A comparative study of nitrogen uptake and nitrification rates in sub-tropical, polar and upwelling waters

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dc.contributor.advisorWaldron, Howarden_ZA
dc.contributor.advisorClark, Darrenen_ZA
dc.contributor.authorPhilibert, Marie Catherine Raïssaen_ZA
dc.date.accessioned2016-02-05T07:26:59Z
dc.date.available2016-02-05T07:26:59Z
dc.date.issued2015en_ZA
dc.descriptionIncludes bibliographical referencesen_ZA
dc.description.abstractNitrification is the oxidation of ammonium to nitrate through a two step biological process. Nitrification in the euphotic zone has, in the past, been considered negligible even though quantifying this process correctly is important when linking carbon export to nitrate uptake by phytoplankton. However, studies of both nitrogen uptake and nitrification rates in surface waters are rare. This thesis presents such data for the Southern Ocean and St-Helena Bay, located in the Southern Benguela upwelling system. Using 15N tracers, Nitrogen uptake and regeneration rates were measured in the Southern Ocean (during a winter cruise in July 2012 and a summer cruise in February-March 2013) and St-Helena Bay (during three studies in November 2011, March 2012 and March 2013). In St-Helena Bay, the upwelling (bloom) cycle was one of the main drivers of the nitrogen cycle. As the bloom cycle started, nitrate uptake rates (5:47 ± 670:48nmol∙L⁻¹∙h⁻¹) and nitrite regeneration (4:36 - 1:28 ± 22:83 - 1:63nmol∙L⁻¹∙h⁻¹) were high but the contribution of nitrification to the nitrate demand was low. Nitrite regeneration at this time could have been driven by phytoplankton excretion. In contrast, at the end of the bloom cycle, nitrate uptake rates were low and was exceeded by nitrate regeneration rates (25:34 - 6:16 ± 82:74 - 34:41nmol∙L⁻¹∙h⁻¹). Nitrite regeneration decreased and was most likely due to ammonium oxidation at this stage of the upwelling cycle. Nitrification in the Southern Ocean was more variable than in St-Helena Bay. It was only detected at five stations out of fifteen and the accuracy of the high nitrite oxidation rates (37:21 - 9:13 ± 217 - 88nmol∙L⁻¹∙h⁻¹) observed can only be assessed with repeat measurements. Nitrate uptake rates ranged from 0.07 to 57:00nmol∙L⁻¹∙h⁻¹ while ammonium uptake rates ranged from 0.81 - 160:94nmol∙L⁻¹∙h⁻¹. The nitrogen uptake rates were similar for both seasons. Using multivariate statistical approach, it was found that during winter, in the Southern Ocean, light and ammonium availability were the most important factors regulating nitrogen uptake while in the late summer, changes in the mixed layer depth had a larger effect. This study provides new observational data for two undersampled regions and contributes to further the mechanistic understanding of the factors regulating nitrogen uptake and nitrification in the Southern Ocean and St-Helena Bay.en_ZA
dc.identifier.apacitationPhilibert, M. C. R. (2015). <i>A comparative study of nitrogen uptake and nitrification rates in sub-tropical, polar and upwelling waters</i>. (Thesis). University of Cape Town ,Faculty of Science ,Department of Oceanography. Retrieved from http://hdl.handle.net/11427/16794en_ZA
dc.identifier.chicagocitationPhilibert, Marie Catherine Raïssa. <i>"A comparative study of nitrogen uptake and nitrification rates in sub-tropical, polar and upwelling waters."</i> Thesis., University of Cape Town ,Faculty of Science ,Department of Oceanography, 2015. http://hdl.handle.net/11427/16794en_ZA
dc.identifier.citationPhilibert, M. 2015. A comparative study of nitrogen uptake and nitrification rates in sub-tropical, polar and upwelling waters. University of Cape Town.en_ZA
dc.identifier.ris TY - Thesis / Dissertation AU - Philibert, Marie Catherine Raïssa AB - Nitrification is the oxidation of ammonium to nitrate through a two step biological process. Nitrification in the euphotic zone has, in the past, been considered negligible even though quantifying this process correctly is important when linking carbon export to nitrate uptake by phytoplankton. However, studies of both nitrogen uptake and nitrification rates in surface waters are rare. This thesis presents such data for the Southern Ocean and St-Helena Bay, located in the Southern Benguela upwelling system. Using 15N tracers, Nitrogen uptake and regeneration rates were measured in the Southern Ocean (during a winter cruise in July 2012 and a summer cruise in February-March 2013) and St-Helena Bay (during three studies in November 2011, March 2012 and March 2013). In St-Helena Bay, the upwelling (bloom) cycle was one of the main drivers of the nitrogen cycle. As the bloom cycle started, nitrate uptake rates (5:47 ± 670:48nmol∙L⁻¹∙h⁻¹) and nitrite regeneration (4:36 - 1:28 ± 22:83 - 1:63nmol∙L⁻¹∙h⁻¹) were high but the contribution of nitrification to the nitrate demand was low. Nitrite regeneration at this time could have been driven by phytoplankton excretion. In contrast, at the end of the bloom cycle, nitrate uptake rates were low and was exceeded by nitrate regeneration rates (25:34 - 6:16 ± 82:74 - 34:41nmol∙L⁻¹∙h⁻¹). Nitrite regeneration decreased and was most likely due to ammonium oxidation at this stage of the upwelling cycle. Nitrification in the Southern Ocean was more variable than in St-Helena Bay. It was only detected at five stations out of fifteen and the accuracy of the high nitrite oxidation rates (37:21 - 9:13 ± 217 - 88nmol∙L⁻¹∙h⁻¹) observed can only be assessed with repeat measurements. Nitrate uptake rates ranged from 0.07 to 57:00nmol∙L⁻¹∙h⁻¹ while ammonium uptake rates ranged from 0.81 - 160:94nmol∙L⁻¹∙h⁻¹. The nitrogen uptake rates were similar for both seasons. Using multivariate statistical approach, it was found that during winter, in the Southern Ocean, light and ammonium availability were the most important factors regulating nitrogen uptake while in the late summer, changes in the mixed layer depth had a larger effect. This study provides new observational data for two undersampled regions and contributes to further the mechanistic understanding of the factors regulating nitrogen uptake and nitrification in the Southern Ocean and St-Helena Bay. DA - 2015 DB - OpenUCT DP - University of Cape Town LK - https://open.uct.ac.za PB - University of Cape Town PY - 2015 T1 - A comparative study of nitrogen uptake and nitrification rates in sub-tropical, polar and upwelling waters TI - A comparative study of nitrogen uptake and nitrification rates in sub-tropical, polar and upwelling waters UR - http://hdl.handle.net/11427/16794 ER - en_ZA
dc.identifier.urihttp://hdl.handle.net/11427/16794
dc.identifier.vancouvercitationPhilibert MCR. A comparative study of nitrogen uptake and nitrification rates in sub-tropical, polar and upwelling waters. [Thesis]. University of Cape Town ,Faculty of Science ,Department of Oceanography, 2015 [cited yyyy month dd]. Available from: http://hdl.handle.net/11427/16794en_ZA
dc.language.isoengen_ZA
dc.publisher.departmentDepartment of Oceanographyen_ZA
dc.publisher.facultyFaculty of Scienceen_ZA
dc.publisher.institutionUniversity of Cape Town
dc.subject.otherOceanographyen_ZA
dc.titleA comparative study of nitrogen uptake and nitrification rates in sub-tropical, polar and upwelling watersen_ZA
dc.typeDoctoral Thesis
dc.type.qualificationlevelDoctoral
dc.type.qualificationnamePhDen_ZA
uct.type.filetypeText
uct.type.filetypeImage
uct.type.publicationResearchen_ZA
uct.type.resourceThesisen_ZA
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