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A palaeolimnological investigation of changes in water quantity and quality in Lake 239, Experimental Lakes Area, in northwestern Ontario indicates marked changes in limnological conditions during the Holocene. Water quantity changes are based on the analysis of diatoms and inferences of lake-level changes from a nearshore sediment core. Changes in water quality are based on the analysis of diatom assemblages and associated quantitative inferences of total phosphorus (TP) from a deep central core, as well as changes in diatom accumulation rates, the proportion of chrysophyte scales to diatoms and changes to organic matter content. Lake levels at least 8 m lower than today were inferred from the nearshore core and were concurrent with an increase in nutrient-rich diatom assemblages, an increase in diatom accumulation and a decrease in the chrysophytes relative to diatoms in the central core. Fragilaria crotonensis and Aulacoseira subarctica were two of the dominant nutrient-rich taxa that contributed to an increase in diatom-inferred TP during the mid Holocene. Results from this study provide strong empirical evidence that water levels much lower than today can dramatically change trophic status, as occurred during the dry mid Holocene, and may provide a good analogue for understanding future impacts of climatic changes due to anthropogenic activities.

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Charcoal accumulation rates and fire-return intervals were calculated from total charcoal and charcoal morphotypes over the Holocene, from a well-dated sediment core from Lake 239 located in north-western Ontario, and compared with previously published independent climate reconstructions. Both total and morphotype analysis show a two-to-three fold increase in accumulation rates in the early-to-mid Holocene (range: 1 to 6 pieces cm–2 year–1) compared with the early and late Holocene (range: 0 to 2 pieces cm–2 year–1). Fire-return intervals and fire frequencies calculated during these periods, based on peak analysis, showed very different trends. The fire-return interval based on Type M charcoal, a morphotype associated with primary charcoal deposition, was high during the early and late Holocene, and low from ~7500 to 4000 cal year BP, with high inferred fire frequency during the warm mid-Holocene (~12.5 fires per 1000 years), compared with <5 fires per 1000 years over the rest of the Holocene, whereas fire-return interval and fire frequency based on total charcoal did not show patterns consistent with climate. These results suggest that a two- to three-fold increase in fire frequency would not be unexpected in the future under a predicted warmer climate.