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 intraspecific variation in these responses remains poorly understood. In this study, we examined the intraspecific variation 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 intraspecific variation on 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 intraspecific diversity and 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.

Climate change is altering local temperatures and resource availability of many ecosystems. We explore the impacts of these changes on the metabolism of phytoplankton, organisms that are crucial as the base of aquatic food webs and engines of aquatic biogeochemical cycling. Specifically, we investigate the metabolic responses of six freshwater phytoplankton species---representing green algae, diatoms, and cyanobacteria---to warming in combination with light, phosphorus, and nitrogen, the three most critical resources for phytoplankton. Using direct-infusion mass spectrometry, a high-throughput metabolomics approach, we identify interactive effects of temperature and resource availability on phytoplankton metabolism. We detect thousands of metabolites involved in key pathways, including amino acid, carbohydrate, and lipid metabolism. Our results show that resource limitation had a stronger effect on metabolism than temperature across all species. While each resource induced distinct metabolic changes, nitrogen and phosphorus limitation triggered more similar responses, whereas light limitation resulted in a unique metabolic profile. We also identified a core set of metabolic pathways involved in all responses, alongside resource-specific pathways. These findings provide mechanistic insights into how phytoplankton metabolically respond to key environmental drivers, enhancing our understanding of their responses to future climate conditions.