Paleolimnological assessment of long-term water-quality changes in south-central Ontario lakes affected by cottage development and acidification

Abstract

Sediment diatom assemblages were used to evaluate water-quality changes since preindustrial times in south-central Ontario lakes receiving acidic deposition and moderate shoreline (mainly cottage) development. Canonical correspondence analysis identified significant relationships between surface sediment diatoms and environmental factors. Relationships were sufficiently strong to develop weighted-averaging regression-calibration models for inferring lake water pH and total phosphorus concentration ((TP)) from diatoms. These models were accurate to within ±0.21 pH units and ±4.2 µg TP·L -1 . Postindustrial pH and (TP) changes were inferred from surface and pre-1850 sediment diatom assemblages. Results suggest that presently acid lakes (pH 7) has increased, as observed in other regions receiving elevated acidic deposition. Diatom-inferred (TP) suggests that cultural eutrophication has not been widespread. In almost all lakes, present-day (TP) is not higher than before European settlement. In many mesotrophic lakes, preindustrial (TP) was higher than at present. Several factors could account for declining lake water (TP), including (i) lake and (or) watershed acidification processes that reduce P loading, or increase P loss rates; and (ii) reductions in nutrient loading from watersheds as a result of reforestation. Resume : Nous nous sommes servis des assemblages de diatomees dans les sediments pour evaluer les changements de la qualite de l'eau, depuis la periode preindustrielle, dans des lacs du centre-sud de l'Ontario qui ont connu des depots acides et un developpement du littoral modere (principalement construction de chalets). L'analyse de correspondance canonique a releve des relations significatives entre la presence des diatomees dans les sediments superficiels et les facteurs environnementaux. Les relations etaient suffisamment fortes pour que nous elaborions des modeles de regression-calibration a moyenne ponderee permettant de calculer le pH de l'eau du lac et la concentration de phosphore total ((PT)) a partir des diatomees. Ces modeles etaient exacts jusqu'a ±0,21 unite de pH et ±4,2 µg PT·L -1 . Nous avons