In spring 2022, a harmful algal bloom in Golfo Nuevo, Argentina, coincided with widespread exposure to phycotoxins. Here we examined trophic transfer of toxins spanning from phytoplankton to higher trophic levels to quantify toxin transfer and bioaccumulation. High toxin levels were detected in phytoplankton, and elevated levels in mesozooplankton, mussels, and fish confirmed trophic transfer. The highest toxin levels in mesozooplankton occurred in southern right whale feeding areas, and fecal samples confirmed toxin uptake by whales. We also documented a mass mortality event in sea lions associated with algal-derived toxins, including evidence for maternal toxin transfer. During this period, ~10% of the local population was treated for gastrointestinal symptoms potentially linked to the bloom. These findings provide field-based evidence of harmful algal bloom-derived toxin dynamics in the Southwestern Atlantic, highlighting the need for sustained interdisciplinary monitoring and environmental risk assessment.