Lake water chemistry and population of origin interact to shape fecundity and growth in Daphnia ambigua

Freshwater environments vary widely in ion availability, owing to both
natural and anthropogenic drivers. Field and laboratory work point to the
importance of overall salinity as well as cation depletion, in shaping the
physiology, behavior, and ecology of freshwater taxa. Yet, we currently
have a poor understanding of the degree to which populations may vary in
response to ion availability. Using Daphnia collected from three lakes
that differ greatly in salinity and calcium availability, we conducted a
laboratory reciprocal transplant experiment to assess how animals
representing these populations vary in fecundity, body size, and survival
when reared in lake water from each environment. The lake water
environment and population of origin strongly interacted to shape Daphnia
growth and reproduction. Surprisingly, we found only modest evidence that
lake water with abundant calcium (5.5 mg/L vs. 1.2-2.3 mg/L) increased
Daphnia growth or reproduction. In contrast, water from a relatively
ion-rich lake (400 µS/cm specific conductance) strongly boosted Daphnia
fecundity over lower-ion lake water (20-50 µS/cm), especially for the
population originating from the high-ion environment. Our results suggest
that ion-poor conditions common in regions around the world may exert
stress on freshwater organisms, even for populations inhabiting these
environments. Meanwhile, moderate salt enrichment may not prove harmful
but could even benefit freshwater taxa in these ion-poor regions. The
context dependence of how and when lake water chemistry affects Daphnia
and other freshwater taxa deserves greater attention, in both ion-depleted
and ion-rich conditions. Daphnia are key members of lake food webs and
serve as an important model for ecology, evolution, and toxicology
research. Consideration of how lake water chemistry may influence how
Daphnia populations respond to abiotic and biotic stress may improve the
ability to evaluate and predict ecological and evolutionary dynamics in
lakes of varying chemical composition.