoxia.Meanwhile,these results could promote the elucidation of the circle of biogenic elements and the ecosystem structure and function in the target area.