Evidence of local adaptation in a freshwater diatom indicates higher sensitivity to nutrient limitation as water temperature rises

Climate change is reshaping freshwater ecosystems by warming waters and
modifying nutrient dynamics. These combined environmental changes exert
novel gradients of selection on phytoplankton populations and communities.
Temperature and phosphorus availability are individually critical
determinants of growth in phytoplankton, and can have interactive impacts
on population and community dynamics. While we understand how
interspecific variation in thermal and resource-use traits of
phytoplankton can affect community composition in response to changing
environments, the extent of local adaptation in these responses among
strains of the same species remains poorly understood. In this study, we
examined the local adaptation in the temperature- and
phosphorus-dependences of growth in the freshwater diatom Fragilaria
crotonensis. We predict that the growth of this diatom is locally adapted
to the environmental conditions of the lakes of origin. To test this, we
isolated strains from eight Swiss lakes (one strain per lake) with
distinct historical temperatures and nutrient status. We estimated the
growth rate of each strain under combined gradients of temperature and
phosphorus availability. We fitted Monod curves to the growth rate data,
and we quantified the minimal phosphorus requirements (P*),
half-saturation constants (Ks), and maximum growth rates (μmax) for each
strain as a function of temperature. We also fitted thermal performance
curves and quantified activation energies (Ea) and cumulative performance
across the thermal gradient as a function of phosphorus availability. We
observed large variation among strains in the dependency of P* on
experimental temperature, with strains from phosphorus-rich lakes showing
stronger increases in phosphorus requirements with warming. These patterns
imply local adaptation to phosphorus availability. Our findings highlight
a potentially critical role for local adaptation in shaping phytoplankton
responses to global change and call for a greater recognition of this
trait variation in making predictions of community-level responses to
future climate.