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.

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The presence of volatile methylsiloxanes (VMS) in the environment has raised concerns among environmental chemists and regulators about their persistence and the risks they may pose to the environment. This thesis explores the partitioning and persistence of VMS in aquatic environments. In Paper I, we reported new measurements of the organic carbon/water (KOC) and dissolved organic carbon/water (KDOC) partition ratios of three cyclic volatile methylsiloxanes (cVMS) and of three polychlorinated biphenyls (PCBs), which were used as reference chemicals. We combined new measurements with existing data to construct polyparameter linear free energy relationships (PP-LFER) that describe the KOC and KDOC of diverse sets of chemicals. The findings suggest that cVMS do not conform to single-parameter regressions that relate the chemicals’ KOC to their octanol/water partition ratio (KOW). PP-LFERs can accurately describe the KOC and KDOC of cVMS but only if cVMS are included in their training sets. In Paper II, we studied the effect of salinity on the KOC and KDOC of three cVMS, two linear volatile methylsiloxanes (lVMS) and three PCBs. We also evaluated the predictive power of the PP-LFERs constructed in Paper I by testing them on the newly measured KOC values of lVMS. The KOC and KDOC increased with increasing salinities similarly to those of the PCBs. PP-LFERs that were trained with datasets that included siloxanes could predict the KOC and KDOC of other siloxanes more accurately than PP-LFERs without siloxanes in the training set. In Paper III, we evaluated the effect of temperature on the KOC of VMS and we compared our measurements of the enthalpy of sorption to organic carbon (ΔHOC) to existing measurements of the enthalpy of phase change between octanol and water (ΔHOW). Due to the scarcity of ΔHOC data in the literature it is common practice in modeling calculations to use ΔHOW instead when correcting for temperature changes. The KOC of cVMS increased with decreasing temperatures. Moreover, our results indicate that ΔHOC and ΔHOW may be intrinsically different and hence replacing ΔHOC with ΔHOW in modeling calculations could lead to substantial errors, especially for VMS. In Paper IV, we explored the environmental fate of VMS in aquatic environments using multimedia models. In particular, we assessed the differences that may occur in calculations of persistence due to (i) the reported KOC measurements of VMS differing by one log unit (ii) the influence of salinity on KOC, and (iii) the differences in the reported ΔHOC and ΔHOW measurements of VMS. The calculated residence times for decamethylcyclopentasiloxane (D5) in a site-specific scenario for a Norwegian fjord receiving siloxanes in wastewater ranged from 200 to 1000 days, and demonstrated that the selection of KOC values can result in substantially different calculated persistence. Future partitioning measurements of VMS in the real environment and mass-balance modeling studies in aquatic environments combined with field measurements could help us to deepen our understanding about their persistence and to assess the risks VMS may pose to the environment. / <p>At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 1: Manuscript. Paper 3: Submitted.</p>

The bioavailability of metabolizable organic matter within marine sediments is one of the more important driving mechanisms controlling benthic pelagic communities. Interactions between organic material and mineral surfaces within the sediment, such as adsorption, can cause organic matter to be unavailable for degradation by organisms; therefore for this study we have used the relationship of organic carbon-to-sediment surface area as an indicator of available organic carbon in northern Gulf of Mexico sediments. We have determined that these sediment interactions demonstrate a significant association with benthic fauna abundances; however they are not the most dominant environmental variables. It may be the combination of biogeochemical parameters, such as organic carbon content, sediment surface area, grain size, water depth and other geophysical variables, that is the ultimate control on the bioavailability of metabolizable organic matter in the northern Gulf of Mexico.

The study investigated seasonal and spatial distributions and biogeochemical processes of dissolved and particulate organic matter in the upper layer of northern South China Sea (SCS). Concentrations of dissolved organic carbon (DOC), nitrogen (DON) and phosphorus (DOP) in the euphotic zone of northern SCS were in the range of 55-147 £gM, 2.4-9.9 £gM and 0.13-0.38£gM, respectively. A maximum concentration of DOC, DON and DOP, was found in the station close to the Pearl River due to freshwater input. The concentration of DOC decreased generally with distance away from the continent, but the ratio of DOC/TOC increased with distance primarily due to trophic dynamics. Concentrations of DOM were generally the highest in the surface layer and decreased with depth, but their C/N/P ratios increased with depth, indicating that both nitrogen and phosphorus were preferentially decomposed over carbon. Below the mixed layer, DOC degradation contributed only about 16% of AOU (apparent oxygen utilization). Inverse correlation between DOM and density was significant in the upper layer suggesting that the distributions of DOC, DON and DOP were largely controlled by vertical mixing. Inverse correlation was also significant between DOM and AOU, showing the effect of microbial decomposition on DOM in deep water. Concentrations of POC, PON and POP in the euphotic zone were in the range of 1.8-17.7£gM, 0.18-2.45£gM and 10-163 nM, respectively. Relatively high concentrations of POC, PON and POP in the surface water of inner shelf were also likely attributed to the input of the freshwater. Significant correlation between POC abundance and Chl-a suggested that phytoplankton abundance may control the distribution of POC. The ratio of ¡µPOC/¡µPON in the euphotic zone ranged from 4.57 to 7.3, implying various effects of bacteria and planktons on POM. A simple one-dimensional vertical eddy diffusion model was applied to estimate downward fluxes of DOC and POC and upward fluxes of nutrients across the boundary of euphotic layer and/or thermocline base. Total downward fluxes of organic carbon (OC) were compared with total upward nutrient-derived OC fluxes. The results suggested that additional nutrient sources in the euphotic layer were required to balance OC budgets. The ratios of DIN/DIP were much smaller than the Redifield N/P Ratio of 16:1, suggesting a status of N-limitation in the euphotic zone. The DOC/DON ratio, however, was much higher than the Redfield ratio. These results implied that DOM must play an important role in modulating nutrient cycling and food web dynamics in the euphotic layer.

The bioavailability of metabolizable organic matter within marine sediments is one of the more important driving mechanisms controlling benthic pelagic communities. Interactions between organic material and mineral surfaces within the sediment, such as adsorption, can cause organic matter to be unavailable for degradation by organisms; therefore for this study we have used the relationship of organic carbon-to-sediment surface area as an indicator of available organic carbon in northern Gulf of Mexico sediments. We have determined that these sediment interactions demonstrate a significant association with benthic fauna abundances; however they are not the most dominant environmental variables. It may be the combination of biogeochemical parameters, such as organic carbon content, sediment surface area, grain size, water depth and other geophysical variables, that is the ultimate control on the bioavailability of metabolizable organic matter in the northern Gulf of Mexico.

Recalcitrant soil carbon is a poorly understood component of total soil organic carbon (SOC). Although the turnover rate of the recalcitrant fraction is slow, warming temperatures are expected to speed the decomposition of recalcitrant SOC resulting in an increase of atmospheric CO₂ in the future. Several studies show that the oldest SOC is associated with the smallest mineral particles (clays), making direct spectroscopic analysis of old carbon difficult. To overcome the difficulty of analyzing natural samples, we created synthetic soils to examine the association between clay surfaces and specific biomolecules based on the hypothesis that clays with higher surface charge will more strongly bond organic molecules, and also that certain molecules will be better stabilized by clay. We used kaolinite, montmorillonite, or quartz (sand) as a synthetic soil inside 12 mL septum-capped vials, added either dissolved glucose or vanillic acid to each mineral, inoculated with soil microbes, and then purged the vials with a CO₂-free atmosphere. We incubated them and measured the concentration and [delta]¹³C of CO₂ that accumulated in the vials. Respiration rates were significantly higher in experiments containing vanillic acid than in those containing glucose. Respiration rates were lowest in experiments containing montmorillonite. We repeated the experiment using dilute H₂O₂ as an oxidant, and adding vanillic acid, glucose, or glycine. Vials with montmorillonite showed lower rates of CO₂ accumulation than kaolinite, and both glycine- and glucose-containing experiments had less CO₂ than vanillic acid-experiments. We conclude that the montmorillonite protected the organic matter from oxidation better than sand or kaolinite. Both clays protected organic matter better than sand. In all experiments with clay, the respired CO₂ had lower [delta]¹³C values than bulk substrate. This carbon isotope fractionation is likely due to preferential desorption, followed by oxidation, of 12C- as opposed to 13C- bearing organic molecules. The mineral-organic interaction is a strong bond that explains the old age of labile organic compounds in soils. These results indicate that the clay fraction of soils must be considered for accurate prediction of future land-atmosphere carbon fluxes. / text

As the largest carbon reservoir in ecosystems, soil accounts for more than twice as much carbon storage as that of vegetation biomass or the atmosphere. The goal of this study is to examine spatial patterns of soil organic carbon (SOC) in Canadian forest area at an eco-region scale and to explore its relationship with different ecological variables. In this study, the first Canadian forest soil database published in 1997 by the Canada Forest Service was analyzed along with other long-term eco-climatic data (1961 to 1991) including precipitation, air temperature, Normalized Difference Vegetation Index (NDVI), slope, aspect, and elevation. Additionally, an eco-region framework established by the Environment Canada was adopted in this study for SOC distribution assessment. Exploratory spatial data analysis techniques, with an emphasis on spatial autocorrelation analysis, were employed to explore how forest SOC was spatially distributed in Canada. Correlation analysis and spatial regression analysis were applied to determine the most dominant ecological factors influencing SOC distribution in different eco-regions. At the national scale, a spatial error model was built up to adjust for spatial effects and to estimate SOC patterns based on ecological and ecosystem property factors. Using the significant variables derived in the spatial error model, a predictive SOC map in Canadian forest area was generated. Findings from this study suggest that high SOC clusters tend to occur in coastal areas, while low SOC clusters occur in western boreal eco-region. In Canadian forest area, SOC patterns are strongly related to precipitation regimes. Although overall SOC distribution is influenced by both climatic and topographic variables, distribution patterns are shown to differ significantly among eco-regions, thus verifying the eco-region classification framework for SOC zonation mapping in Canada.

Arsenic contamination of circum-Himalayan groundwater is leading to one of the greatest humanitarian disasters of modern times, poisoning at least 70 million people who are mostly poor and rural. The groundwater is hosted in Holocene aquifers consisting of Himalayan sediments deposited by the great Asian rivers in deltaic environments. Arsenic is released when organic material (OM) reacts with the iron-oxide minerals co-deposited in the sediments onto which arsenic is adsorbed. The source of OM is one of the most important questions facing researchers and policy makers. There are generally accepted to be three potential sources of OM: 1) sedimentary bound OM (SOM) co-deposited with sediments; 2) thermally mature petroleum upwelled from reservoirs below the aquifers; 3) dissolved organic carbon (DOC) some of which might be drawn in to the aquifer through modern pumping and irrigation. In this thesis the nature of organic material in the aquifer is researched and the processes and timescales which lead to arsenic release are studied. Here evidence for a new conceptual model of arsenic release is presented. Isotopic tracing combined with a new geochemical model and organic geochemical techniques, shows that OM driving arsenic release pre-dates agriculture in the region and was from natural grasslands in the early Holocene. The geochemical model utilises strontium isotopes to correct the radiocarbon age of dissolved inorganic carbon (DIC) to find only the age and isotopic signature of DIC from oxidation of organic material. This shows that DIC from oxidation of OM was from the early Holocene and had an isotopic signature consistent with the early Holocene SOM in this region. A study of the sediments in the region built upon a geomorphological history shows that the most oxidised SOM is from early Holocene sediments. Thus both techniques separately indicate that pre-agricultural organic material drove arsenic release. This conceptual model however reveals the "arsenic sand paradox", because whilst release is from early Holocene clays, today highest concentrations of arsenic are in younger sands. Explaining this paradox is the most important next step leading on from this research.