Abstract
Marine ecosystems are facing multiple anthropogenic global changes, including ocean acidification, warming, and reduced nutrient supplies. Together, these will challenge phytoplankton including large centric diatoms such as Coscinodiscus sp., a group that is important to ocean food webs and carbon export. We investigated the interactive effects of warming, elevated CO 2 , and nitrate availability on Coscinodiscus growth, elemental stoichiometry, and Fe and C uptake rates in a four‐treatment factorial experiment combining two CO 2 levels (∼400 ppm and 800 ppm) and two temperatures (16°C and 20°C) across seven nitrate concentrations (1–100 μ mol L −1 ). Higher temperatures led to higher maximum growth rates ( μ max ), but also higher half‐saturation constants for nitrate ( K 1/2 ), while elevated CO 2 increased K 1/2 only at the warmer temperature. Lower μ max / K 1/2 ratios under warming and rising CO 2 indicated a higher nitrate requirement at these conditions. High temperature decreased cellular P and Si contents and consequently increased N : P and C : Si ratios, especially at ambient CO 2 . Fe : C uptake ratios responded positively to lower nitrate levels, lower CO 2 , and warming. Significant interactions between nitrate availability and temperature or CO 2 were observed for specific growth rates, chlorophyll a and Si contents, Fe : C, N : P, and Si : C, while temperature and CO 2 interactions were only significant for μ max / K 1/2 and cellular P content. The mutual interactions among CO 2 concentrations, temperature, and nitrate supply may all affect future growth, physiology, and carbon export by Coscinodiscus sp., however, in general warming and nitrate availability appear to be more influential than CO 2 .