Benthic cyanobacteria proliferations drive anatoxin production throughout the Klamath River Watershed, California. Klamath River

Blooms of toxin-producing cyanobacteria are an enduring public health threat in lakes and rivers. In addition to more commonly studied planktonic taxa in lakes, attached cyanobacteria covering river beds and lake littoral zones can produce anatoxins, potent neurotoxins of growing concern. However, relative to planktonic blooms, the geographical and temporal extent and ecology of anatoxin-producing benthic cyanobacteria are poorly documented. In summer 2021, we surveyed sites throughout the Klamath River watershed in Northern California, USA for anatoxins from benthic mats using visual surveys, composite mat samples, water samples, and samples of transported coarse particulate organic matter (CPOM). Benthic anatoxins were widespread, adding to a growing body of evidence that anatoxins from benthic cyanobacteria may be more common than previously recognized. Compared to water column and CPOM samples, anatoxin concentrations were highest in benthic mats. Detection frequency, as indicated by anaC gene copies, was high in both transported CPOM and benthic mats (74 and 86%, respectively), and transported CPOM anatoxin concentrations reflected benthic mat anatoxin concentrations and cover. Relationships between observed taxa, toxin concentrations, and genetic source tracking indicated that Microcoleus was the dominant anatoxin producer. Clear, low nutrient tributaries supported anatoxin concentrations as high as, and in some cases higher than, mainstem sites that had higher nutrients. Weak, negative relationships among water quality parameters and anatoxins suggest that some aspects of the tributary streams not captured in this analysis promote the proliferation of benthic Microcoleus and associated anatoxin production. Monitoring benthic mats, including the use of nets targeting sloughed benthic material, can inform public health notifications and document changes in the proliferation of benthic cyanobacteria and associated cyanotoxin production in rivers.