The Role of Heat and Drought during Occasional Blooms of Picoplankton and Nanoplankton in a Large Drinking Water Reservoir (allatoona Lake, Georgia)

In the late summers of 1993 and 2007, phytoplankton communities in Allatoona Lake shifted from typically diverse assemblages to near monospecific blooms of small cell-size species (a 0.5 X 2 μm cyanobacterium and a 2.5 X 15 μm diatom, respectively). These changes resulted in major increases in the amount of flocculating chemicals used to treat drinking water. Water temperatures in the epilimnion during the late summer in each year were the highest measured in these two decades, and these warmest years were associated with very low inflows into the reservoir. Limited nutrient data also indicate an unusual shift to nitrogen limitation during the 1993 bloom. Literature based on physiological studies suggests that small cells (having relatively less surface areas) may have an advantage when the limiting nutrient shifts and temperatures increase, providing an explanation for these blooms of small cells consistent with the meteorological and limnological changes observed during the blooms.