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Impacts of the 2014 severe drought on the Microcystis bloom in San Francisco Estuary.

  • Lehman, P W1
  • Kurobe, T2
  • Lesmeister, S3
  • Baxa, D2
  • Tung, A3
  • Teh, S J2
  • 1 Interagency Ecological Program, California Department of Fish and Wildlife, 2109 Arch Airport Road, Stockton, CA, 95206, USA. Electronic address: [email protected]
  • 2 Department of Anatomy, Physiology and Cell Biology, School of Veterinary Medicine, 1089 Veterinary Medicine Dr., Vet Med 3B, University of California, Davis, CA, 95616, USA.
  • 3 Division of Environmental Services, California Department of Water Resources, 3500 Industrial Blvd., West Sacramento, CA, 95691, USA.
Published Article
Harmful algae
Publication Date
Mar 01, 2017
DOI: 10.1016/j.hal.2017.01.011
PMID: 28366405


The increased frequency and intensity of drought with climate change may cause an increase in the magnitude and toxicity of freshwater cyanobacteria harmful algal blooms (CHABs), including Microcystis blooms, in San Francisco Estuary, California. As the fourth driest year on record in San Francisco Estuary, the 2014 drought provided an opportunity to directly test the impact of severe drought on cyanobacteria blooms in SFE. A field sampling program was conducted between July and December 2014 to sample a suite of physical, chemical, and biological variables at 10 stations in the freshwater and brackish reaches of the estuary. The 2014 Microcystis bloom had the highest biomass and toxin concentration, earliest initiation, and the longest duration, since the blooms began in 1999. Median chlorophyll a concentration increased by 9 and 12 times over previous dry and wet years, respectively. Total microcystin concentration also exceeded that in previous dry and wet years by a factor of 11 and 65, respectively. Cell abundance determined by quantitative PCR indicated the bloom contained multiple potentially toxic cyanobacteria species, toxic Microcystis and relatively high total cyanobacteria abundance. The bloom was associated with extreme nutrient concentrations, including a 20-year high in soluble reactive phosphorus concentration and low to below detection levels of ammonium. Stable isotope analysis suggested the bloom varied with both inorganic and organic nutrient concentration, and used ammonium as the primary nitrogen source. Water temperature was a primary controlling factor for the bloom and was positively correlated with the increase in both total and toxic Microcystis abundance. In addition, the early initiation and persistence of warm water temperature coincided with the increased intensity and duration of the Microcystis bloom from the usual 3 to 4 months to 8 months. Long residence time was also a primary factor controlling the magnitude and persistence of the bloom, and was created by a 66% to 85% reduction in both the water inflow and diversion of water for agriculture during the summer. We concluded that severe drought conditions can lead to a significant increase in the abundance of Microcystis and other cyanobacteria, as well as their associated toxins. Copyright © 2017 The Authors. Published by Elsevier B.V. All rights reserved.

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