LAUSR

The frequency, intensity and duration of cyanobacterial harmful algal blooms are expected to increase with climate change. Here we tested the null hypothesis that successive severe drought years would not differ in the magnitude, community composition and controlling factors for Microcystis blooms during 2014 and 2015, the third and fourth most severe drought years on record in the San Francisco Estuary, California, USA. Identical sets of physical, chemical and biological data were collected every 2 wk at 10 stations between August and November for each year. Primary producer biomass, abundance, biovolume, community composition and toxin production were quantified. Contrary to expectation, the surface and subsurface Microcystis bloom in 2014 was at least an order of magnitude greater than in 2015, the drier and warmer year. In addition, the 2015 drought had a greater percentage of other cyanobacteria (non-Microcystis) and eukaryotic phytoplankton than 2014. Median water quality conditions were similar between years, but correlations among physical, chemical and biological variables often differed in magnitude and direction. PRIMER DISTLM (BEST) analysis identified water temperature, the landward extent of saltwater intrusion and outflow as variables that accounted for the most variation in Microcystis surface biovolume (R2 = 0.48) or subsurface abundance (R2 = 0.45). We conclude that the magnitude of Microcystis blooms may not always increase with drought severity or prolonged drought, and are dependent on within-year spatial and temporal variation.