ABSTRACT: Mid-Holocene lake level decline and eutrophication in Rawson Lake (Lake 239), Experimental Lakes Area, northwestern Ontario.

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A paleolimnological study of lake-level changes and lake eutrophication in Rawson Lake (Lake 239), a headwater lake in the Experimental Lakes Area in northwestern Ontario indicates large fluctuations have occurred over the Holocene. Quantitative estimates of lake level from near-shore cores are based on a diatom-inferred depth model that was developed from surface sediments collected along several depth transects in Rawson Lake. Changes in lake trophic status 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. Lowering of lake level began in the early Holocene and during the mid-Holocene declined by at least 8 m. An 8-m decline would result in a 42 percent decline in surface area and 58 percent decline in the volume of Rawson Lake. Concurrent with the lake-level declines was an increase in inferred total phosphorus (TP) and an increase in diatom accumulation 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. These results provide the first substantive evidence of large declines in both water quantity and water quality in northwestern Ontario during the mid-Holocene. Present-day conditions of higher lake levels and dominance of the oligotrophic planktonic, Cyclotella stelligera in both the near-shore and central cores was established by around 3000 years ago. The transition of this site from the aridity of the mid-Holocene to more modern conditions occurred over a protracted period. Sites further to the west in the Canadian prairies appear to exhibit more abrupt and a later transition out of the arid mid-Holocene conditions. The variation in timing of the onset of modern lake conditions likely reflects the variability in geological and hydrological setting, as well as the geographic variance in timing of climatic change at specific locations.

Bibliography of Canadian Geomorphology