Dissolved organic matter cycling in and above oxygen minimum zones.

Loginova, Alexandra N. (2016) Dissolved organic matter cycling in and above oxygen minimum zones. (PhD/ Doctoral thesis), Christian-Albrechts-Universität Kiel, Kiel, Germany, 164 pp.

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Eastern Boundary Upwelling Systems (EBUSs) represent very productive areas in the World Ocean. The major part of organic carbon, fixed by primary producers, is rapidly reworked and utilized by heterotrophic communities, causing water column deoxygenation. The deoxygenation, caused by biological processes, is also promoted by the strong water column stratification and sluggish circulation that are characteristic for EBUSs. Dissolved organic matter (DOM) represents the major pool of organic carbon in the global ocean. Therefore, high concentrations of DOM, accumulated near the surface as result of phytoplankton release may significantly affect the biogeochemical cycles of oxygen (O2) and other elements in and above oxygen minimum zones (OMZs) and vice versa. However, DOM distribution, composition and cycling in EBUSs and adjacent OMZs are largely understudied. This doctoral thesis includes two case studies that address DOM distribution and cycling in these marine systems. The first part of the thesis investigates the distribution, composition and potential effects of DOM supply on O2 in and above the OMZ in the Eastern Tropical South Pacific (ETSP) region off coast of Peru. The second part of the thesis is focused on DOM production and quality in response to OMZ-induced changes in nutrient availability in the euphotic zone of the Eastern Tropical North Pacific (ETNA). Both studies suggest that OMZs and DOM quantity and quality are tightly linked. Thus, in the first study diapycnal fluxes of dissolved organic carbon (DOC) across the oxycline (20-50 m depth) were found to be equal to 144±250 mg DOC m-2day-1, where dissolved hydrolysable amino acids (DHAA) and dissolved carbohydrates (DCHO) contributed ~5% and ~20%, respectively [Manuscript I]. The attenuation of the DOC flux was potentially responsible for ~180% of O2 consumption across the oxycline, while ~40% of O2 attenuation could be explained by consumption of labile (LDOM) and semi labile (SLDOM) DOM. Therefore, it can be concluded that DOM consumption is important in sustaining the upper boundary of OMZ. On the other hand, composition and optical properties of DOM were only moderately related to changes in O2 concentrations. DOM fluorescence revealed a rapid decrease in protein-like DOM and an accumulation of humic-like components at depths, where oxygen concentrations switched from oxygenated to anoxic [Manuscript II]. The rapid utilization of proteinaceous DOM was generally supported by changes in diagenetic state of DOM [Manuscript I]. Herewith, several differences in the bio-availability of LDOM and SLDOM components were discussed in comparison to other areas, where the water column is oxygenated. At depth, anoxic sediments were recognized as an additional DOM source in the OMZ. The distribution of the DOM plume, released by sediments could be traced in high resolution up to the euphotic zone, suggesting a direct link of sediment release and surface productivity [Manuscript II]. In the second study, DOM revealed sensitivity to, both, dissolved inorganic nitrogen (DIN) and dissolved inorganic phosphorus (DIP) concentrations, which may change under low O2 conditions within OMZs due to N-loss processes and sediment DIP release [Manuscript III]. Fresh bioavailable DOM accumulated under high DIN and DIP, while DOM reworking and,
therefore, its optical properties were closer linked to DIN concentrations. This study suggests that N-loss processes, occurring within OMZs, as well as future OMZ expansion and intensification may substantially change DOM quality by decreasing ocean color and DOM reworking. The data included in this thesis indicate that DOM is a highly dynamic pool, which is not only influenced by biogeochemical processes, but also affected by physical mixing. Results show that DOM processing co-determines the consumption of O2 and also may reflect changes in elemental cycles. Therefore, the detailed investigation of DOM composition and its relation to production and removal processes may improve our understanding of biogeochemical cycling in and above OMZs.

Document Type: Thesis (PhD/ Doctoral thesis)
Thesis Advisor: Engel, Anja and Marandino, Christa A.
Research affiliation: OceanRep > GEOMAR > FB2 Marine Biogeochemistry > FB2-CH Chemical Oceanography
OceanRep > SFB 754
OceanRep > GEOMAR > FB2 Marine Biogeochemistry > FB2-BI Biological Oceanography
Projects: SFB754
Date Deposited: 17 Jan 2017 11:36
Last Modified: 02 Nov 2022 09:04
URI: https://oceanrep.geomar.de/id/eprint/35722

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