The freshwater diatom Asterionella formosa Hassall is a widely distributed species whose populations have been increasing in the recent sediments of numerous lakes worldwide. This taxon’s proliferation has commonly been associated with nutrient enrichment and, more recently, with twentieth century increases in atmospheric nitrogen deposition. However, a growing number of studies have reported increases in A. formosa in the absence of increased nutrients and have thus raised questions about whether alternative causative factors (such as recent climate warming) may be important. Here we assess the relative importance of nutrients and climate warming on A. formosa by comparing regional air temperature trends and 20 years of annually measured lake water nutrients (total inorganic nitrogen, total nitrogen, and total phosphorus) with past changes in relative abundance of this taxon from a sediment core obtained from a soft-water, Boreal Shield lake (George Lake, Killarney, Ontario). Measured lake water total phosphorus has always been very low and remains so to the present (<10 µg L−1). Total inorganic nitrogen (TIN) and total nitrogen (TN) concentrations, however, show distinct declining trends (p < 0.05) at George Lake since the 1990s, when substantial increases in mean annual and mean winter air temperatures (p < 0.01) were also recorded. These declines in lake water nitrogen concentrations match declines in atmospherically deposited nitrogen in this region. Since the late-1980s and early-1990s, there has been a marked increase in the relative abundance of A. formosa, which had been virtually absent from the lake’s sediment record in earlier intervals. A. formosa trends closely track increasing regional air temperatures, but also clearly coincide with declines in lake water nutrients and decreases in regional nitrogen deposition (p < 0.01). These trends have also occurred in other lakes in the region. Our results show that increases in nitrogen deposition and/or nutrient enrichment are not pre-conditions for increases in the relative abundance of A. formosa. Instead, warming-related changes including longer open water periods, changes in lake mixing regimes, and lake thermal properties might have contributed to the recent increase in the relative abundance of A. formosa in this lake and likely elsewhere.