Paleolimnological analyses using diatoms preserved in a 34-cm sediment core from the presently eutrophic Collins Lake, southeastern Ontario, were used to determine the lake's natural trophic state, as well as the impacts of past watershed activities on water quality. The presence of eutrophic diatoms (especiallyFragilaria crotonensisandAulacoseira ambigua) and low chrysophyte cyst to diatom frustule ratios in the pre-setdement section of the sediment core suggested that the lake was naturally somewhat productive. Further eutrophication occurred, as demonstrated by relative increases in eutrophic diatom taxa such asAulacoseira granulata, F. crotonensis, andAsterionella formosa, with the onset of European settlement, deforestation of the catchment vegetation, and agricultural practices. Finally, in the early to mid-1970s, land-use changes from primarily rural to more residential and recreational resulted in further nutrient enrichment of Collins Lake, likely from increased point-source nutrient inputs from private septic systems and household phosphate-based fertilizers. Total phosphorus concentrations inferred using a quantitative diatom-based transfer function confirmed the above qualitative assessments of the recent trophic state changes of Collins Lake. However, the naturally-productive nature of Collins Lake, as inferred from this paleolimnological study, contradicts the results obtained from applying the Ontario Ministry of the Environment's Ontario Trophic Status Model (TSM), which hindcast oligotrophic conditions for the lake, prior to European settlement. These paleolimnological data have important implications for lake management issues.