The timing of spatial variability alters movement patterns and population dynamics in Chamydomonas reinhardtii

Background/Question/Methods

While the vast majority of environments exhibit temperature changes across space (i.e., thermal spatial heterogeneity), various possibilities exist for how this spatial variation unfolds across time. An environment’s degree of spatial variation has well-appreciated impacts on population, predator-prey, and community diversity dynamics, among others. Likewise, temporal variation in environmental conditions has multiscale consequences, including those driven by physiological and evolutionary mechanisms. Research conducted in natural environments unavoidably encompasses consequences of both spatial and temporal variation, but the interactions between these forms of variation are rarely explicitly considered. Here, we experimentally generate temporally fluctuating thermal environments that have either a negative, positive, or neutral relationship between the mean spatial environmental temperature and the degree of thermal spatial heterogeneity. Daily mean temperature and mean spatial variation are held constant between the three environments. We test the hypothesis that the timing of spatial variation relative to diel temperature cycles can meaningfully alter movement patterns and population dynamics in the motile green algae Chlamydomonas reinhardtii. We predict that a positive relationship between thermal spatial variation and mean temperature will promote faster growth rates and be associated with a greater amount of individual movement, relative to when the two variables are neutrally or negatively related.

Results/Conclusions

Characterizing environments with differing relationships between mean spatial temperature and spatial heterogeneity is not feasible using common summary statistics such as mean temperature or mean spatial variability. Despite this, our results indicate that changing the timing of spatial heterogeneity across treatments had meaningful consequences for the movement and dynamics of C. reinhardtii populations. Contrary to initial expectations, C. reinhardtii growing in environments with positive relationships between spatial variability and mean temperature showed reduced population growth rates, more directed movement as indicated by a reduced absolute turning angle, and decreased negative thermotaxis over time, when compared to the other treatments. We additionally document substantial local variation in the dynamics of natural spatial variability by collecting summer water temperature measurements from twelve ponds in the Mount Saint Helens watershed, WA, USA. Our results collectively suggest that the timing, in addition to the magnitude or configuration of spatial heterogeneity, is an underappreciated but salient feature within the broader fabric of spatiotemporal variation.