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