Dissolved Organic Matter in the Canadian Arctic & Sub-Arctic: Importance of DOM Quality & Quantity in a Warming Climate
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Dissolved organic matter (DOM) is a ubiquitous component of aquatic and terrestrial systems and an important constituent that can influence aquatic health and drinking water quality. For instance, DOM can act as an important nutrient for microbes or react with chlorine during treatment of drinking water supplies to form harmful disinfection by-products. As DOM is comprised of thousands of different organic molecules, the degree that DOM reacts with its surroundings depends not only on the amount of carbon, but also on its composition. A changing climate can influence DOM concentration and composition in a number of ways, such as by altering the residence time within the watershed or changing rates of DOM processing through warming temperatures. Recently, changes to DOM quality and quantity have been observed across surface waters in the northern hemisphere, which can complicate future drinking water treatment options. Organic-rich areas underlain with permafrost are found across the circumpolar north, leading to uncertainty over the effects of permafrost degradation on carbon release and fate, particularly upon downstream ecosystems and drinking water resources. Better prediction as to how DOM will respond under a warming climate requires an understanding of the variability in the amount and composition of DOM, as well as the drivers of DOM reactivity. However, the few arctic or sub-arctic locations with comprehensive DOM datasets can be difficult to compare due to the use of various DOM characterization techniques. Further, data are lacking, or entirely missing, in many areas of Canada’s western sub-arctic. The overall goal of this thesis is to use field and laboratory measurements to quantify the heterogeneity encountered in DOM concentration and composition from a variety of hydrologic environments in three Canadian sub-arctic and arctic ecoregions, and to link this variability to DOM lability. Statistical analyses of long-term monitoring of river hydrology and water quality provides a quantitative measure to the response of a watershed to a warming climate. Northern areas are quickly responding to a warming climate, yet few long-term monitoring records exist and most focus on large rivers draining directly into the Arctic Ocean. The Government of Northwest Territories have been collecting monthly river water quality parameters for the past 30+ years, providing a comprehensive dataset to quantify changes occurring to the Northwest Territories (NT) and define baseline conditions to help assess future change. Trend analysis was applied to mean annual and monthly air temperature, total precipitation, discharge, concentration, and load for rivers draining the taiga shield (Yellowknife and Cameron Rivers) and taiga plains (Marian River) during the past 30 years. Mean annual air temperatures have significantly increased during the past 80 years (3.2 x 10-2 ºC/yr), with significant increased winter monthly average temperatures (January to April). Large inter-annual variability was found in monthly average discharge for the Yellowknife and Cameron rivers, yet no significant change was found to the mean annual discharge during the past 80 years (the Marian River is ungauged). Winter flows have also increased over time within the Yellowknife River. Significant increases to mean monthly cation and anion concentrations occurred within the Yellowknife and Cameron rivers, but they did not result in significant changes to the annual loads. Baseline conditions can be easily determined for the Marian and Cameron rivers due the unchanging hydrologic and geochemical record. However, the Yellowknife River exhibits a uni-directional change in water quality, indicative of enhanced subsurface flow pathways even with no significant change to its discharge. These results indicate that annual variability in river discharge is important for determining differences in geochemical fluxes with a warming climate. DOM composition can be quantified using a wide range of analytical techniques that vary in information obtained, cost, and analytical complexity. The overall objective was to use a readily available suite of DOM characterization techniques from surface and subsurface environments to determine which simple parameters explain the most variability within our DOM dataset, and use these parameters to create a simple, effective way to compare compositional differences in DOM. Samples were collected from surface and subsurface waters across northern freshwaters in Canada. DOM composition was quantified via absorbance, elemental ratios, and size-exclusion chromatography. Overall, DOM concentrations ranged from 0.5 mg C/L in surface water at high arctic sites to 273 mg C/L in NT subsurface environments. Composition measures that best explained DOM variability and were most unrelated in approach were specific absorbance at 255 nm (SUVA), ratio of dissolved organic carbon to dissolved organic nitrogen (DOC:DON), slope of absorbance between 275 to 295 nm (S275-295), and humic substances fraction (HSF). Application of principal component analyses (PCA) quantified these independent measures of composition to explain up to 61% of the variability within the first three PCA axes. These four measures were used to create a ‘Composition Wheel’ to facilitate comparison of DOM across samples. A wide range in SUVA, S275-295, DOC:DON, and HSF values were observed across DOM concentrations and spatial scales. Overall, subsurface DOM composition was similar across all locations, defined by high amounts of humics, DOC:DON, and SUVA, and low S275-295, while surface water DOM contained a variety of compositions across sites. Composition Wheels provide a simple way to visualize and compare DOM composition and quality, as well as efficiently communicate differences in DOM composition to a variety of scientific and interested audiences. Microbial degradation is often the most important driver of DOM fate within aquatic systems. However, few measures of DOM microbial degradation rates are found among sub-arctic and arctic freshwater studies. The overall objective was to use a 30-day incubation experiment to determine how DOM composition influences microbial DOM degradation, and quantify microbial degradation rates for various surface and subsurface waters in the taiga shield (Yellowknife, NT), southern arctic (Daring Lake, NT), and northern arctic (Lake Hazen Watershed, NU). Proportion of DOM loss ranged from 1 to 27% across all samples with no clear association to initial DOM composition or hydrologic site. First-order degradation rates ranged from 0.4 to 11.2 x10-3 d-1, and were highest from DOM in the taiga shield subsurface and a southern arctic pond. Samples from the northern arctic contained the lowest rates and DOM loss, suggesting high arctic DOM may have undergone processing prior to the incubation experiment to more southern locations or no processing if the inoculum contained no viable microbes. Metrics of DOM composition responded differently to microbial degradation across all samples. Absorbance based measures (S275-295, and SAC420) have poor relationships to the proportion of DOM loss and 1st-order microbial degradation rates, whereas molecular size-based groupings were stronger predictors of degradation rate and proportion of DOM loss. Further, SUVA was a sensitive indicator of the microbial-induced change in DOM, and not a good predictor of biodegradability. DOM from all three northern ecoregions contained some degree of microbial-labile components that may not be well reflected using most measures of the initial DOM composition. Hence, the amount of DOM lost, degradation rate, and the uniqueness in the response of DOM at each location indicates a location-specific definition of DOM lability. Photolysis is an important degradation pathway for DOM in northern systems due to the prolific number of shallow, exposed surface waters characteristic of many arctic landscapes. However, few DOM photodegradation rates have been published from Canadian arctic freshwaters. The objective of this data chapter was to determine how differences in DOM composition influence photo-lability and photodegradation rate from arctic and sub-arctic surface and subsurface waters in Canada. Degradation rates, calculated from the total DOM loss, were highest in subsurface samples (linear: 2 to 25 x 10-3 m2/E; 1st-order: 4.1 to 17 x10-4 m2/E). Degradation rates were used to calculate total DOM loss after 500 E/m2 of photosynthetic active radiation (PAR) across all samples, equivalent to 18 and 13 days of sunlight in the high arctic and subarctic, respectively. Southern arctic subsurface and creek lost the highest amount of DOM (58 and 38%, respectively) while most other samples lost between 13 to 19%. The lowest proportion of DOM loss was observed from a taiga shield river (4%) likely due to pre-exposure among the landscape. No significant correlations were found between initial DOM composition and photolytic degradation rate. Alternatively, initial measures of SUVA and SAC420 predicted the proportion of photolytic DOM loss. Photolytic-induced changes to DOM were similar across all samples: decreased values of SUVA, DOC:DON, and SAC420, and increased values of S275-295. Hence, photolysis uniformly altered DOM regardless of initial composition or sample location. Specific measures of DOM composition, such as SUVA and SAC420, provide sensitive indicators of photolytic processes and can be used to estimate the degradation rate and proportion of DOM loss. Chlorine reacts with DOM to form harmful disinfection by products (DBP), yet the extent DOM forms DBP may depend upon the amount and composition of DOM. The objective of this data chapter was to determine how differences in DOM composition from sub-arctic freshwaters influences DBP formation. Samples were collected from Yellowknife, Wekweètì, and Daring Lake, NT, and a microbial and photolytic degradation experiment to understand how drivers of DOM fate influence DBP formation. Further, public water quality data records were used to determine the prevalence of DOM and DBP across NT water treatment records. DOM composition was characterized using overall concentration, SUVA, S275-295, DOC:DON, and size-exclusion chromatography determined fractions of humic substances (HSF). Concentrations of trihalomethanes (THM) and haloacetic acids (HAA) were measured with equivalent chlorine residuals 24 hours after chlorine addition. Public water quality records indicate both DBP and DOM were ubiquitous in NT water sources and generally below health guidelines. DOM composition plays an important role for disinfection demand as no strong relationship was found between chlorine demand and DOM concentration. Simple measures of DOM composition, such as SUVA and S275-295, resulted in stronger correlations with DBP concentration than overall DOM concentration. In particular, high molecular weight and aromatic humics, representative of terrestrial-like DOM sources, formed higher DBP concentrations. Microbial degradation led to higher DBP yields normalized to DOM mass while photolysis had little effect. We show various compositions of DOM from across the NT lead to different but predictable differences in DBP concentration. Data and interpretations from all chapters were brought together to form a conceptual diagram of DOM evolution in the NT. Different DOM compositions and changes related to microbial and photolytic degradation aided with categorization of different samples along a tri-axis plot of compositional end members: terrestrial, photolytic, and autochthonous. Not all DOM composition metrics respond the same way during degradation. Specific indicators of processes were identified from the degradation experiments: SUVA and SAC420 both responded oppositely to microbial and photolytic degradation, while S275-295 only increased for photolysis. Although these processes appeared to align with the variation observed among high arctic DOM composition and sources, notable differences in high arctic DOM composition demonstrate the need for revision to incorporate other high arctic sites to the current DOM conceptual diagram. The conceptual diagram identified zones of ‘labile’ DOM composition, as well as zones of ‘high-risk’ DOM that have the potential to form high concentrations of DBP. The conceptual diagram provides a framework to focus and continue developing the impact of various drivers on DOM quantity and quality under different climate scenarios.
Cite this version of the work
Pieter Jan Karel Aukes (2019). Dissolved Organic Matter in the Canadian Arctic & Sub-Arctic: Importance of DOM Quality & Quantity in a Warming Climate. UWSpace. http://hdl.handle.net/10012/14975