Seasonal variation in thermal plasticity of an alpine lake Daphnia population

Background: Temperature changes dramatically throughout the growing season in temperate latitudes, and seasonal changes in temperature are especially pronounced in alpine lakes where water stratifies into distinct thermal layers during summer. Hypothesis: Populations are expected to maintain a greater degree of plasticity in more heterogeneous environments, such as when lake stratification occurs. Organism: We studied seasonal variation in plasticity of a population of Daphnia, a key grazer in alpine lakes. Methods: We isolated maternal lines of Daphnia pulicaria from Blue Lake (Sierra Nevada, CA) at four different times throughout the growing season, then measured phenotypic traits and survivorship after individuals were reared at two temperatures (17°C and 24°C). Results: We found mixed evidence for the role of thermal variation in maintaining plasticity. Thermal plasticity for offspring number and age at maturity varied seasonally; however, inconsistent with our hypothesis, neither response was related to stratification. Similarly, we observed lower plasticity for the clutch interval when the lake experienced peak thermal stratification in mid-summer compared with early-fall conditions. In contrast, plasticity for critical maximum temperature (CTmax) was highest during peak stratification. As CTmax is a direct measurement of upper thermal limits, it should be related to maximum temperature experienced in the water column. Thus, this result is consistent with a positive correlation between thermal variation and plasticity. Conclusion: Our results suggest that the degree of plasticity in response to temperature varies throughout the season in relation to thermal stratification, with different life-history traits showing distinct seasonal patterns of plasticity.