Arcellaceans (thecamoebians) as indicators of land-use change: settlement history of the Swan Lake area, Ontario as a case study

Arcellaceans have been used as indicators of a variety of paleolimnological conditions including pH, eutrophication, oxygen level, and heavy metal contamination, but there has only been limited application of the group to climate and land-use change research. The limnological evolution of Swan Lake in southwestern Ontario was documented using arcellaceans as proxies, and compared to the results of a palynological analysis, with which it closely correlated. The palynological record documents the rapid disappearance of forest by around 1850 as the area was cleared for agriculture and settlements. The change was characterized by a shift in the palynological record from the Woodland to High Diversity assemblages. Similarly the arcellacean fauna changed from the Pre-European Settlement Assemblage to the distinctive low diversity, stressed environment European Deforestation Assemblage. The introduction of high-yield chemical fertilizers in the post World War II era, and the resultant eutrophication of the lake, was clearly recognizable in the palynological record as indicated by the dramatic increase in the algae Pediastrum (High Nutrient Assemblage). This change in farming practice was also identifiable with arcellacean proxies, as indicated by the appearance of the algalphilic Eutrophication Assemblage. The arcellacean Ecologically Destabilized Assemblage dominated the lake for short intervals during the transition between the Pre-European Settlement and European Deforestation assemblages and again at the transition between the European Deforestation and Eutrophication Assemblages, indicative of periods of ecological destabilization as the lake adjusted to new trophic inputs. A stressed environment arcellacean Ecologically Destabilized Assemblage found in a portion of the core estimated to have been deposited between AD 1350 and AD 1700 may provide evidence of an earlier phase of deforestation associated with Huron Indian agricultural practices. The close correlation between the palynological and arcellacean proxy data clearly demonstrates the potential of arcellaceans as land-use change proxies, and indicates that changes in land-use had an almost immediate impact on the Swan Lake ecosystem. This observation raises concerns about the impact that rapid urbanization is having on the environmentally sensitive Oak Ridges Moraine watershed, of which Swan Lake is a part.