COS 113-1 - Coping with stress: Freshwater salinization alters life history traits of zooplankton

Background/Question/Methods

Exposure to novel stressors can affect survival, growth, and reproduction leading to reduced population growth and stability. Theory suggests individuals who tolerate stressors put more energy into survival and less energy into growth and reproduction. We tested whether rising the salinity of fresh waters resulting from anthropogenic sources would lead to such tradeoffs in freshwater zooplankton and if salt type and concentration mattered. Additionally, tested how food availability altered the life-history response to salinization. We cultured Daphnia in solutions of either NaCl or CaCl₂ – two major salts contributing to freshwater salinization worldwide. After 13 – 17 generations of salt exposure, we conducted a life history assay to quantify the survival, growth, and reproduction of individuals. Our experiment utilized 18 treatments crossing NaCl and CaCl­₂ road salts with two salt concentrations, two food densities, and two population exposure histories (‘naïve’ – cultured without salt, ‘exposed’ – cultured with salts). Treatments were replicated ten times each for a total of 180 individual zooplankton that were monitored daily from birth to death for 1) lifespan, 2) maturation rate, 3) inter-brood timing, 4) average brood size, and 5) body size at death.

Results/Conclusions

Our results show that individuals from salt-exposed populations have different life history trait responses than naïve populations. Relative to the control, high CaCl­₂ concentrations caused naïve individuals to have 63% shorter lifespans, 43% longer brood intervals, and 40% smaller brood sizes. Exposed individuals in high CaCl­­₂ concentrations had 80% longer lifespans relative to their naïve counterparts. We identified that this increased lifespan came at a tradeoff of 15% longer brood intervals. The naïve individuals in high NaCl concentrations had a 36% decline in brood size and 30% longer time to maturation relative to the control. An exposure history to NaCl further affected life history with 24% shorter lifespans, 23% smaller broods, and 31% longer brood intervals relative to the naïve. We conclude that multigenerational exposure to salt can impact survival in both positive and negative ways, and these changes come at a cost to reproduction. Exposure history appears to result in both adaptive and maladaptive changes in life history traits. This study provides a mechanism for the drastic decline and slow recovery we observe in zooplankton populations exposed to elevated salinity among fresh waters.