Upper water column nitrification processes and the implications of euphotic zone nitrification for estimates of new production

dc.contributor.authorGrundle, Damian Shaun
dc.contributor.supervisorJuniper, S. K.
dc.degree.departmentDept. of Biologyen_US
dc.degree.levelDoctor of Philosophy Ph.D.en_US
dc.description.abstractI used a specific inhibitor approach to systematically measure NH4+ oxidation rates through the euphotic zone of three distinct oceanographic regimes. Study sites included Saanich Inlet, a highly productive British Columbia fjord, the Line P oceanographic transect in the NE subarctic pacific, and the Bermuda Atlantic Time-series Study (BATS) station in the oligotrophic, sub-tropical Sargasso Sea. Nitrate uptake rates were also measured at select stations on a number of research cruises. NH4+ oxidation rates were found to proceed throughout the euphotic zone in each of my study regions, and, overall, euphotic zone NH4+ oxidation rates ranged from undetectable to 203 nmol L-1 d-1. A general characterization of the rates observed in each of my study regions shows that euphotic zone NH4+ oxidation rates were typically highest in Saanich Inlet, intermediate along Line P, and lowest at BATS. The observation that NH4+ oxidation occurred throughout the euphotic zone in each of my study regions was in contrast to the traditional assumption of no euphotic zone nitrification, and it should now be considered a ubiquitous process in the euphotic regions of the ocean. Results found that euphotic zone nitrification could have potentially supported, on average, 15, 53 and 79% of the phytoplankton NO3- requirements in Saanich Inlet, and along Line P and at BATS, respectively, and this underscores the need for a major re-evaluation of the new production paradigm. Light, substrate concentrations, and potentially substrate supply rates were all found to play a role in regulating NH4+ oxidation, albeit to varying degrees, and I discuss the influence that each of these variables may have had on controlling NH4+ oxidation rates at regionally specific scales in Chapters 2 (Saanich Inlet), 3 (Line P) and 4 (BATS). Finally, a cross study-region comparison of results showed that the relative degree by which new production estimates were reduced, when euphotic zone nitrification was taken into consideration, decreased exponentially as total NO3- uptake rates increased; the relationship I describe between these two variables may potentially provide a simple and rapid means of estimating the extent to which new production may have been overestimated at regionally specific and global scales. My Line P sampling program also provided me with an opportunity to conduct the first investigation of intermediate depth N2O distributions along the Line P oceanographic transect. My results demonstrated that nitrification is the predominant production pathway for N2O in the NE subarctic Pacific. N2O distributions along Line P were variable, however, and I also consider the role of different transiting water masses and potential far-field denitrification in contributing to this variability. Finally, I estimated sea-to-air fluxes of N2O and based on these results I have demonstrated that the NE subarctic Pacific is a “hotspot” for N2O emissions to the atmosphere.en_US
dc.identifier.bibliographicCitationGrundle, D.S., D.A. Timothy, and D.E. Varela. 2009. Variations of phytoplankton productivity and biomass over an annual cycle in Saanich Inlet, a British Columbia fjord. Continental Shelf Research 29, 2257-2269.en_US
dc.identifier.bibliographicCitationGrundle, D.S., and S.K. Juniper. 2011. Nitrification from the lower euphotic zone to the sub-oxic waters of a highly productive British Columbia fjord. Marine Chemistry 126, 172-181.en_US
dc.identifier.bibliographicCitationGrundle, D.S., R. Maranger, and S.K. Juniper. 2012. Upper water column nitrous oxide distributions in the NE subarctic Pacific. Atmosphere-Ocean 50, 475-486.en_US
dc.rights.tempAvailable to the World Wide Weben_US
dc.subjectBiogeochemical Oceanographyen_US
dc.subjectNew Productionen_US
dc.subjectNitrous Oxideen_US
dc.subjectSaanich Inleten_US
dc.subjectSargasso Seaen_US
dc.subjectNE Subarctic Pacificen_US
dc.subjectCarbon Exporten_US
dc.subjectDissolved Oxygenen_US
dc.titleUpper water column nitrification processes and the implications of euphotic zone nitrification for estimates of new productionen_US


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