Connectivity of toxigenic Pseudo-nitzschia species assemblages between the Northeast U.S. continental shelf and an adjacent estuary

•Toxigenic Pseudo-nitzschia spp. may enter a northeast U.S. estuary from offshore sources more often in winter than in summer.•Domoic acid on the continental shelf was elevated in winter compared to summer.•Several Pseudo-nitzschia species often co-occur with domoic acid and may be responsible for toxin production in this region.•Toxin was detected with relatively low macronutrients, warmer sea surface temperatures in winter, and cooler temperatures in summer.•Offshore Pseudo-nitzschia spp. and domoic acid should be monitored for inflow events that may seed HABs in coastal ecosystems. Pseudo-nitzschia harmful algal blooms have recently caused elevated domoic acid in coastal environments of the Northeast United States. In 2017, the toxigenic species P. australis was observed in Narragansett Bay, Rhode Island, a temperate estuarine ecosystem, for the first time since 2009 when DNA monitoring for Pseudo-nitzschia species began. This highly toxic species likely contributed to toxin-related shellfish harvest closures and is hypothesized to have been introduced by an offshore source. Little is known about offshore Pseudo-nitzschia spp. populations in the Northeast Continental Shelf marine ecosystem or how often toxigenic species enter Narragansett Bay through physical processes. Here, we collected filtered biomass samples from multiple time series sites within Narragansett Bay and along the Northeast U.S. Shelf Long-Term Ecological Research transect in winter and summer to investigate the frequency and seasonality of potential Pseudo-nitzschia spp. inflow from the continental shelf to the estuary. Species were taxonomically identified using DNA sequencing of the ITS1 region and domoic acid concentrations were quantified by liquid chromatography with tandem mass spectrometry and multiple reaction monitoring. During six years of sampling, Pseudo-nitzschia species assemblages were more similar between Narragansett Bay and the Northeast shelf in winter than summer, suggesting greater ecosystem connectivity in winter. These winter assemblages were often accompanied by higher domoic acid. Several Pseudo-nitzschia species co-occurred most often with domoic acid and were likely responsible for toxin production in this region, including P. pungens var. pungens, P. multiseries, P. calliantha, P. plurisecta, P. australis, and P. fraudulenta. Domoic acid was detected during periods of relatively low macronutrient concentrations in both seasons, warmer sea surface temperatur