PALEOLIMNOLOGICAL ASSESSMENT OF LONG-TERM WATER-QUALITY CHANGES IN SOUTH-CENTRAL ONTARIO LAKES AFFECTED BY COTTAGE DEVELOPMENT AND ACIDIFICATION

Sediment diatom assemblages were used to evaluate water-quality change s since preindustrial times in south-central Ontario lakes receiving a cidic deposition and moderate shoreline (mainly cottage) development. Canonical correspondence analysis identified significant relationships between surface sediment diatoms and environmental factors. Relations hips were sufficiently strong to develop weighted-averaging regression -calibration models for inferring lake water pH and total phosphorus c oncentration ([TP]) from diatoms. These models were accurate to within +/-0.21 pH units and +/-4.2 mu g TP . L(-1). Postindustrial pH and [T P] changes were inferred from surface and pre-1850 sediment diatom ass emblages. Results suggest that presently acid lakes (pH < 6) have acid ified, and the pH of alkaline lakes (pH > 7) has increased, as observe d in other regions receiving elevated acidic deposition. Diatom-inferr ed [TP] suggests that cultural eutrophication has not been widespread. In almost all lakes, present-day [TP] is not higher than before Europ ean settlement. In many mesotrophic lakes, preindustrial ITP] was high er than at present. Several factors could account for declining lake w ater [TP], including (i) lake and (or) watershed acidification process es that reduce P loading, or increase P loss rates; and (ii) reduction s in nutrient loading from watersheds as a result of reforestation.