Fifty Shades of Brown: Variability of Dissolved Organic Matter in Forested Streams across Spatial and Temporal Scales
Creators: Orlova, Julia
Publisher: name: University of Alberta Library
Year: 2024

<p>Dissolved organic matter (DOM) in stream water plays a critical role in shaping aquatic ecosystems, influencing water quality and acting as a food source to microorganisms, and affects drinking water treatment. Understanding variations in the concentration and composition of DOM and environmental controls on DOM is crucial for predicting the effects of climate change and land use on changes in stream DOM. Yet, with so many factors potentially affecting DOM and vast differences among forested regions, our understanding of DOM in streams across the boreal region is incomplete. The three studies (Chapters 2-4) in this thesis were designed to further our understanding of variations in and controls on stream DOM across spatial and temporal scales of heterogeneous forested regions.<br>
Chapter 2 examines stream DOM at a sub-continental scale. Using a combination of analytical techniques, including absorbance and fluorescence spectroscopy, liquid chromatography, mass spectrometry, and flow field-flow fractionation, we analyzed DOM composition in 52 streams across six forested ecozones in Canada. I discovered three primary axes of variation in DOM composition: DOM aromaticity, driven by the abundance of wetlands; DOM oxygenation, related to variations in climate; and biopolymer content, linked to the presence of lakes in catchments. Although DOM composition varied greatly seasonally and among the streams within the same ecozone, inter-site variability in DOM composition was often greater than intra-site and temporal variations, suggesting the profound influence of landscape and climatic characteristics.<br>
Chapter 3 focuses on a single ecozone, the Boreal Plains, which is differentiated from other boreal regions by its flat terrain, subhumid climate and heterogeneity in glacial deposits resulting in complexity of surface water – groundwater interactions and variable hydrologic connectivity of terrestrial sources to streams. This chapter examined DOM concentration and composition in 17 forested streams. Spatial variations in stream DOM at the regional scale arose from the presence of lakes, as well as coarse-grained surficial geology, associated with reduced DOM concentration and aromaticity. Temporally, distinct trends emerged, including increases in DOM throughout the summer season, likely tied to soil warming that promotes decomposition of organic matter, and short-term dilution or flushing related to storm events, and declines during droughts. The results suggested that even at the regional scale, variations in stream DOM concentration and composition were substantial and could be predicted based on catchment characteristics, hydrology and season.<br>
Finally, Chapter 4 shifts the focus from terrestrial sources of DOM to aquatic processes that transform DOM – photodegradation and biodegradation. Changes in DOM concentration and composition due to these processes were studied in water from five Boreal Plains streams using laboratory incubations. Photodegradation was particularly influential, aligning stream samples more closely with lake DOM composition, revealing the importance of this process for DOM transformation in the Boreal Plains streams, as well as emphasizing the importance of lakes in this landscape.<br>
Collectively, these studies underscore the large variability in stream DOM at sub-continental and regional scales and contribute valuable insights into the controls of stream DOM composition. At all scales, they highlight the importance of lakes and wetlands in a catchment, and temperature. The differences in terrestrial sources of DOM and in-stream processing have implications for carbon cycling, downstream water quality and treatability. </p>