A two-part paleolimnological study was conducted to: i) understand the important factors that are related to the present-day distribution of scaled-chrysophytes; ii) investigate changes in the scaled-chrysophyte assemblages in the Experimental Lakes Area northwestern Ontario, in response to recent climate warming. Scaled-chrysophyte assemblages were analyzed in the modern sediments of 40 ELA lakes to determine their relationship to measured environmental variables. The sediment record from 210Pb dated cores from six ELA lakes were analyzed at a sub-decadal resolution to evaluate if chrysophytes were changing in a consistent fashion and if these changes could be accounted for by measured climatic factors. Ordination analysis of the modern chrysophyte flora was significantly related to pH, lake depth, and the degree of thermal stratification, as well as water temperature. Mallomonas punctifera ‘small’ and Mallomonas acaroides were indicators of warm surface-waters. High-resolution analysis of six ELA lakes revealed pronounced shifts in the chrysophyte assemblages over the last ca. 150 years. The most notable shift in the chrysophyte assemblage was characterized by an overall shift towards higher relative abundances of colonial taxa. In several lakes increases in unicellular warm-water taxa were also observed. Breakpoint Analysis identified significant changes in the chrysophyte assemblages beginning in the late-1800s to mid-1900s in most lakes. An interclass correlation coefficient (ri) was used to assess the temporal coherency of the chrysophyte assemblages over the past ca. 100 years. All lakes displayed a similar directional change which was significantly coherent (p<0.05). A Brien’s Test identified sub-sets of lakes that were temporally coherent and homogenous. The high coherency of two groups, (Group A, grand mean=0.89, p-value=5.3x10-15; Group B, grand mean=0.38, p-value=0.038), suggests the dominance in extrinsic factors in governing the lake responses. The average PCA axis-1 scores of Group A (r-value=0.62, p=0.03) and Group B (r-value=0.60, p=0.038) were significantly correlated to regional mean annual temperature. Collectively, the results of this study suggest that changes observed in the scaled-chrysophyte assemblages in the ELA region are consistent with recent climate warming. / Thesis (Master, Biology) -- Queen's University, 2011-09-29 16:42:06.753