Global climate change involves an increase in oceanic CO2 concentrations as well as thermal stratification of the water column, thereby reducing nutrient supply from deep to surface waters. Changes in inorganic carbon (C) or nitrogen (N) availability have been shown to affect marine primary production, yet little is known about their interactive effects. To test for these effects, we conducted continuous culture experiments under N limitation and exposed the bloomforming dinoflagellate species Scrippsiella trochoidea and Alexandrium fundyense (formerly A. tamarense) to CO2 partial pressures (pCO2) ranging between 250 and 1000 μatm. Ratios of particulate organic carbon (POC) to organic nitrogen (PON) were elevated under N limitation, but also showed a decreasing trend with increasing pCO2. PON production rates were highest and affinities for dissolved inorganic N were lowest under elevated pCO2, and our data thus demonstrate a CO2-dependent trade-off in N assimilation. In A. fundyense, quotas of paralytic shellfish poisoning toxins were lowered under N limitation, but the offset to those obtained under N-replete conditions became smaller with increasing pCO2. Consequently, cellular toxicity under N limitation was highest under elevated pCO2. All in all, our observations imply reduced N stress under elevated pCO2, which we attribute to a reallocation of energy from C to N assimilation as a consequence of lowered costs in C acquisition. Such interactive effects of ocean acidification and nutrient limitation may favor species with adjustable carbon concentrating mechanisms and have consequences for their competitive success in a future ocean.