Beschreibung:
<jats:p>The effect of variable concentrations of dissolved molecular carbon dioxide, [CO<jats:sub>2,aq</jats:sub>], on C:N:P ratios in marine phytoplankton was studied in batch cultures under high light, nutrient‐replete conditions at different irradiance cycles. The elemental composition in six out of seven species tested was affected by variation in [CO<jats:sub>2,aq</jats:sub>]. Among these species, the magnitude of change in C:N:P was similar over the experimental CO<jats:sub>2</jats:sub> range. Differences in both cell size and day length‐dependent growth rate had little effect on the critical CO<jats:sub>2</jats:sub> concentration below which a further decrease in [CO<jats:sub>2,aq</jats:sub>] led to large changes in C:N:P ratios. Significant CO<jats:sub>2</jats:sub>‐related changes in elemental ratios were observed at [CO<jats:sub>2,aq</jats:sub>] ≪ 10 μmol kg<jats:sup>-1</jats:sup> and correlated with a CO<jats:sub>2</jats:sub>‐dependent decrease in growth rate. At [CO<jats:sub>2,aq</jats:sub>] typical for ocean surface waters, variation in C:N:P was relatively small under our experimental conditions. No general pattern for CO<jats:sub>2</jats:sub>‐related changes in the elemental composition could be found with regard to the direction of trends. Either an increase or a decrease in C:N and C:P with increasing [CO<jats:sub>2,aq</jats:sub>] was observed, depending on the species tested. Diurnal variation in C:N and C:P, tested in <jats:italic>Skeletonema costatum</jats:italic>, was of a similar magnitude as CO<jats:sub>2</jats:sub>‐related variation. In this species, the CO<jats:sub>2</jats:sub> effect was superimposed on diurnal variation, indicating that differences in elemental ratios at the end of the photoperiod were not caused by a transient buildup of carbon‐rich storage compounds due to a more rapid accumulation of carbohydrates at high CO<jats:sub>2</jats:sub> concentrations. If our results obtained under high light, nutrient‐replete conditions are representative for natural phytoplankton populations, CO<jats:sub>2</jats:sub>‐related changes in plankton stoichiometry are unlikely to have a significant effect on the oceanic carbon cycle.</jats:p>