or : What Makes the Difference in Highly Stratified Fjords?

and which follows it seasonally, are the main producers of lipophilic toxins in temperate coastal waters, including Southern Chile. Strains of the two species differ in their toxin profiles and impacts on shellfish resources. is considered the major cause of diarrhetic shellfish poisoning (DSP) outbreaks in Southern Chile, but there is uncertainty about the toxicity of and little information on microscale oceanographic conditions promoting their blooms. During the austral summer of 2020, intensive sampling was carried out in two northern Patagonian fjords, Puyuhuapi (PUY) and Pitipalena (PIT), sharing dominance and near detection levels. Dinophysistoxin 1 (DTX 1) and pectenotoxin 2 (PTX 2) were present in all net tow samples but OA was not detected. Although differing in hydrodynamics and sampling dates, shared behavioural traits in the two fjords: cell maxima (>10 cells L) in the interface (S ~ 21) between the estuarine freshwater (EFW)) and saline water (ESW) layers; and phased-cell division (µ = 0.3-0.4 d) peaking after dawn, and abundance of ciliate prey. Niche analysis (Outlying Mean Index, OMI) of with a high marginality and much lower tolerance than indicated an unfavourable physical environment for (bloom failure). Comparison of toxin profiles and niches in three contrasting years in PUY-2020 ( bloom), 2018 (exceptional bloom of ), and 2019 (bloom co-occurrence of the two species)-shed light on the vertical gradients which promote each species. The presence of FW (S < 11) and thermal inversion may be used to provide short-term forecasts of no risk of blooms and OA occurrence, but associated with DTX 1 pose a risk of DSP events in North Patagonian fjords.