The development of pH during the spring bloom in 2011 and 2012 was investigated in Disko Bay, West Greenland. During the spring phytoplankton bloom pH reached 8.5 at the peak of the bloom. Subsequently, the pH decreased to 7.5. Microcosm experiments were conducted on natural assemblages sampled at the initiation of the spring bloom each year and manipulated to cover pH levels in the range of 8.0-9.5 to test the immediate tolerance of Arctic protist plankton to elevated pH under nutrient-limiting (2011) and nutrient-rich conditions (2012). The most pronounced effect of elevated pH was found for heterotrophic protists, whereas phytoplankton proved more robust. Two out of three heterotrophic protist species were significantly affected if pH increased above 8.5, and all heterotrophic protists had disappeared at pH 9.5. Based on Chl a measurements from the two sets of experiments, phytoplankton community growth was significantly reduced at pH 9.5 during nutrient-rich conditions, while pH had little impact on nutrient-limited phytoplankton growth. The results were supported by cell counts that revealed that phytoplankton growth during nutrient-rich conditions was significantly reduced from an average of 0.49 d-1 at pH 8.0 to an average of 0.27 d-1 at pH 9.5. In comparison, only one out of four tested phytoplankton species were significantly affected by elevated pH under nutrient-limited conditions. Sudden pH fluctuations, such as those occurring during phytoplankton blooms, will most likely favour pH-tolerant species, such as diatoms.