Environmental Drivers and Trends of Alexandrium catenella and Paralytic Shellfish Toxins in Monterey Bay, California, from 2001 to 2025

Alexandrium catenella, a harmful algal species responsible for paralytic shellfish poisoning (PSP), is frequently found along the California coast. Although it is prevalent, the environmental and climatic variables underlying long-term variability in A. catenella abundance, bloom occurrence, and paralytic shellfish toxins (PSTs) in Monterey Bay, California, remain poorly understood. This study analyzed a 24-year dataset (2001–2025) collected weekly at the Santa Cruz Municipal Wharf in Santa Cruz, California. Principal component analysis (PCA) and generalized linear models (GLMs) were used to examine the relationships among environmental variables, basin-scale climate indices, and the presence of A. catenella and PSTs. Year-round (annual and seasonal (spring/summer, fall, winter) models indicated that the occurrence of A. catenella was associated with warmer, lower-nutrient conditions and lower discharge rates from the San Lorenzo River. Long-term variations in A. catenella and PST occurrences were also influenced by basin-scale climate patterns, particularly the PDO-ONI and NPGO climate modes. These results show that the dynamics of A. catenella in Monterey Bay are affected by environmental conditions, seasonal oceanographic variability, and large-scale climate processes.