Paleolimnological approaches were used to assess the ecological and environmental implications of diatom assemblage compositional changes recorded over the last ∼ 200 yr from four sites in the Lake of the Woods (LoW), Ontario, Canada. Comparisons between a reference site (Whitefish Bay) and three disturbed sites (Bigstone Bay, Paleolimnological Project site No. 1, and Forrest Island) provide insights into the effects that multiple stressors (dam construction, total phosphorus [TP] changes, and recent warming) have had on the ecology of this highly complex freshwater system. Overall patterns of diatom compositional changes from highresolution 210Pb-dated sediment cores revealed a strong temporal coherence ca. 1910 and then again over the last few decades among all sites. Hydromanagement activities at the turn of the 19th century and recent warming over the last few decades played key roles in the LoW diatom changes. Diatom compositional changes at all sites were significantly related to trends in nearby air temperature records over the past century and to changes in lake ice phenology over the past ∼ 40 yr from Whitefish Bay. Turn-of-the-century hydromanagement activities do not appear to have had long-term effects on diatom-inferred TP (DI-TP), particularly at the disturbed sites. Clear decreases in DI-TP over the last few decades were evident at all sites, but at the reference site these decreases were well below pre-disturbance levels, which we link in part to recent warming. Substantial changes in climate will amplify the effects of multiple stressors on lake water properties that have major implications on algal communities. © 2010, by the American Society of Limnology and Oceanography, Inc.