Cross-shelf hydrography differentially shapes phycotoxin distribution across the outer Patagonian continental margin (SW Atlantic).

Microalgal toxins have been studied for decades, yet their occurrence and ecological role in offshore environments remain poorly understood due to the scarcity of observations. The external margin of the Patagonian continental shelf (Southwest Atlantic, Argentina) is a highly productive region supporting intensive fisheries. Phytoplankton phenology and biomass have been assessed largely through remote sensing, while direct assessment of toxin-producing microalgae remains limited, despite recent reports of exceptionally intense blooms. Here, we investigated the co-occurrence of multiple phycotoxins across the transition between the outer Patagonian shelf and adjacent oceanic waters, integrating toxin profiles with protistan community structure and hydrographic conditions. Hydrographic analyses were consistent with differences between stratified shelf waters and mixed, nitrate- and phosphate-richer off-shelf waters. Dinoflagellates dominated on the shelf, including two high-biomass blooms of Karenia sp. and the Amphidomataceae family, whereas diatoms increased toward the off-shelf area, as observed for Pseudo-nitzschia. All detected phycotoxins, including saxitoxins, spirolides, azaspiracid-2, domoic acid, pectenotoxins, and dinophysistoxin-1, were concentrated on the stratified shelf, despite occasionally high abundances of potential toxin-producing taxa in off-shelf waters. Additionally, most phycotoxins were detected within the 20-250 µm plankton size-fraction, whereas little or no toxins were detected in organisms > 250 µm. The predominance of toxins within the 20-250 µm plankton fraction is consistent with observations obtained during an early stage of trophic transfer under active bloom conditions, before substantial toxin accumulation in the > 250 µm plankton fraction. Overall, our findings highlight the combined influence of hydrographic retention, bloom dynamics and trophic processes on phycotoxin distribution.