Surface waters receive large amounts of dissolved organic matter (DOM) via runoff from land. The DOM is rich in organic carbon that serves as an energy source for the aquatic biota. During uptake of this energy, aquatic organisms mineralize organic carbon. The resulting inorganic carbon is partially released to the atmosphere as carbon dioxide and methane that are greenhouse gases, and which are of concern for the ongoing global warming. The rate at which organic carbon is mineralized depends strongly on DOM quantity and quality that vary with respect to both time and space. In this thesis, DOM quantity and quality were addressed using spectroscopic methods that build on the absorptive and fluorescent properties of chromophoric DOM (CDOM). New techniques to measure CDOM absorption and fluorescence were applied and further developed that allowed us to present novel CDOM variability patterns. Addressing the lake-rich Scandinavian landscape, strong focus was placed on water retention by lakes that tightly links to lake DOM quantity and quality. An analysis of 24,742 lakes from seven large Swedish river systems indicated that the majority of lakes in Sweden exchange their water within one year. From headwaters to the Sea, summed lake volumes in the catchments of lakes were found to increase at rates comparable to discharge, which indicated effective water renewal along flow. A strong relationship between lake water retention and CDOM was apparent and further investigated based on samples from a lake district to a regional scale. Results from in situ high-frequency monitoring of CDOM absorption in a eutrophic humic lake showed intra-annual variability patterns known from oligotrophic lake systems. The patterns for CDOM absorption contrasted results obtained for synchronously measured partial pressures of carbon dioxide that showed diurnal signals. Measurements of CDOM fluorescence and DOC concentrations indicated lake-internal DOM production. A comparison of these results with results from addressing 560 lakes distributed across Sweden, showed that a well-calibrated CDOM fluorescence measurement captures signals from lake-internal DOM production. I conclude that improved CDOM fluorescence measurements are promising to address lake-internally produced DOM.

The repercussions of rapid climate-change are felt worldwide, but particularly in Arctic and Subarctic regions. Evidence of recent changes in water chemistry is being recorded in Arctic aquatic ecosystems, bringing further attention to contaminant dynamics in these environments. I assessed the role of dissolved organic matter (DOM) in controlling the bioavailability of mercury (Hg), a top priority among Arctic contaminants, to aquatic food webs using a bacterial bioreporter under oxic conditions. Experiments were performed under pseudo- and non-equilibrium conditions, in both defined media and water samples from tundra lakes with a large gradient in DOM. Inorganic HgII was considerably more bioavailable under non-equilibrium conditions than when DOM was absent or when HgII and DOM had reached pseudoequilibrium (24h). Under these enhanced uptake conditions, HgII bioavailability followed a bell shaped curve as DOM concentrations increased, both for defined media and field samples, suggesting that complexation kinetics and binding thresholds on DOM determine HgII bioavailability to methylating bacteria, and likely MeHg concentrations, the bioaccumulative neurotoxic form of Hg. Experiments also suggest that DOM may alter cell wall properties to facilitate the first steps toward HgII internalization via facilitated or active transport, and yet without altering overall cell wall permeability. While further research on ternary (HgII-cell-DOM) interaction is warranted, I propose a molecular shuttle model for DOM in facilitating bacterial HgII uptake, and the existence of a short-lived yet critical time window (<24h) during which DOM facilitates the entry of newly deposited HgII from the atmosphere into aquatic food webs.

Insect pests are the most widespread disturbance in Canadian forests, but resulting impacts of forest defoliation on stream ecosystem functions are poorly understood. This study investigated the effects of a spruce budworm outbreak on water quality and the structure and function of microbial communities in streams of 12 catchments across a gradient of cumulative defoliation severity in the Gaspésie Peninsula, Québec, Canada. Bi-weekly stream habitat sampling was conducted spring to fall 2019-2021, with stream flow rates measured and water samples collected and analyzed for water chemistry parameters, nutrients, and dissolved organic matter (DOM) structure and quality. Algal communities were assessed at the same time by measuring in-situ biomass. Bacteria and fungi communities on leaf packs were assessed by incubating six leaf packs for five weeks (mid-August- late September) in one stream reach per watershed. Microbial community composition of leaf packs was determined using metabarcoding of 16S and ITS rRNA genes, and functions were examined using extracellular enzyme assays, leaf litter decomposition rates, and taxonomic functional assignments. This study determined that cumulative defoliation increased stream temperatures, flow rates, and SUVA (DOM aromaticity), but not nutrients. It increased algal biomass and altered microbial community composition, with a stronger influence on bacteria than fungi. The observed increases in SUVA and algal biomass corresponded with changes to bacteria carbon cycling functions, which indicated that microbes were preferentially selecting carbohydrates produced by algae rather than the aromatic compounds from increased terrestrial inputs. There were no changes to other bacteria or fungi functions and no changes to taxonomic or functional diversity. Overall, results indicate that forest pest outbreaks alter carbon inputs to streams and the structure and function of stream microbial communities associated with carbon cycling. / Thesis / Master of Science (MSc) / Terrestrial and aquatic landscapes are tightly linked, and forest disturbances can influence stream ecosystems. Insect pests defoliate millions of hectares of forests each year, but the resulting impacts on stream ecosystems are poorly understood. This study investigated the effects of a spruce budworm outbreak on water quality and microbial communities in streams in Gaspésie, QC, Canada. Microbial communities are critical to the functioning of stream ecosystems as they convert energy (e.g., carbon) into useable forms for other organisms. Results indicate that defoliation altered stream flow rates, temperatures, and carbon composition, as well as the microbial communities involved in carbon cycling processes. Carbon is essential to aquatic food webs and this improved understanding of how carbon flow is altered by a widespread forest disturbance can inform pest management decisions for spruce budworm outbreaks.

Understanding hydrologic controls on carbon accumulation and export within geographically isolated wetlands (GIW) has implications for the success of wetland restoration efforts intended to produce carbon sinks. However, little is known about how hydrologic connectivity along the aquatic-terrestrial interface in GIW catchments influences carbon dynamics, particularly regarding dissolved organic matter (DOM) transport and transformation. The organic matter (carbon) that accumulates in wetland soils may be released into water, generating DOM. DOM is mobile and reactive, making it influential to aquatic metabolism and water quality. To understand the role of different soil horizons as potential sources of DOM, extractable soil organic matter (ESOM) was measured in soil horizons collected from upland to wetland transects at four Delmarva Bay GIWs on the Delmarva Peninsula in the eastern United States. ESOM quantity and quality were analyzed to provide insights to organic matter sources and chemical characteristics. Findings demonstrated that ESOM in shallow organic horizons had increased aromaticity, higher molecular weight, and plant-like signatures. ESOM from deeper, mineral horizons had lower aromaticity, lower molecular weights, and protein-like signatures. Organic soil horizons had the largest quantities of ESOM, and ESOM decreased with increasing soil depth. ESOM quantities also generally decreased from the upland to the wetland, suggesting that continuous soil saturation leads to a decreased quantity of ESOM. Despite wetland soils having lower ESOM, these horizons are thicker and continuously hydrologically connected to wetland surface water, leading to wetland soils representing the largest potential source of DOM to the Delmarva Bay wetland system. Knowledge of which soil horizons are most biogeochemically significant for DOM transport in Delmarva and other GIW systems will become increasingly important as climate change is expected to alter the hydrologic connectivity of wetland soils to the surface water-groundwater continuum and as wetlands are more frequently designed for carbon sequestration. / Master of Science / Wetlands store carbon in their plant biomass and soils, which helps to mitigate the effects of climate change by keeping carbon out of the atmosphere. Carbon builds up in wetland soils because the continuously wet conditions slow down the microbial processes that would otherwise break down the organic matter (carbon) and release it to the atmosphere via greenhouse gas emissions. However, the organic matter that accumulates in wetland soils may be released into water, generating dissolved organic matter (DOM). This DOM has the potential to flow out of the wetland, providing a source of energy to aquatic organisms or impacting downstream water quality. Not all wetlands are continuously connected to other water bodies. Geographically Isolated Wetlands (GIW) are wetlands that you could walk all the way around and keep your feet dry. Despite lack of continuous surface water connections, GIWs may still influence downstream water quality via groundwater flow paths or seasonal surface water connections. This variable connectivity makes GIWs a unique setting to study carbon storage and fluxes in wetland soils. The potential for soil-derived DOM generation was studied by extracting organic matter from soils along a wet to dry gradient in Delmarva Bay GIWs. Shallow soils had the largest quantities of extractable soil organic matter (ESOM) and this organic matter is likely sourced from plant inputs to the soil. ESOM from deeper soils was more similar to the microbes that consume and alter the organic matter as it cycles deeper into the soil. Soils located in the wetland basin had less ESOM because continuous saturation depleted the pool of ESOM. Despite lower values of ESOM, wetland soils are very thick and continuously saturated, making these soils the largest potential contributor of soil-derived DOM to Delmarva Bay GIWs. This work furthers our understanding of how hydrology drives carbon cycling in GIWs and will inform wetland restoration efforts designed to create carbon sinks.

Colored dissolved organic matter (CDOM) is a part of total dissolved organic matter (DOM) and plays an important role in marine carbon cycling. Thus, a better description of the fates of CDOM may increase our understanding of DOM sources and sinks in marginal seas. This study aims to explore the distributions and possible controlling factors of DOM in the Taiwan Strait (TS) and the East China Sea (ECS) in the summer season. The TS is a marginal sea of interest as it receives freshwater from both Taiwan Island and Mainland China and it also connects water transports between ECS, South China Sea (SCS) and Kuroshio water (KW). The concentration of DOM is slightly higher in the western side (China coast) than in the eastern side (Taiwan coast), and the highest concentration is always found in the Minjiang plume, revealing a significant impact of river discharge. The absorption and fluorescence properties of CDOM varied to a large degree with space arisen from the mixing of source waters including freshwater (from Taiwan and Mainland China), Changjiang diluted water (CDW), SCS water and KW. The KW and river-plume waters (Changjiang, Minjiang) have the lowest and highest values for absorption coefficient (a(325)) and fluorescence intensity of terrestrial humic-like CDOM (Ft: Ex/Em= 320-360/420-460), respectively. Both a (325) and Ft correlated inversely with salinity. Although the spectral slope (S) varied only within a small range, it still can be used to differentiate water masses in TS, as the S value is generally lower in coastal waters than in SCS and KW. Meanwhile, Ft is highly correlated with surface DIN and Si in TS, showing that these parameters are strongly influenced by terrestrial inputs. There are six types of water masses mixing in the ECS in summer named CDW, Yellow Sea water (YSW), China coastal upwelling water, KW, Taiwan Strait Warm Current (TSWC) and Kuroshio upwelling water. Generally, the concentrations of DOC, DON and DOP are the highest in the surface water and decrease with depth due to strong degradation below the surface. The DOC/DOP and DOC/DON ratios of the most stations are higher than the Redfield ratio, showing a carbon enrichment of DOM in the ECS. The optical properties of CDOM show particular DOM characteristics in different water masses and reveal clearly the sources of DOM over various zones. After the operation of Tree-Gorges Dam, the CDW covering zone is likely reduced as reflected from the spatial patterns of salinity and absorption coefficient. The factor analysis implies that the terrestrial inputs and influence may be the dominant factor in constraining DOM and CDOM distributions in the TS and ECS.

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Dissolved organic matter (DOM) plays a vital role in biogeochemical processes and can flux CO2 to the atmosphere when labile fractions are degraded, hence DOM degradation is increasingly studied. Some studies have suggested that fluorescence-derived substrate characteristics are useful metrics for estimating bioavailability (as prerequisite condition for biodegradability), however, recent findings on soil organic matter emphasize the importance of ecosystem scale factors such as physical separation of substrate from soil microbial communities and soil physiochemical cycles driving organic matter stability. I extend this principle to soil derived DOM and hypothesize that such environmental conditions, covariant with season, land use and landscape position, impact the composition of soil DOM and activity and abundance of the microbial community, which together govern DOM biodegradability. As a result, DOM bioavailability may not reliably be predicted using substrate characteristics alone. To test these hypotheses, I assessed aqueous soil extracts for water extractable organic carbon (WEOC) content, biodegradability, microbial biomass and fluorescence spectroscopy on water extractable organic matter (WEOM) across a range of environmental conditions in northern Vermont, USA. My results indicate that changes in environmental conditions affect composition, quantity, and biodegradability of DOM. WEOC concentrations were highest in the fall and lowest in the summer, while no significant differences were found between land covers or landscape position, however, DOM biodegradability was significantly higher in the agricultural (AG) site across seasons. Despite a shift in utilized substrate from less aromatic DOM in summer to more aromatic DOM in winter, biodegradability was similar for all seasons. The only exception were cold temperature incubations where microbial activity was depressed, and processing was halted. These results indicate that bioavailability cannot be reliably predicted based on fluorescence-based metric alone, rather, my core findings illustrate a complex picture of how environmental conditions, landscape characteristics, and substrate composition interact to drive the biodegradability of labile carbon pools in the soil environment. This thesis includes i) a background and comprehensive review of literature to inform the reader of any relevant topics, ii) a paper submitted for publication to Biogeochemistry (Chapter 2), and iii) supplemental information containing figures and tables pertinent to the paper.

Copper-induced toxicity in aqueous systems depends on its speciation and bioavailability. Dissolved organic matter (DOM) and reduced sulfur species can complex copper, influencing speciation and decreasing bioavailability. DOM composition in estuaries can vary, depending on allochthonous, autochthonous, or wastewater source. At a molecular level, variability in DOM quality potentially results in different copper binding affinities. The aim of this study was to characterize and quantify DOM and reduced sulfur in estuaries and investigate possible correlations between these parameters and the capacity to complex copper, reducing its toxicity. This study will have implications on the development of marine-specific toxicity prediction models. DOM was characterized in seventy-one estuarine samples through DOC concentration and fluorescence measurements, combined with spectral resolution techniques, to quantify humic-, fulvic-, tryptophan-, and tyrosine-like fractions. Reduced sulfur was measured by the chromium-reducible sulfide (CRS) technique. Acute copper toxicity tests were done on a subset of samples expressing extreme DOC, fluorescent allochthonous, autochthonous, and CRS concentrations. The results showed significant differences in DOM quality, independent of DOC concentration. In terms of total fluorescent material, humic-like material ranged from 9.48% to 66.1%, followed by fulvic-like with a range of 14.5% to 63.2%, and 0.00% to 36.5% for tryptophan-like and 0.64% to 25.2% for tyrosine-like material. CRS was widely variable among the samples; concentrations ranging from 0.5 nM to 7800 nM. The toxicity results suggested DOC was a very good predictive measure of copper EC50 in estuaries (r2 = 0.84) independent of DOM quality. Furthermore, CRS was saturated at low copper concentrations indicating strong binding sites for copper, suggesting that while CRS is protective, it does not bind copper at toxicologically relevant concentrations and therefore is not a good predictive measure of copper toxicity.

Copper-induced toxicity in aqueous systems depends on its speciation and bioavailability. Dissolved organic matter (DOM) and reduced sulfur species can complex copper, influencing speciation and decreasing bioavailability. DOM composition in estuaries can vary, depending on allochthonous, autochthonous, or wastewater source. At a molecular level, variability in DOM quality potentially results in different copper binding affinities. The aim of this study was to characterize and quantify DOM and reduced sulfur in estuaries and investigate possible correlations between these parameters and the capacity to complex copper, reducing its toxicity. This study will have implications on the development of marine-specific toxicity prediction models. DOM was characterized in seventy-one estuarine samples through DOC concentration and fluorescence measurements, combined with spectral resolution techniques, to quantify humic-, fulvic-, tryptophan-, and tyrosine-like fractions. Reduced sulfur was measured by the chromium-reducible sulfide (CRS) technique. Acute copper toxicity tests were done on a subset of samples expressing extreme DOC, fluorescent allochthonous, autochthonous, and CRS concentrations. The results showed significant differences in DOM quality, independent of DOC concentration. In terms of total fluorescent material, humic-like material ranged from 9.48% to 66.1%, followed by fulvic-like with a range of 14.5% to 63.2%, and 0.00% to 36.5% for tryptophan-like and 0.64% to 25.2% for tyrosine-like material. CRS was widely variable among the samples; concentrations ranging from 0.5 nM to 7800 nM. The toxicity results suggested DOC was a very good predictive measure of copper EC50 in estuaries (r2 = 0.84) independent of DOM quality. Furthermore, CRS was saturated at low copper concentrations indicating strong binding sites for copper, suggesting that while CRS is protective, it does not bind copper at toxicologically relevant concentrations and therefore is not a good predictive measure of copper toxicity.