THE ENVIRONMENTAL PARAMETERS AND TRENDS DRIVING TOXIGENIC ALEXANDRIUM CATENELLA BLOOMS IN MONTEREY BAY, CALIFORNIA FROM 2001-2019by Regina L. Radan

An 18-year data set of samples collected weekly from the Santa Cruz Municipal Wharf (SCW) in Santa Cruz, California, was used to investigate the environmental parameters driving Alexandrium catenella abundance and toxicity in northern Monterey Bay, California. Alexandrium catenella is the causative organism for Paralytic Shellfish Poisoning (PSP) in California coastal waters and is routinely monitored. A semi-quantitative index of relative abundance and quantitative cell counts of A. catenella (cells L-1) reveals an increase in relative and absolute abundance of A. catenella from 2003-2007, and again from 2016-2019. Using a Generalized Linear Model (GLM), both annual and seasonal models were developed using 12 biological and physical parameters to explain the dependent variables, A. catenella abundance, and Paralytic Shellfish Toxins (PST) concentration. Together, (-) NO3- (µM) and sea surface temperature (ºC) (p = 0.0031), (-) San Lorenzo River discharge (m-3s-1) (p = 0.0002), and (+) chlorophyll-a (µg L-1 ) (p <0.0001), were all strongly correlated with the presence of A. catenella at the SCW in the annual model. Spring and fall/winter models showed similar patterns. Blooms of A. catenella are defined as abundances greater than one standard deviation from the climatological mean (1349 cells L-1). Blooms were strongly correlated with (-) San Lorenzo River discharge (p = 0.0192) and (+) chlorophyll-a (p < 0.0001) in the annual model but showed different patterns for the seasonal models. When lagging PSTs by three weeks, the annual model identified a strong negative correlation with NO3- and a positive correlation with sea surface temperature (ºC) (p=0.0024). This study demonstrates the importance of examining more than one environmental factor when determining the drivers for toxic blooms. These models could potentially be used to forecast future blooms and the presence of PSTs in Monterey Bay, California, and elsewhere where dinoflagellates are common members of the phytoplankton assemblage.