Describing presence and abundance of Alexandrium catenella with co-occurring environmental conditions in the Puget Sound

Abstract

Harmful algal blooms (HABs) produced by the marine dinoflagellate Alexandrium catenella pose significant risks to public health, marine ecosystems, and shellfish industries in Puget Sound due to the species’ production of paralytic shellfish toxins (PSTs). Despite ongoing monitoring efforts, uncertainty remains regarding the environmental drivers and spatio-temporal variability of blooms. Here, I investigated patterns of presence and abundance of A. catenella using a multi-year environmental DNA (eDNA) dataset collected by the Washington Ocean Acidification Center (WOAC) from 2019 to 2024. Water samples from 14 stations across five basins were analyzed using droplet digital PCR (ddPCR) to quantify concentrations of A. catenella. Environmental associations were evaluated using Bayesian hurdle models and multivariate clustering approaches incorporating temperature, salinity, pH, dissolved oxygen, and dissolved inorganic nitrogen (DIN). Presence of A. catenella varied across sampling months, with higher detection probabilities observed in July and September relative to April. Interannual variability was also evident, with increased presence of A. catenella in recent years.Environmental analyses controlling for temporal variability (month and year) indicated that presence of A. catenella was positively associated with DIN and dissolved oxygen, while the abundance was most strongly linked to oxygen concentration. Cluster analysis revealed that detections were more likely to occur under nutrient-rich, marine-influenced conditions. Spatially, A. catenella was detected more frequently and in higher abundances in northern and central Puget Sound. Findings demonstrate that A. catenella population dynamics are driven by multiple interacting environmental and seasonal factors rather than individual drivers. This study highlights the value of eDNA-based monitoring for detecting HAB species and characterizing favorable environmental conditions, with implications for improved management under a changing climate.