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Upland peat catchments are usually assumed to function as carbon sinks, however, there have been extensive studies witnessing increasing trends in concentrations and fluxes of organic carbon in UK rivers over the last few decades. A number of controls on dissolved organic carbon (DOC) release from peatlands, such as climatic changes and landuse management, have been proposed. This study examines the effects of land use and management on organic carbon exports in the Dyke catchment of the River Halladale (northern Scotland) with a nested catchment approach. This study provides insight into the processes controlling the DOC dynamics in the Dyke catchment, and the impact of disturbance caused by landuse changes such as afforestation and tree felling for restoration. The results from factor analysis, end-member mixing, absorbance (E4/E6), and hysteresis analyses on stream water chemistry from individual sub-catchments identified the major hydrological pathways during storm events, and based on these results, conceptual models were developed to explain DOC evolution during storm events. At all the sites studied, nearsurface soil water was identified as the major controlling end-member for stream DOC concentrations. The calculated annual flux of DOC from the Dyke catchment, up-scaled from the results of the individual sub-catchments, is 521.6 kg C ha-1 yr-1, which is significantly (~5 times) higher than the previously published value (103.4 kg C ha-1 yr-1) for the River Halladale catchment (Hope et al., 1997). In this study, it is shown that about 57 - 95% of the DOC export occur during 5 - 10% of the high flows, therefore, it is crucial that quantitative records of DOC export are developed using high frequency storm event measurements, as well as lower frequency low flow sampling. Climatic changes related to precipitation, temperature, coupled with water yield capacity of the sub-catchments, are identified as significant controls on DOC fluxes, rather than landuse change, as the intact site releases more organic carbon per unit area compared to the disturbed and re-wetted site undergoing restoration. In addition, the results from this study provide landowners, policy makers and organisations with the evidence they require for initiating future peatland restoration works, as felling of forestry coupled with drain-blocking is shown to be an effective restoration technique that may help a catchment to eventually return to a near-pristine state.

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Thesis: Ph. D., Joint Program in Oceanography/Applied Ocean Science and Engineering (Massachusetts Institute of Technology, Department of Civil and Environmental Engineering; and the Woods Hole Oceanographic Institution), 2015. / Cataloged from PDF version of thesis. / Includes bibliographical references. / Microorganisms play a central role mediating biogeochemical cycles in the ocean. Marine dissolved organic matter (DOM) - a reservoir of organic solutes and colloids derived from plankton is a major source of carbon, nutrients, and energy to microbial communities. The biological transformation and remineralization of DOM sustains marine productivity by linking the microbial food web to higher trophic levels (the microbial loop) and exerts important controls over the cycles of carbon and bioessential elements, such as nitrogen and phosphorus, in the sea. Yet insight into the underlying metabolism and reactions driving the degradation of DOM is limited partly because its exact molecular composition is difficult to constrain and appropriate microbial model systems known to decompose marine DOM are lacking. This thesis identifies marine microorganisms that can serve as model systems to study the metabolic pathways and biochemical reactions that control an important ecosystem function, DOM turnover. To accomplish this goal, bacterial isolates were obtained by enriching seawater in dilution-to-extinction culturing experiments with a natural source of DOM, specifically, the high molecular weight (HMW) fraction (>1 kDa nominal molecular weight) obtained by ultrafiltration. Because it is relatively easy to concentrate and it is fairly uniform in its chemical composition across the global ocean and other aquatic environments, HMW DOM has the potential to serve as a model growth substrate to study the biological breakdown of DOM. The phylogeny, genomes, and growth characteristics of the organisms identified through this work indicate that HMW DOM contains bioavailable substrates that may support widespread microbial populations in coastal and open-ocean environments. The availability of ecologically relevant isolates in culture can now serve to test hypothesis emerging from cultivation-independent studies pertaining the potential role of microbial groups in the decomposition of organic matter in the sea. Detailed studies of the biochemical changes exerted on DOM by selected bacterial strains will provide new insight into the processes driving the aerobic microbial food chain in the upper ocean. / by Oscar Abraham Sosa. / Ph. D.