Impact of elevated pH on succession in the Arctic spring bloom

The development of pH during the spring bloom of 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 and subsequently decreased to 7.5. Microcosm experiments were conducted on natural assemblages sampled at the initiation of the spring bloom each year and pH levels were manipulated 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 pro- nounced effect of elevated pH was found for heterotrophic protists, whereas phytoplankton proved more robust. Two out of 3 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 2 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 which revealed that phyto - plankton 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 1 out of 4 tested phyto- plankton species was 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.