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Quantify The Sources Of Oxygen Consuming Organic Material In The Hypoxic Zone In The East China Sea Off The Changjiang Estuary

Posted on:2015-03-15Degree:MasterType:Thesis
Country:ChinaCandidate:H J WangFull Text:PDF
GTID:2250330428963759Subject:Marine Chemistry
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Hypoxia, typically defined as a water body with dissolved oxygen (DO)<2~3mg/L is of great environmental and ecological concerns, and has been received wide attention among both the scientific community and the general public. Decision making towards mitigation and/or remediation of hypoxic ecosystems is however controversial and challenging due primarily that the direct cause of hypoxia in the coastal ocean remains unclear. It is well understood that bottom hypoxia is primarily driven by organic material degradation but whether the oxygen consuming organic material is solely sourced from eutrophication induced primary by production in the surface water that would subsequent sink and get remineralized remains contentious despite decades of research. In this study, we chose a well-studied hypoxic zone in the East China Sea off the Changjiang estuary to examine the ultimate sources of the organic material that consumes DO by measurements of the carbon isotopic composition of both particulate organic carbon (δ13CPOC), the oxygen consumer, and dissolve inorganic carbon (δ13CDIC), the end product of POC oxidation.Our fieldwork was conducted in August2011during a recovery process of the subsurface hypoxia in the East China Sea off the Changjiang Estuary. With the aid of a three end-member mixing model based on potential temperature and salinity conservation, we were able to predict the relative contribution of dissolve inorganic carbon (DIC) from different water masses with different δ13CDIC signatures. We derived that the isotopic composition of the remineralized organic carbon (δ13COCx) that consumed oxygen in the hypoxic zone were-17(±3)‰. Assuming that the end member of the on-site production of organic material has an isotopic composition of-18.5(±1)‰, and terrestrial organic carbon is featured with-24.5(±0.5)‰, we derived that86%ranging from36~100%of organic material responsible for oxygen consumption in the subsurface waters of the hypoxic zone was autochthonous organic material, which can be attributed to the river-plume induced primary production.In this thesis, we also did another case study to assess the sources of organic matter that consumed DO in a small lake (Lovers’ Lake) in the campus of Xiamen University, China which had experienced seasonal hypoxia in summer. We had been conducting time series sampling since the summer of2011in this small lake to constrain the temporal variations of δ13CDIC and δ13CPOC along with other biogeochemical parameters. From late spring until early fall, water column was stratified and DO was depleted below3m. δ13CDIC in the surface water was around-4‰but remained stable at-12‰below4m, while δ13CPOC in the surface water was-27‰reaching its minimum at3m (-32‰) then increased slightly with depth to-30‰. If we also assumed that remineralization was the only process causing the difference in δ13CDIC between summer and winter below the thermocline, we derived the isotopic composition of the oxygen consuming organic material,δ13COCx as-27.7‰, which was almost identical to the summer suface δ13CPOC. Therefore, the degradation of the autochthonous organic material almost took the full responsibility for the seasonal bottom hypoxia.However, during an extreme case of the lake after being impacted by Typhoon "Nanmadol", allochthonous rather than autochthonous organic material may largely be responsible for the oxygen consumption. These samples were collected on September1-15,2011after a typhoon. During these15days, DO and δ13CDIC decreased and DIC increased in the subsurface water. Based on a simple isotope mass balance, the degradation of the allochthonous organic material should take65%responsibility for the oxygen consumption after the typhoon event, while autochthonus organic carbon only took the other responsiblity.Both cases demonstrated in a quantitatively way to calculate the isotopic signature of remineralization of organic matter which caused bottom hypoxia. The new approach was adopted also has applicability to other systems. We found that the new production enhanced by Changjiang nutrients should take the main responsibility for seasonal bottom hypoxia, while the terrestrial organic carbon averagely only take14%for bottom hypoxia in Changjiang Estuary. Applying the similar research approach, we found that the surface plankton bloom in summer should almost take the full responsibility for bottom hypoxia in the small lake under study, while the terreistiral organic carbon occasionally should be responsible for the middle layer hypoxia recovery after strong precipitation. This study confirmed that nutrients are the primary cause to the downstream hypoxia in the subsurface water body, and should have a strong implication for nutrient management.
Keywords/Search Tags:Changjiang Estuary hypoxia, Lake hypoxia, Carbon stable isotope, Eutrophication
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