Benthic overwintering cyanobacteria seed stock plays an important role in estuarine bloom dynamics
Cyanobacteria harmful algal blooms (CHABs) are a global phenomenon and increasing threat in freshwater and estuarine environments. Planktonic CHABs in low hydraulic residence time environments such as estuaries are often derived from upstream blooms that get transported downstream. Upstream locations with longer residence times (e.g., reservoirs, lagoons, marinas, or dead-end channels) tend to accumulate benthic overwintering cells that serve as inoculum for subsequent summertime blooms. In this study we investigated sediment samples collected across a variety of habitat types to evaluate which areas represent sinks and sources of the potentially toxic cyanobacterium genera Microcystis. We also collected summer and fall water samples to better understand the fate and transport of Microcystis throughout the upper San Francisco Estuary also known as the Sacramento-San Joaquin Delta (Delta). The Delta has many dead-end channels and marinas (i.e., static peripheral areas) that could provide optimal conditions for overwintering Microcystis seed stock. Quantitative polymerase chain reaction (QPCR)was used to quantify total Microcystis in the sediments to determine seed stock at the end of the bloom season in November, and to determine seed stock remaining in April that would be available to inoculate summertime blooms. To study bloom origin, succession, and transport dynamics DNA from discrete water and sediment samples was PCR amplified using cyanobacteria-specific 16S-23S ITS rDNA primers. We developed a novel PCR assay that targeted the microcystin synthase B (mcyB) gene to assess toxigenic Microcystis strain composition. Due to hydrologic conditions and previous reports, static peripheral areas were predicted to have the highest seed stock in April and study results confirmed this hypothesis. These same sites also generally had the highest concentrations of summertime Microcystis in the surface waters relative to the flowing river sites and areas classified as flooded islands. Thus, the abundance and distribution of the seed stock appears to play a central role in summertime Delta CHABs. There was also evidence that Microcystis strains in sediment and water samples from static peripheral areas were connected to distal locations in the Delta. Combined this study demonstrated that overwintering benthic seed stock can be used as an important tool to better understand Microcystis dynamics in large hydrologically complex estuarine systems.