August was the end of the dramatic and traumatic domoic acid event that extended along the coast from San Luis Obispo to San Diego. However, California Sea Lions continued to strand well into August, suggesting either delayed intoxication, or ongoing offshore blooms. The several strandings associated with potential DA toxicosis reported in August shifted into central California, as far north as Sonoma County, as opposed to the southern California nexus of strandings from May-July. This distributions of strandings aligned with C-HARM estimates of ongoing bloom and domoic acid potential, given that particulate DA risk from C-HARM was predicted to be high along the entire coast until October with the broadest offshore extent in central California. Towards mid to late October, heightened pDA risk shifted to central and southern California. Meanwhile, cellular DA probabilities were only high in northern California for August and September and then shifted south to central and southern California, hovering at 50-60% probability for the month of October. As partially reported in the last bulletin, levels of the less toxigenic Pseudo-nitzschia ‘delicatissima’ size class were at bloom levels at all southern California HABMAP pier sites in July. Both size classes of Pseudo-nitzschia bloomed at Cal Poly Pier in July. The less toxigenic size class bloomed again at this site in early September and late October, while the more toxignenic ‘seriata’ group approached bloom levels several times from August to October at Cal Poly Pier and was well above bloom levels at all Monterey and Santa Cruz Wharfs during the July to early August toxic bloom period. Abundances of both size classes remained low at the northern California HABMAP sites all summer and early fall. 

Domoic acid measurements from HABMAP were not well aligned with the frequency of high Pseudo-nitzschia ‘seriata’ levels at many sites in the July bloom period. There was one relatively high hit for DA at Stearns Wharf in Santa Barbara in mid-July during the bloom and two very high DA levels recorded at Cal Poly Pier in July and mid-August. No DA measurements are available for the northern California sites. 

CDPH state monitoring of the toxigenic ‘seriata’ size class identified relative hot spots in the Humboldt Bay area in September and October, aligning with advisories for consumption of razor clams for northern California (Del Norte County) due to the risk of Amnesic shellfish poisoning. Interestingly, no advisories for domoic acid were issued in the central or southern California regions during the large stranding event from May to late July/early August. Very high levels approaching or exceeding 10,000 cells/L of Alexandrium spp. were recorded by HABMAP at all central California sites from July to the end of October. CDPH sampling at Morro Bay corroborated high central California levels, leading to CDPH and CDFW advisories on recreational harvests of mussels, scallops, and clams in the San Luis Obispo County region. That warning was only just lifted on 12 December 2023.

Interestingly, the IFCBs (robotic microscopes) at southern California HABMAP pier and mooring sites captured dinoflagellate blooms, mostly of the genus Ceratium/Tripos, aligning with “red tides” witnessed along the South Coast in late summer. At Santa Cruz, diatoms were thriving in August, most likely associated with the prolonged upwelling period along the central and northern coast that extends well into August. However, there were few Pseudo-nitzschia present in the Santa Cruz IFCB imagery during that time. That community shifted to an almost monospecific bloom of Akashiwo spp. by September as waters presumably warmed and stratified in late summer.