California water regulators roll out updated benthic cyanobacteria monitoring guidance, set 15% cover posting threshold

State Water Resources Control Board staff on a May webinar introduced revised guidance and a field standard operating procedure (SOP) for monitoring benthic harmful cyanobacterial blooms (HCBs), including interim numeric thresholds and a tiered monitoring approach using solid-phase adsorption toxin tracking (SPAT).

Hannah Merges, a California Sea Grant fellow placed with the State Water Resources Control Board’s Office of Information Management and Analysis working with the SWAMP unit, said the updated guidance sets a visual percent-cover threshold of 15% for posting a yellow “toxic algae” advisory: “Anything above that 15% cover is gonna trigger a signage posting,” Merges said during the presentation.

The guidance updates standardize three monitoring types — event response, follow-up monitoring and routine monitoring — and prioritizes rapid, low-cost percent-cover visual assessments for immediate posting decisions. The tiered approach uses SPAT samplers…