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Paleolimnological Reconstruction of Long-Term Trends in Phototrophic Communities in Prairie Reservoirs



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Anecdotal evidence suggested that Lake Diefenbaker, a large river-valley reservoir in Southern Saskatchewan, Canada, has been experiencing increased pelagic algal bloom frequency and intensity in recent years. This has raised concerns regarding possible deteriorating water quality, including taste and odour issues, and the potential production of harmful cyanobacterial toxins. Due to limited historical environmental monitoring data, paleolimnological investigations of Lake Diefenbaker sediments were conducted to assess spatial and temporal environmental trends. Lines of investigation included trends in organic input and changes within the phototrophic (algal) community, with a primary focus on cyanobacteria and associated toxins. Sediment cores were collected from Lake Diefenbaker (the test case site) and two smaller Prairie lakes, Buffalo Pound Lake, Saskatchewan (eutrophic lake with recurring cyanobacterial blooms) and Ross Lake, Manitoba (historically known to have inputs of raw sewage), both serving as positive reference sites for method development purposes (extraction of environment DNA and fecal sterols, respectively). These sediment cores were sectioned at 1-cm increments and analyses were conducted for the presence of fecal sterols, sedimentary algal pigments, algal biotoxins and preserved environmental DNA. Inference of historical changes in fecal pollution, algal and cyanobacterial community composition and toxin synthesis genes were completed using a variety of analytical methodologies (e.g. high performance liquid chromatography, high-resolution gas chromatography mass spectroscopy, and Q-exactive Orbitrap liquid chromatography mass spectroscopy) and next-generation sequencing techniques. In Ross Lake, trends in raw and primary treated sewage was reconstructed through the analyses of coprostanol and cholesterol. Concentrations of coprostanol increased from <1 µg g-1 in older sediments, to 252.3 µg g-1 organic carbon at the peak (approximately in the early 1930s). Furthermore, ratios of coprostanol to cholesterol >1, peaking at 3.6 were consistent with anecdotal information that municipal sewage was discharged into Ross Lake during the early years of urbanization, prior to changes in treatment of sewage and discharge practices that began in 1951. These techniques were then applied to Lake Diefenbaker sediment cores to investigate the presence of sewage input. For Buffalo Pound Lake, high-throughput next-generation sequencing techniques were used to reconstruct and identify the presence of potentially harmful cyanobacteria, including Microcystis, Dolichospermum, and Planktothrix. Furthermore, the abundance of the microcystin synethetase A gene confirmed the presence of potentially toxic cyanobacteria and that the genetic potential to produce microcystins has been present since reservoir formation. These findings demonstrate a novel means to infer long-term dynamics of the cyanobacterial community in inland waters and highlights the power of paleo-16S-high-throughput sequencing to rapidly identify problematic organisms with high resolution. These techniques were also applied on Lake Diefenbaker sediments. In Lake Diefenbaker, sedimentary pigment analyses combined with other lines of evidence (e.g. diatom remains and physicochemical parameters) suggested spatial and temporal trends in reservoir ecology. Distinct ecological regions of Lake Diefenbaker were identified, likely due to differences in the morphology and hydrology which exist along the longitudinal axis of this, and other similar river-valley reservoirs. Sediments from up-reservoir locations suggested relatively consistent primary production, nutrient loading and trophic status throughout the temporal coverage of the collected sediment cores. In general, primary productivity increases with distance down-reservoir, until available nutrients are depleted in the mid-reservoir region of Lake Diefenbaker. In addition, sediments from the furthest down-reservoir locations suggest increasing primary production over the last two decades, particularly in the Qu'Appelle arm. Reconstruction of the cyanobacterial community using next-generation sequencing revealed the presence of potentially problematic cyanobacteria (e.g. Dolichospermum, Microcystis and Planktothrix) in the Qu'Appelle and Gardiner arms, taxa capable of producing toxins and taste-and-odour compounds. The continued presence of these nuisance cyanobacterial taxa and the increasing relative abundance of the mcyA gene in Lake Diefenbaker suggested that the genetic potential to produce microcystins have been present since formation of the reservoir, and that this potential has varied over time. Altogether, the combination of traditional analytical methodologies and next-generation sequencing techniques represents a novel approach to identify relationships between environmental conditions and cyanobacterial communities in freshwater ecosystems. Further, the results detailed in this thesis identify the practicality of utilizing paleolimnological approaches to reconstruct historical environmental trends in spatially-complex aquatic systems where long-term monitoring data are lacking or absent.



Paleolimnology, cyanobacteria, next-generation sequencing, algae



Doctor of Philosophy (Ph.D.)


Toxicology Centre




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