Nutrient‐dependent thermal response in growth and stoichiometry of Antarctic phytoplankton

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Abstract Rising temperatures and altered nutrient supply are expected to influence future phytoplankton thermal performance and community dynamics. The thermal response of phytoplankton has been shown to be influenced by nutrient availability, but information is still limited, particularly for the coastal Southern Ocean. Additionally, environmental interactions are often tested with single species but rarely address comparisons between populations and communities. To fill this gap, we experimentally investigated the thermal response in maximum growth rate (μmax), carrying capacity (K) and stoichiometry of three Antarctic phytoplankton species (Chaetoceros simplex, Geminigera cryophila, and Phaeocystis antarctica) in interaction with three nitrogen : phosphorus (N : P) ratios and identified the thermal traits of each species in monocultures and a mixed assemblage. Phytoplankton growth rate (μmax) showed a typical unimodal response to temperature with a thermal optimum around 4°C and significantly higher μmax with nutrient addition. The strongest temperature sensitivity to nutrient supply was observed around Topt and an ambient N : P supply ratio. Thermal traits, however, showed only minor changes with nutrient supply, indicating saturated nutrient concentrations. Nutrient addition significantly increased K while the thermal response of K was constant. Phytoplankton N : P was species‐specific and significantly altered by the temperature × nutrient interaction. Species‐specific growth performance varied whether the species grew in monocultures or in communities, demonstrating that monoculture thermal performance does not necessarily predict species thermal responses in multispecies assemblages. Overall, the results indicate that increasing temperatures will push Antarctic phytoplankton closer to their thermal optimum, altering phytoplankton growth, stoichiometry, and species interactions.