Lake acidification can strongly impact the structure and function of lake ecosystems, causing extirpation of some species while other organisms are able adapt to changing pH. We followed the genetics of a population of white sucker (Catostomus commersonii) before (1975), during (1976-1983), and after (1984-1994) experimental whole-lake acidification in the Experimental Lakes Area, Ontario Canada. During this time, pH changed from 6.8 to 5.0, causing recruitment failures from 1981-1985. We hypothesized that the experimental acidification created a genetic bottleneck, reducing the genetic diversity and thus adaptive potential of succeeding generations. To investigate, we extracted DNA from archived pectoral fin-rays fixed in epoxy that had been used to non-destructively age fish samples. The allelic composition of 12 highly variable microsatellite loci was evaluated in relation to year class for both the acidified lake (Lake 223) and a nearby control lake (Lake 260).
Results/Conclusions
White sucker in both the acidified and control lakes had remarkably high genetic diversity throughout the study period. No evidence was found for a reduction in genetic diversity following acidification. In fact, average heterozygosity per year class increased from 0.70 to 0.90 following lake acidification. Post-acidification year classes were genetically differentiated from pre-acidification year classes, and accounted for 9.5% of total molecular variance. The results are incompatible with a genetic bottleneck event, and suggest immigration from a genetically differentiated source coincided with the start of acidification. This is surprising because the source population for these alleles is unclear and because pH changed slowly for the first few years of acidification. Our findings reveal a previously unknown consequence of a classic lake acidification experiment and suggest a role for compensatory immigration in sustaining fish populations impacted by severe environmental stress.