SUMMARY: From October through December, the phytoplankton community along the Southern and Central California coast alternated between diatom- and dinoflagellate-dominated conditions, with Prorocentrum, Margalefidinium, and Akashiwo dominating the HAB community.
Pseudo-nitzschia was generally present at low levels across HABMAP and IFCB monitoring sites, with localized increases at select locations. Elevated particulate domoic acid (pDA) was detected only once at Santa Cruz Wharf in mid-October (0.07 ng/mL on October 15) and CalPoly San Luis Obispo in late December (0.04 ng/mL on December 22).
C-HARM predicted elevated probabilities of Pseudo-nitzschia abundances exceeding the 10,000 cells/L threshold from October through December across much of Central and Northern California, in both coastal and offshore waters, with much less activity predicted for the Southern California Bight (which is in contrast to last spring). Very high probabilities (>0.8) of particulate domoic acid (pDA) concentrations above alert levels were forecast over extensive regions of California coastal and offshore waters. In December, these high pDA probabilities became pronounced and persistent along the Northern California coast, particularly near Humboldt Bay. These model predictions are consistent with elevated domoic acid (DA) levels detected in Dungeness crab meat at a number of locations along Northern California coast in late October, as reported by the California Department of Public Health (CDPH).
IMPORTANT NOTE: Elevated Pseudo-nitzschia abundances have been reported throughout January at Santa Barbara, Santa Cruz, CalPoly and Monterey Wharf. At Santa Cruz, the most recent samples showed elevated pDA concentrations of 0.044 ng/mL on January 21 and 0.174 ng/mL on January 28. Santa Cruz is currently the only site with late-January data, so it is not yet possible to assess how widespread this recent increase may be.
ANIMAL IMPACTS: Marine mammal strandings associated with suspected DA exposure remained low during this period, with 15 cases reported across Southern and Central California, the majority of which are believed to reflect chronic rather than acute exposure.
Read more in the full October-December 2025 HAB Bulletin report!
