A geochemical perspective on the impact of development at Alta Lake, British Columbia, Canada

Small, temperate lakes are important to communities across Canada, but commonly lack the long-term water quality records that are required to assess the impact of development and environmental change. We measured a combination of bulk sediment geochemistry and stable isotopes of carbon and nitrogen in a 210Pb- and 14C-dated sediment core from Alta Lake, Whistler, British Columbia. Channel avulsion at AD 1754 ± 81 on the 21-Mile Creek alluvial fan resulted in increased deposition of organic matter with a signal of autochthonous production. A decrease in δ13C beginning at AD 1949 ± 17 indicates an increase in pelagic productivity relative to benthic productivity and/or sewage input to Alta Lake. A corresponding increase in δ15N indicates an increase in sewage-derived nutrients during the same time period. These changes were synchronous and coincide with the onset of rapid development in the Whistler Valley. Increased metal deposition (Cu, As, and Zn) during the same period resulted from watershed disturbance during highway construction. Cu, As, and Zn concentrations correlated positively with C concentrations and negatively with Ti and K, suggesting removal of metals from the water column is closely related to autochthonous organic matter deposition. Fe/Mn ratios and Mo concentrations were expected to respond to hypolimnetic oxygen availability in response to decreased flushing rate and increased productivity following the loss of inflow from 21-Mile Creek, but did not display a clear time-stratigraphic signal. Collectively, these data suggest that the combination of bulk geochemistry and stable isotopes was an effective suite of variables for inferring environmental change and the effects of development on Alta Lake.