HUMAN IMPACT AT CRAWFORD LAKE, ONTARIO, CANADA: THE SEDIMENT RECORD OVER THE LAST MILLENNIUM

Here we present results from a varved 80-cm sediment sequence from Crawford Lake (Ontario, CA). High-resolution sampling, multi-proxy analyses and an accurate age model, based on varve counts and over 20 AMS 14C dates, allow us to document the ecosystem response to regional climate change and human impact periods over the last 1000 yrs. Pollen analysis records farming periods, forest succession and climate change.

Poaceae, Zea and Ustilago distinguish farming periods. The Iroquoian Indian period, 1274-1516, also has domestic Helianthus and Cucurbita and weedy Portulaca; eight Iroquoian villages are within two kilometers of the lake. The charcoal-laden sediment of the Canadian period, 1820-2000, features introduced Rumex acetocella and native Ambrosia. Forest biomass reconstructed with R-values shows succession of Acer-Fagus deciduous forest to Acer-Quercus forest during Iroquoian period and since 1650 mixed forest of Acer, Quercus, Pinus and Tsuga. In the Canadian period, Betula and Ulmus flourished after logging. Since 1960, Ulmus declined due to disease. Transfer functions indicate temperature and precipitation peaked during the Iroquoian period. O-isotope analyses of inorganic CaCO3, a proxy for temperature, confirm a cooling trend from 1500-1900.

Sedimentation rates and C-isotope values increased dramatically at the initiation of the Iroquois period - and then remain high - indicating prolonged ecosystem impact. In the Canadian period, organic C increased, while CaCO3 and its C-isotope values decreased.

Diatom assemblages shift during both periods. Stephanodiscus peaks at the start of the Iroquois period. Assemblages were more complex in the Canadian period where initial peaks of Cyclotella michiganiana and Synedra nana gave way since 1970 to Asterionella formosa and a return of Cyclotella bodanica v. lemanica, a species largely absent since the Iroquois period. Overall diatom abundance and rotifer abundance (five species) peak during both periods of human disturbance, indicating enhanced nutrients. Interestingly, there is a lag between diatom abundance and sedimentological changes at the start of Canadian period, indicating that lakes having experienced a degree of cultural modification may be less susceptible to further anthropogenic perturbations than pristine environments.