Regional and fine-scale local adaptation in salinity tolerance in Daphnia inhabiting contrasting clusters of inland saline waters

Freshwater salinisation is an important threat to biodiversity, ecosystem
functioning, and the provision of ecosystem services. Therefore,
understanding the capacity of species to adapt to salinity gradients is
crucial. Clusters of naturally saline habitats represent ideal test cases
to study the extent and scale of local adaptation to salinisation. We
studied local genetic adaptation of the water flea Daphnia magna, a key
component of pond food webs, to salinity in two contrasting landscapes - a
dense cluster of sodic bomb crater ponds and a larger-scale cluster of
soda pans. We show regional differentiation in salinity tolerance
reflecting the higher salinity levels of soda pans versus bomb crater
ponds. We found local adaptation to differences in salinity levels at the
scale of tens of metres among bomb crater pond populations but not among
geographically more distant soda pan populations. The population-level
salinity tolerance range was reduced in more saline bomb crater ponds
through an upward shift of the minimum salt tolerance observed across
clones and a consequent gradual loss of less tolerant clones in a nested
pattern. Our results show genetic adaptation to salinity gradients at
different spatial scales and fine-tuned local adaptation in neighbouring
habitat patches in a natural landscape.