We employed zooplankton resting eggs to track population-level shifts in acid
tolerance over the last century in a boreal shield lake recovering from acidification. Shifts
in mean and variance of ecological tolerances of species that occupy ecosystems recovering
from anthropogenic stress are important to consider because of their potential to
influence eco-evolutionary dynamics at community and ecosystem levels. In a laboratory
experiment, we compared juvenile survival of Leptodiaptomus minutus copepods hatched
from resting eggs from three time periods (80- to 100-year- old: pre-industrial, 20- to 50-
year- old: lake acidification, and 8- to 10-year- old: recovery of lake-water back to
pH C 6.0) under several pH treatments. Mitochondrial DNA was used to confirm species
identity and to test for population bottlenecks as a possible mechanism to explain our
results. We expected that nauplii hatched from eggs deposited prior to industrialization
(lake-water pH & 6.0) and from the period of pH recovery (lake-water pH C 6.0) would
have lower mean and more variable survival at acidic pH compared to nauplii hatched
from the period of peak lake acidification (lake-water pH & 4.7). Our results, which are
likely a combination of both genetic and environmental effects, suggest support for this
hypothesis. Nauplii hatched from eggs deposited during the period of acidification in
George Lake had reduced variation in pH tolerance compared to the recovery period. This
was likely driven by strong selection rather than genetic drift because we found no evidence
of a population bottleneck. However, we could not detect differences in the variance
of naupliar survival between pre-industrial and acidic time periods. Trends in mean
naupliar survival from different time periods matched findings from other field-based
studies that detected a relationship between lake acidification history and acid tolerance in
L. minutus.