Abstract
A recent study hypothesized that the near‐zero temperatures that generally prevail in Arctic waters negate the influence that different nitrogen (N) sources can otherwise have on the growth and elemental stoichiometry of marine micro‐algae. Here we test this hypothesis experimentally by evaluating how temperature (0–9°C) affects the growth and elemental stoichiometry of an ecologically relevant Arctic diatom Chaetoceros gelidus growing on different N sources (ammonium, nitrate, urea) at saturating irradiance. Following an initial acclimation period in which steady growth rates were achieved under each experimental treatment, changes in cellular concentrations of chlorophyll a and particulate carbon (C), nitrogen (N), phosphorus (P), and biogenic silica (Si) were monitored. While N source effects on growth rate became manifest as temperature rose above 0°C, the estimated optimal growth temperature was similar in all cases ( T opt = 8.3°C). A positive effect of temperature on the N : P ratio occurred only at 6°C. Above this temperature, the N : P ratio decreased to values close to those observed at 0°C and 3°C. By contrast, the C : N ratio remained nearly invariant between 0°C and 6°C but increased substantially at 9°C. Overall, the results suggest that the presently widespread and successful diatom C. gelidus possesses the ability to remain competitive despite ongoing environmental changes and that its responses to warming and the availability of different N sources may impact elemental fluxes in the future.