Abstract
In Rhode Island (RI), harmful algal blooms (HABs) of the toxic diatom, Pseudonitzschia, are an emergent public health and economic threat. These HAB events involve the synthesis of a phytoplankton toxin, domoic acid (DA), that bioaccumulates in filter- and benthic-feeding organisms, transferring through pelagic food webs with potential for humans to be exposed to DA via consumption of tainted seafood. In this study, the aim was to identify RI shellfish growing areas (GAs) experiencing high Pseudo-nitzschia abundance, and to determine DA production of Pseudo-nitzschia cells through quantification of DA in a subset of plankton and shellfish samples. There was also an attempt to identify the key environmental drivers of increased Pseudo-nitzschia abundance and particulate DA (pDA) in RI GAs. From 22 February 2024 to 26 September 2024, a total of 195 phytoplankton samples were collected from 16 GAs using a 20 µm-mesh plankton net. Pseudo-nitzschia cells were microscopically enumerated within 30 hours using light microscopy and cellular abundance was calculated. DA was quantified using liquid chromatography tandem mass spectrometry (LC/MS-MS) in 99 phytoplankton samples as well as blue mussel (Mytilus edulis) and northern quahog (Mercenaria mercenaria) meat from areas experiencing frequent abundances greater than 20,000 cells L−1 using liquid chromatography tandem mass spectrometry (LC/MS-MS). Relationships with water temperature, salinity, and dissolved nutrients were investigated using Spearman's rank correlation tests. Pseudo-nitzschia was present in 59.5% of samples, with 13 samples from 11 GAs experiencing abundances greater than 20,000 cells L−1. Levels of pDA were detected on 9 April 2024, 29 April 2024, and 20 May 2024 to 4 June 2024. A bloom event affecting Narragansett Bay, Ninigret Pond, and Quonochontaug Pond, was identified from 20 May 2024 to 4 June 2024, where Pseudo-nitzschia abundance reached a maximum of 178,095 cells L−1, with frequent abundances above 20,000 cells L−1. Although pDA was detected during this bloom event, DA was not detected in shellfish meats. Significant positive correlations were observed between Pseudo-nitzschia abundance and pDA, Pseudo-nitzschia abundance and water temperature, pDA and water temperature, and pDA and salinity. Pseudo-nitzschia abundance was significantly negatively correlated with dissolved nitrogen (NO₃ + NO₂-, NO₂-, NH3 and total N) and silicate (SiO2). Overall, these results indicate while there was a persistence of toxic Pseudo-nitzschia in RI waters during 2024, DA was not present in shellfish meats. Taken together this work aids in our understanding of Pseudo-nitzschia bloom dynamics in a region with emerging HAB events.