High-Latitude Responses of Diatom and Chironomid Assemblages to Regional Climate Changes and Other Environmental Stressors
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High-latitude freshwater ecosystems are amongst the most vulnerable to climate change impacts. However, the response to warming is rarely straightforward and may be modulated by local or regional factors, producing variability in biotic responses. This thesis examines three important drivers of local to regional-scale variability in the climate response of diatoms and chironomids, specifically: 1) the modulation of regional climate by the Wrangell-St. Elias ice fields over the Holocene in the Yukon; 2) the role of ice cover as the dominant driver of diatom assemblages in the High Arctic; and 3) the interaction between climate and nutrient dynamics at the archeological site Kukulek, Alaska. In the Yukon, paleolimnological records from three lakes at varying distances from the Wrangell-St. Elias ice fields were used to investigate the possible influence of large cryospheric features over the Holocene. The lake proximate to the ice fields showed a response to the Holocene Thermal Maximum, a warm period of similar magnitude to expected warming, suggesting that the moderating effect of the ice fields evident in recent records (spanning ~150 years) might deteriorate as the ice fields ablate. In the High Arctic, using a comparative study design we explicitly contrasted the diatom response to warming in four groups of sites characterized by different ice cover regimes. The diatoms shifted in a predictable manner from a low diversity, epipelic and epilithic assemblage, to a more diverse, largely epiphytic, assemblage likely in response to warming-associated changes in ice cover. At the Kukulek archeological site, marine-derived nutrients from historical hunting activities were traced by stable isotopes entering the adjacent pond, potentially tracing two periods of disturbance in the sediment record: the 1878-1880 AD famine and the archeological excavation in the 1930s. Warming temperatures, coincident with the late 19th century disturbance, may have compounded the fertilization of this pond as thawing permafrost released nutrients from the Kukulek site, affecting the diatom species composition. Collectively, this research increases our ability to effectively use diatom and chironomid assemblages as paleoecological proxies for climate and other environmental changes at high latitudes. Understanding context is key for improved paleoclimate reconstructions in this under-studied region.