Global climate change is threatening both our water quality and quantity. Specifically, the influence of climate change on freshwater lakes includes decreased water availability, increased evapotranspiration, changes in nutrient availability, and shifts in species composition. Understanding the changes that are occurring to our freshwater ecosystems is imperative to understanding the full impact of climate change on both the environment and society. This thesis examines a shift in sedimentary diatom assemblages towards increased relative abundances of Discostella species, a phenomenon that has been documented across the Northern Hemisphere. One of the central tenants of this work is that it has been done at the Experimental Lakes Area (ELA) in the boreal region of Canada, a study site that is uniquely devoid of large-scale anthropogenic disturbances. An analysis of the long-term monitoring records at the ELA show that Discostella species are primarily spring bloomers, and recent increases in their abundance is linked to changes in spring thermal conditions. To link the changes recorded in the sedimentary records of eight lakes to climate change, and assess whether Discostella species are in fact showing a response to climate change, a novel approach utilizing the theory of temporal synchrony was applied to eight paleo records. Discostella stelligera was found to be synchronous in all eight lakes studied, suggesting that a broad-scale forcing factor is influencing its abundance. Further, it was significantly correlated with annual and winter temperatures, supporting a link to changes in spring thermal conditions as a possible explanation. To assess the relative sensitivity of the study lakes, particularly in comparison to other boreal and temperate region lakes, an analysis was conducted of the timing of the first change in Discostella species. Several statistical techniques were employed and all approaches showed that the study lakes from ELA responded earlier in comparison to other lakes studied from other boreal and temperate regions (ca. 1900 vs ca. 1970), but later than Arctic and Sub-Arctic lakes. We suggest that this may be due to the lack of local disturbances at the ELA and/or the comparably small size of our study lakes. / Thesis (Ph.D, Biology) -- Queen's University, 2014-06-30 15:03:04.447

The vast majority of Adirondack lakes have been impacted since pre-industrial time through either regional (e.g. acidification) or local (e.g. fish stocking, liming, seepage of road salt, cultural eutrophication) environmental disturbances. Inferring changes related to climate are difficult due to the possible complexities associated with multiple stressors. Thirty minimally-disturbed ‘reference’ lakes were identified from a database of 1469 lakes using the following selection criteria: <5% shoreline development; a circumneutral pH; and no records of introduction of non-native piscivores (protecting against eutrophication, chronic lake acidification and modifications of trophic structure, respectively). As such, these ‘reference’ lakes may be useful for isolating the effects of regional changes, including climate warming, from other anthropogenic influences. A ‘top-bottom’ paleolimnological analysis of diatom assemblages was undertaken to identify changes since pre-industrial times. The uppermost sediment layer was used to represent present-day diatom assemblages, and a sediment sample from a depth of 20cm was used to represent pre-industrial conditions. Discostella stelligera, a planktonic diatom taxon, is presently the dominant species in 50% of the lakes, and showed the greatest change. D. stelligera increased by >10% in 11 of the 30 study lakes. The measured present-day chemical and physical limnological variables in the study lakes could not predict changes in the abundance of D. stelligera, suggesting that unmeasured variables, such as decreased ice cover, and changes in mixing and stratification patterns, are likely driving the observed changes in diatom assemblages. Heavily silicified tychoplanktonic Aulacoseira species declined by an average of 8.3% across the dataset. In three of the relatively deep lakes with higher concentrations of phosphorus, another planktonic diatom, A. formosa, was found to increase in abundance. Additionally, some lakes that were dominated by a high proportion of benthic taxa experienced increases in the relative abundance of benthic taxa over time. Observed changes in the relative abundances since pre-industrial times are significantly higher than the variation between replicate cores, and between two ‘pre-industrial’ sediment intervals. Further studies on the timing of changes in diatom assemblages are necessary to confirm that such changes are consistent with recent changes in climate in the Adirondacks. / Thesis (Master, Biology) -- Queen's University, 2014-05-29 21:33:42.24