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

Abstract

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.

High‐resolution imaging particle analysis of freshwater cyanobacterial blooms

“…Other investigations have similarly shown that FlowCambased Microcystis spp. cell abundance estimates based on these volumetric measurements were closely correlated with those determined by microscopic analyses (Lehman et al 2013(Lehman et al , 2015(Lehman et al , 2017Wang et al 2015). In comparison, light microscopy-based total cell counts for Aphanizomenon were often underestimated when using FlowCam (Fig.…”

“…Then, manual cell counts of a few of the captured images are performed to estimate the average ABD per cell. Next, the total ABD is divided by the average ABD per cell to compute a total cell count per unit sample volume (e.g., Lehman et al ). Further, total cell counts for each taxon can then be converted into biomass by matching morphologies with best fitting geometric shapes and their volumetric formula (Hillebrand et al; Sun and Liu ).…”

“…In comparison, its use in freshwater studies is less common (Reilly‐Mathews ; Tarrant et al ; Spaudling et al ). Most studies have focused on more easily enumerated single‐celled or relatively large marine species of diatoms and dinoflagellates; however, FlowCam has also been used to reliably quantify the abundance of smaller cells of potentially toxic colonial cyanobacteria belonging to the genus Microcystis along with other phytoplankton taxa (Lehman et al , , ; Wang et al ).…”

Colony formation in the cyanobacterium Microcystis

“…Microcystis spp. predominate in the plankton of some of the world's largest lakes, such as Lake Erie in North America and Lake Taihu in China (Lehman et al, ; Levy, ; Zhu et al, ). Other cyanobacterial species are also known to dominate freshwater ecosystems, such as Dolichospermum (also known as Anabaena ) spp.…”

Determining the Exposure Pathway and Impacts of Microcystis on Threadfin Shad, Dorosoma petenense, in San Francisco Estuary

“…Although toxicity detected in the threadfin shad could not be attributed to exposure to Microcystis because there were likely multiple antecedent interactions associated with the study site being a highly altered estuary, our study showed that threadfin shad were directly exposed to the toxin microcystin from Microcystis blooms throughout the bloom season and not just during the subsequent senescence when microsystins would be released from lysed cells. Microcystis blooms are an increasing concern globally as well as in the San Francisco Estuary, with increased trajectory projections in the frequency and severity of Microcystis blooms in relation to climate change (Intergovernmental Panel on Climate Change 2014; Dettinger et al 2016; Lehman et al 2017). The novel approach detailed in our study can be modified to determine dietary exposure of Microcystis to fish.…”