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. Although 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-dependent growth in the freshwater diatom <i>Fragilaria crotonensis</i>. 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 (<i>P*</i>), half-saturation constants (<i>K</i> _s) and maximum growth rates (<i>μ</i> _max) for each strain as a function of temperature. We also fitted thermal performance curves and quantified activation energies (<i>E</i> _a) and cumulative performance across the thermal gradient as a function of phosphorus availability. We observed large variation among strains in the dependence of <i>P*</i> 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.