Ion availability varies considerably among lakes, owing to both natural variation and regional human impacts. Both low ion and high salt environments can cause osmoregulatory stress in freshwater taxa; however, calcium availability can mitigate the physiological challenges associated with both these extremes. The water flea Daphnia, an important zooplankton grazer in freshwater environments, is sensitive to changes in ion chemistry. Evidence of adaptation to high salt conditions and emerging evidence of intraspecies variation in tolerance of calcium limitation in Daphnia show that variation along these ion gradients can also drive natural selection.
We addressed the question of whether lake water ion chemistry interacts with local adaptation to ion availability in shaping toxicity responses to NaCl in Daphnia ambigua, a species commonly found in lakes throughout North America. We isolated Daphnia clonal lineages from three lakes that varied in overall salinity as well as calcium concentrations and performed a laboratory reciprocal transplant toxicity test to assess chronic NaCl toxicity.
Results/Conclusions
Water chemistry, population, and their interaction shaped Daphnia NaCl toxicity responses. In low-ion (conductivity: 60 uS/cm) high calcium (~5.5 mg/L) lake water, exposure to 825 mg/L NaCl did not impact survival (p >0.999), and reproduction decreased a modest 30% (p< 0.001). In contrast, survival decreased (p< 0.001) and reproduction ceased in a low-ion/low-calcium environment (conductivity: 20 uS/cm; Ca2+: 1.8 mg/L). Lake of origin impacted survival in the low-calcium environment, with Daphnia from the high-calcium lake 3x less likely to survive NaCl exposure compared with animals from the low-calcium lake. Meanwhile, Daphnia from a high-salinity lake (conductivity: 500 uS/cm; Ca2+=2.2 mg/L) were more tolerant of NaCl compared with Daphnia from the low-ion/low-calcium lake, when considering the population x environment x NaCl treatment interaction (p=0.002). Overall, this supports that 1) calcium limitation greatly increases NaCl toxicity and 2) local adaptation to naturally high ion conditions can slightly reduce the toxicity of added NaCl.
These findings have implications for toxicity risk assessment and conservation planning to protect freshwater habitats from salt pollution. Toxicity tests are often conducted with high-calcium medium, which may underestimate NaCl toxicity in Daphnia. Also, low-calcium lake environments are likely to be more strongly impacted by severe salt pollution.
