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type of natural waters: Lena River and its channels, lake, permafrost meltwater creeks, permafrost outflows and coastal waters of the Laptev Sea. Molecular systematics allowed identification of source-specific DOM molecular markers and their relative contribution to DOM of different origin. The relative contribution of organic nitrogen was highest in the permafrost-derived samples. Bulk DOM parameters matched well with the molecular DOM fingerprints, especially with focus on the nitrogenous fraction. Molecular diversity in DOM from different origins can be explained by regulation of different environmental processes.
Characterized source-specific and source-dominant molecular markers will allow in future estimation of contribution of different sources into DOM which integrates signals from various sources (e.g. riverine and estuarial DOM). Since we demonstrated significant differences of the DOM molecular composition of melt water creeks and the Lena River, the growing contribution of permafrost-derived DOM might also change the riverine biogeochemistry in the Lena Delta and the Laptev Sea with consequences on the ecosystem.
Summarizing the study on DOM reactivity in the river-sea transition we showed that DOC exhibited a conservative behavior in the Lena River - Laptev Sea transition and passed the mixing zone almost quantitatively, but, simultaneously, thousands of DOM molecular components showed active involvement to the transitional processes. We observed strong physico-chemical gradients of salinity, temperature, pH, nutrients, and DOC on studied transects in the coastal Laptev Sea. The correlation of salinity and molecular peak intensity were successfully applied for the estimation of DOM reactivity in the river-sea transition.
About 40% of DOM molecular pool was highly affected by processes, which occurred in the river-sea transition zone. 27% of all identified DOM molecular components exhibited strongly conservative behavior in the river-sea transition. We called this the “pseudo-conservative” behavior of DOM. Highly conservative components can find further applications as tracers for DOM components of terrestrial origin and for estimation of their persistence in the oceans. Reactive components are the subjects of future research in the
141 context of dominant DOM turnover and mineralization mechanisms and their contribution to the regional carbon cycle. These compounds are potential targets for the exploration of estuarine processes on the molecular level. The interpretation of detailed DOM compositional changes as result of estuarine processes is the aim of further studies.
The study on mobilization, mineralization and turnover of permafrost-derived DOM highlighted several aspects with high relevance for the permafrost carbon cycle. Extremely high and variable DOC and TDN concentrations were found for mud streams and permafrost outflows formed by mobilization of thawed permafrost deposits. DOC concentration in permafrost outflows significantly differed from DOC in adjacent riverine and coastal waters.
Within 18 days of dark microbial incubation ~44% of the DOC was mineralized. Photo incubation resulted in a relatively low DOC decomposition rate which was comparable with the control samples, suggesting minor contribution of direct photo degradation in permafrost-derived DOC mineralization. Observed decomposition rates fit well with the fact that small streams in permafrost areas are substantial sources of CO2 to the atmosphere. Bacterial metabolism regulated not only DOC concentration, but chemical richness and molecular diversity of the permafrost-derived DOM: microbial incubation resulted in a considerable compression of the molecular space with ~18% decrease of chemical richness. Hence reintroduction of reactive permafrost carbon could have critical implications for regional carbon cycling. The diverse chemical nature of labile and relatively recalcitrant components indicates that chemical composition drives availability of molecules for microbial metabolism. All observed quantitative and molecular tendencies are illustrative for DOM processing on the terrestrial-aquatic interface in permafrost OM mobilization processes.
Continuous and reproducible molecular changes for DOM mobilized from permafrost deposits along the terrestrial-aquatic transition were recorded. This fact suggested a significant role of microbial regulation in the molecular composition of DOM in the Lena Delta. Thus, we experimentally and observationally assessed the fraction of DOM which
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actively contributes to carbon cycling with a high potential of transformation to greenhouse gases. In summary, the total contribution of ice complex outflows to the riverine OC at eroding river banks and coastal bluffs was minor. However, it is locally influential, particularly due to high concentrations of biogeochemical constituents in ice complex outflows. Thus, the majority of DOM mobilization and mineralization during permafrost thaw occurs without long-distance transport. Processes on the shores and coasts result in much faster feedback and their potential influence is more relevant in context of hydrology, hydrochemistry and biogeochemistry of the large arctic rivers and coastal Arctic Ocean.
The non-targeted mass-spectrometric characterization of DOM is not a routine tool in DOM research but the number of applications is continuously growing. In the last decade it found many applications not only in pure analytical chemistry of DOM but in characterization of biogeochemical properties and environmental roles of DOM in natural waters. It is important to note, that non-targeted approaches in DOM chemistry are still highly sophisticated, not unified and under development. Different research groups use diverse analytical protocols, which sometimes are not completely transparent and reproducible. Therefore, future methodologies of chemical analysis and data treatment/interpretation procedures have to be unified for an improved development of the field. FT-ICR MS is a semi-quantitative method in organic analytics of DOM and usually peaks are treated only based on their presence/absence. Recent studies showed informativity of peak intensities in mass spectra of DOM, they can be used for data mining in large datasets, but do not reflect concentrations of single molecular components.
In this thesis it was demonstrated that non-targeted analytics can be applied for studies on DOM biogeochemistry in the changing systems like the Arctic. The studies showed that although FT-ICR MS and NMR have many limitations, they can significantly deepen our understanding of DOM roles and behavior in natural waters. Therefore, in case of DOM analysis we are dealing with the puzzle which
143 cannot be assembled completely, but available pieces of the puzzle could be already sufficient to guess the silhouette of the big picture.
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