Anthropogenic disturbances are creating novel environments, threatening biodiversity and ecosystem services. Salinization of freshwater ecosystems from sources such as road salt application and agriculture is one disturbance of growing concern. Zooplankton, particularly Daphnia, are sensitive to salinity increases. Daphnia are essential, sentinel members of aquatic food webs, as they are important feeders on phytoplankton and a vital food source for some fish. Therefore, changes in Daphnia’s abundance or behavior, due to disturbances, could result in drastic consequences and induce trophic cascades. We investigated whether Daphnia pulex could adapt to increased levels of the most common road salt, NaCl. We then investigated subsequent consequences to the circadian rhythm from prolonged exposure to NaCl. First, we exposed Daphnia to five levels of NaCl (15-1000 mg Cl-/L) in a community experiment. Then, we collected Daphnia after 2.5 months and cultured them in the lab under low salt conditions. To test for evolved tolerance, we exposed the Daphnia populations to varying levels of NaCl (30-1900 mg Cl-/L) in a time-to-death experiment. To test for effects on the circadian rhythm, we completed dark experiments on all populations and used qrt-PCR analysis to measure mRNA expression levels of per, a core clock gene, over time.
Results/Conclusions
In the time-to-death experiment, all Daphnia populations demonstrated high survival when exposed to 30 mg Cl-/L. At moderate levels of road salt (1300 mg Cl-/L), populations previously exposed to increased concentrations (i.e., 100-1000 mg Cl-/L) had higher survival than control populations (15 mg Cl-/L). All populations survived poorly when exposed to high concentrations (≥1500 mg Cl-/L). Next, we found that per levels from Daphnia exposed to low-salt concentrations showed 20-hour oscillations. Populations adapted to moderate concentrations of NaCl (100 and 250 mg Cl−/L) showed little difference in the circadian cycle when compared to the control population. However, adaptation to high concentrations (500 and 1000 mg Cl−/L) ablated the circadian rhythm in per expression. Our results show that Daphnia can evolve tolerance to NaCl in just 2.5 months. Given Daphnia’s importance in freshwater ecosystems, evolved tolerance could result in their persistence and mitigate salinization effects. We also show that this evolved tolerance has dramatic effects on Daphnia’s circadian rhythm. Disruptions to the circadian rhythm may have far-reaching implications since they are key to many biological processes, potentially including diel vertical migration. Thus, alterations to Daphnia’s circadian rhythm could alter food webs, affecting biodiversity and important ecosystem services in the ever-changing world.