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Harmful algal blooms (HABs) of toxin-producing microalgae are recurrent in Patagonian fjord systems. Like toxigenic HABs, high-biomass harmful algal blooms (HB-HABs) have important socio-economic repercussions, but most studies have focused on the former. Here we report the formation and development of an intense HB-HAB of Prorocentrum micans that occurred in Northwest Chilean Patagonia in the late summer (February-March) of 2022. Concentrated and extensive brown spots were visible on the water surface, accompanied at the end of February by a strong odour. Prorocentrum micans cells were detected at relatively low densities (up to 215 cells mL -1 ) in January but by February 11 cell densities exceeded 1000 cells mL -1 , reaching a maximum of 8.3 × 10 3 cell mL -1 in the surface layer. The high cell densities at Reloncaví Sound and the Gulf of Ancud were closely associated with narrow-ranging increases in the sea surface temperature (17-18.5 °C) and salinity (29-31 g kg -1 ). Sentinel-2 satellite images from February 22 showed a colour change corresponding to the presence of the brown patches at both locations, consistent with the increases in the normalized index of chlorophyll differences (NDCI) and chlorophyll a concentrations (~50 μg L -1 ). Satellite images from GHRSST indicated warmer waters in Reloncaví Sound and the Gulf of Ancud than in the Gulf of Corcovado, located 170-km to the south. An oceanographic 3-D model (MOSA) showed surface currents with a cyclonic eddy centred in the Gulf of Ancud. This circulation pattern suggested greater water retention in the study area during January and February, with the drifting and rotation of the coastal currents around the eddy maintaining the P. micans bloom. Thus, the elevated cell density of P. micans in the Gulf of Ancud, near the periphery of the eddy, confirm the presence of a material accumulation hotspot for HABs and HB-HABs., Competing Interests: Declaration of competing interest We have not financial interests/personal relationships which may be considered as potential competing interests., (Copyright © 2024 Elsevier B.V. All rights reserved.)

Protoceratium reticulatum is the main yessotoxin-producer along the Chilean coast. Thus far, the yessotoxin levels recorded in this region have not posed a serious threat to human health. However, a bloom of P. reticulatum during the austral summer of 2022 caused the first ban of shellfish collection, due to the high toxin levels. A bloom of P. reticulatum during the austral summer of 2020 allowed an evaluation of the fine-scale distribution of the dinoflagellate during a tidal cycle. High-resolution measurements of biophysical properties were carried out in mid-summer (February 18-19) at a fixed sampling station in Puyuhuapi Fjord, Chilean Patagonia, as part of an intensive 24-h biophysical experiment to monitor the circadian distributions of P. reticulatum vegetative cells and yessotoxins. High P. reticulatum cell densities (>20 × 10 3 cells L -1 ) were found in association with a warmer (14.5-15 °C) and estuarine (23.5-24.5 g kg -1 ) sub-surface water layer (6-8 m). P. reticulatum cell numbers and yessotoxins followed a synchronic distribution pattern consistent with the excursions of the pycnocline. Nevertheless, the surface aggregation of the cells was modulated by the light cycle, suggesting daily vertical migration. The yessotoxin content per P. reticulatum cell ranged from 9.4 to 52.2 pg. This study demonstrates both the value of fine-scale resolution measurements of biophysical properties in a highly stratified system and the potential ecosystem impact of P. reticulatum strains producing high levels of yessotoxins., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2024 Elsevier B.V. All rights reserved.)

Phytoplankton succession is related to hydroclimatic conditions. In this study we provide the first description of a toxic phytoplankton succession in the Patagonian Fjord System. The shift was modulated by atmospheric-oceanographic forcing and consisted of the replacement of the marine dinoflagellate Dinophysis acuta in a highly stratified water column during austral summer by the diatom Pseudo-nitzschia calliantha in a mixed water column during late summer and early autumn. This transition, accompanied by a change in the biotoxin profiles (from lipophilic dinophysis toxins to hydrophilic domoic acid), was induced by the arrival of an intense atmospheric river. The winds in Magdalena Sound may have been further amplified, due to its west-east orientation and its location within a tall, narrow mountain canyon. This work also documents the first known appearance of toxic P. calliantha in Northern Patagonian. The potential impacts of the biotoxins of this species on higher trophic levels are discussed., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2023 Elsevier Ltd. All rights reserved.)

Harmful algal blooms (HABs) in southern Chile are a serious threat to public health, tourism, artisanal fisheries, and aquaculture in this region. Ichthyotoxic HAB species have recently become a major annual threat to the Chilean salmon farming industry, due to their severe economic impacts. In early austral autumn 2021, an intense bloom of the raphidophyte Heterosigma akashiwo was detected in Comau Fjord, Chilean Patagonia, resulting in a high mortality of farmed salmon (nearly 6000 tons of biomass) within 15 days. H. akashiwo cells were first detected at the head of the fjord on March 16, 2021 (up to 478 cells mL -1 ). On March 31, the cell density at the surface had reached a maximum of 2 × 10 5 cells mL -1 , with intense brown spots visible on the water surface. Strong and persistent high-pressure anomalies over the southern tip of South America, consistent with the positive phase of the Southern Annular Mode (SAM), resulted in extremely dry conditions, high solar radiation, and strong southerly winds. A coupling of these features with the high water retention times inside the fjord can explain the spatial-temporal dynamics of this bloom event. Other factors, such as the internal local physical uplift process (favored by the north-to-south orientation of the fjord), salt-fingering events, and the uplift of subantarctic deep-water renewal, likely resulted in the injection of nutrients into the euphotic layer, which in turn could have promoted cell growth and thus high microalgal cell densities, such as reached by the bloom., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2022 Elsevier B.V. All rights reserved.)