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Dissertation zugänglich unter
Elimination of reactive nitrogen in continental shelf sediments measured by membrane inlet mass spectrometry.
Eliminierung von reaktivem Stickstoff in Sedimenten des Kontinentalsockels, gemessen mittels Membraneinlass-Massenspektrometrie
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Denitrifikation , Membrane inlet mass spectrometry , Nordsee , Benguela , Sediment
Freie Schlagwörter (Deutsch):
Freie Schlagwörter (Englisch):
membrane inlet mass spectrometry , nitrogen elimination , sediment , North Sea , Benguela
Emeis, Kay-Christian (Prof. Dr.)
Tag der mündlichen Prüfung:
Kurzfassung auf Englisch:
Nitrogen is one of the mayor building blocks of organisms and thus an important nutrient of primary producers. During the last century, exponentially increasing amounts of reactive nitrogen were produced by industrial nitrogen fixation and subsequently dispersed as agricultural fertilizer. This anthropogenic reactive nitrogen is partially leached from the soil, transported mainly as nitrate by rivers such as Elbe, and finally reaches the coastal ecosystem. The nitrate load contributes to eutrophication and may cause harmful algae blooms, bottom water anoxia and mass extinctions. However, the anthropogenic nitrogen fixation is opposed by microbial processes such as denitrification and anammox, which convert reactive nitrogen back into N2 and thus effectively remove the nitrogen from the ecosystem. The objective of the present study was measuring the elimination of reactive nitrogen in the sediment of the Elbe estuary and the adjacent German Bight (North Sea), and identifying the controlling environmental factors.
The second chapter presents a rarely employed method of N2-profile measurements with membrane inlet mass spectrometry, which was adopted for measurements in North Sea sediment. The newly developed membrane tip featured improved mechanical durability and enabled measurements with less sediment disturbance than previously presented types. Potential interferences of N2/Ar measurement by sediment properties were identified, and a correction of biased measurements was proposed.
The newly-developed membrane-inlet for N2-profiling in sediments was employed along the salinity gradient of Elbe estuary and the adjacent Elbe plume (chapter 3). The obtained N2 fluxes from the sediment into the water column are a proxy of elimination of reactive nitrogen within the sediment. The N2 fluxes correlated with the bottom water nitrate concentrations and sediment TOC content, respectively. An impact of oxygen or temperature was not evident. The calculated nitrate fluxes from the water column into the sediment based on nitrate concentration profiles had a similar pattern, but did not match exactly the N2 fluxes. Especially in permeable sediment, N2 fluxes significantly exceeded the nitrate consumption, and coupled nitrification-denitrification was identified as most probable source of additional N2 production.
The correlation of benthic nitrate consumption and N2 production with the availability of nitrate, oxygen and TOC, as found in the salinity gradient of the Elbe (chapter 3), was used as basis for the derivation of an estimation function of both reaction rates (chapter 4). This estimation function describes the assumed effect of nitrate, oxygen and TOC on benthic nitrate consumption and N2 production. The function was calibrated with the data of 70 stations in the Elbe estuary, North-Frisian Wadden Sea and German Bight, and additionally with compiled published data comprising various ecosystems of the Atlantic. Both calibrations were used in combination with distribution maps of nitrate, oxygen and TOC to estimate the rates of nitrate consumption and N2 production in sediments of the Elbe plume. During winter (February, March 2009), direct denitrification removed 1 % of the Elbe nitrate load. Coupled nitrification-denitrification produced N2 equal to 3-7 % of the nitrate load. During summer (August, September 2009), direct denitrification removed 2-3 % of the nitrate load, whereas coupled nitrification-denitrification produced N2 equal to 19-43 % of the nitrate load. The nitrogen source of coupled nitrification-denitrification is organic matter, which contains nitrogen assimilated as nitrate during primary production. Thus assimilation was identified as the dominant nitrate-removing process. The assimilated nitrogen is subsequently partially eliminated as N2 by ammonification-nitrification-denitrification.
The methods employed for profiling of N2 and nutrients in North Sea sediment were additionally used in sediments of the Benguela Upwelling System off Namibia, southern Africa (chapter 5). Likewise to North Sea sediment, coupled nitrificationdenitrification had a higher contribution to benthic N2 production than direct denitrification of water column nitrate. In contrast to the North Sea, a substantial impact of temperature and oxygen on remineralisation was found in sediments of Benguela Upwelling System, whereas the bulk sediment TOC content had no evident effect. The reason for this apparent contradiction of results from North Sea and Benguela was the high variability of nitrate consumption and N2 production observed under similar conditions, which disguised the effects of parameters with little variance.