Rapid eco-evolutionary responses in perturbed phytoplankton communities.

Biodiversity currently faces unprecedented threats owing to species extinctions. Ecologically, compensatory dynamics can ensure stable community biomass following perturbation. However, whether there is a contribution of genetic diversity to community responses is an outstanding question. To date, the contribution of evolutionary processes through genotype shifts has not been assessed in naturally co-occurring multi-species communities in the field. We examined the mechanisms contributing to the response of a lake phytoplankton community exposed to either a press or pulse acidification perturbation in lake mesocosms. To assess community shifts in the ecological response of morphospecies, we identified taxa microscopically. We also assessed genotype shifts by sequencing the ITS2 region of ribosomal DNA. We observed ecological and genetic contributions to community responses. The ecological response was attributed to compensatory morphospecies dynamics and occurred primarily in the Pulse perturbation treatment. In the Press treatments, in addition to compensatory dynamics, we observed evidence for genotype selection in two species of chlorophytes, Desmodesmus cuneatus and an unidentified Chlamydomonas. Our study demonstrates that while genotype selection may be rare, it is detectable and occurs especially when new environmental conditions are maintained for long enough to force selection processes on standing variation.