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Clear Lake studies and tribal monitoring show seasonal community shifts and toxins in creeks and some tap water
Summary
University of Southern California sequencing found seasonal shifts in Clear Lake’s planktonic microbial community and a strong correlation between Microcystis and microcystin; the Big Valley Band reported high localized benthic toxin detections in creeks and a tap‑water detection in a household served by lake water.
Researchers from the University of Southern California and staff from the Big Valley Band of Pomo Indians presented complementary findings from Clear Lake and nearby creeks.
Isha Ora (presenter name recorded as "Echa Ora" in the transcript), representing USC, summarized a multi‑year DNA sequencing study of planktonic bacteria and microbial eukaryotes at five Clear Lake sites from 2019–2021. The sequencing work showed strong seasonal succession: picocyanobacteria (Cyanobium) dominated summer communities in the western lake arm, while Dolichospermum, Microcystis and other taxa increased later in the season. USC reported that Microcystis abundance was the single genus most strongly correlated with microcystin concentrations in their samples, and that Dolichospermum/Anabaena and other diazotrophs often preceded increases in total nitrogen that then coincided with Microcystis growth.
USC’s eukaryotic sequencing showed chlorophytes, diatoms and cryptophytes contributing to seasonal changes; preliminary correlation analyses suggested some grazers and microbial eukaryotes may co‑occur with or respond to toxin presence, but authors said experimental confirmation is required.
Sarah Ryan, environmental director for the Big Valley Band of Pomo Indians, described the tribal monitoring program that used SPAT passive samplers, microscopy and grab samples to screen creeks flowing to Clear Lake. The tribe reported detecting high localized mat toxin concentrations at shoreline locations in 2024 and noted one hotspot at Lily Cove with a very high mat result (presenters used the figure 16,000 µg/L as a historical peak for a shoreline location in their sampling record). Big Valley staff also said tribal tap‑water testing (a CDC‑funded project) detected cyanotoxins in finished household tap water in at least one home that uses lake‑source water and that this prompted outreach and follow‑up testing.
Big Valley and USC speakers emphasized the complementary nature of the work: sequencing and planktonic monitoring provide large‑scale context and taxonomy, while mats and SPAT data reveal high‑concentration, localized risks to shoreline users and people who use direct lake intake systems.
Both groups called for continued integrated monitoring across lake arms and in inflowing creeks and for coordination between tribal, county and state partners.
