State Water Boards present updated benthic cyanobacteria guidance, sets 15% percent‑cover posting threshold

State Water Resources Control Board staff on a May webinar rolled out updated guidance and a field standard operating procedure (SOP) for monitoring benthic harmful cyanobacterial blooms (HCBs), introducing a 15% percent‑cover visual threshold to guide signage postings and a three‑tier monitoring approach that begins with SPATT passive samplers.

Hannah Merges, a state fellow placed with the State Water Resources Control Board’s Office of Information Management and Analysis (SWAMP unit), said the revised guidance aims to standardize monitoring across California and improve the information used to protect people and domestic animals from cyanotoxin exposure. “Anything above that 15% cover is going to trigger a signage posting,” Merges said, describing the percent‑cover metric as a rapid, low‑cost way to prioritize protective action while lab toxin results are pending.

The guidance emphasizes a tiered approach. Tier 1 uses SPATT (solid phase adsorption toxin tracking) bags deployed about four to seven days to detect rising cyanotoxin signals. If SPATT indicates increasing toxins, responders move to tier 2 visual assessments of benthic mat percent cover over a defined sampling reach (up to 150 feet). Tier 3 is collection of a composite mat sample for laboratory toxin analysis and taxonomic identification.

El…