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In southern Chile, harmful algal blooms (HABs) pose a threat to public health, artisanal fisheries, and the aquaculture industry (mussels and salmon). However, little is known about the environmental factors contributing to outbreaks of HABs in fjord systems. In summer 2017, an oceanographic cruise was carried out to study the physical processes associated with a bloom of the dinoflagellate Karenia spp. in the Gulf of Penas and Taitao Peninsula, Chilean Patagonia, causing a massive mortality of salmon (approximately 170,000 fish, worth USD 390,000). Satellite images from Sentinel-3 were utilized to distinguish between areas with high and low densities of Karenia cells. Cell densities were highest in the waters of the northern Taitao Peninsula (70 × 103 cells L−1), and lowest at the Gulf of Penas. Support vector classification (SVC) based on bands 1 (400 nm), 2 (412.5 nm), and 6 (560 nm) from the Sentinel-3 images and the normalized fluorescence line height (FLH) classified bloom presence/absence with an 83% coincidence rate. The SVC model correctly identified non-bloom areas, with limited false positives, and successfully captured bloom zones where Karenia densities were highest. These results demonstrate the importance of incorporating satellite tools in the design and implementation of monitoring programs for the early detection of HABs, particularly in remote, difficult-to-access areas. 

Abstract Species of the Dinophysis acuminata complex are the main cause of diarrhetic shellfish poisoning worldwide. These mixotrophs perform photosynthesis with plastids stolen from specific ciliate prey. Current transport models forecast advection of established populations, but modelling bloom development and maintenance also needs to consider the prey (Mesodinium spp.) of Dinophysis. Predator and prey have distinct niches, and Dinophysis bloom success relies on matching prey populations in time and place. During autumn 2019, red tides of Mesodinium rubrum in Reloncaví Fjord, Chile, were not followed by Dinophysis growth. The dynamics of Mesodinium–Dinophysis encounters during this and additional multiscale cases elsewhere are examined. Analogies with some classic predator—prey models (match–mismatch hypothesis; Lasker’s stable ocean hypothesis) are explored. Preceding dense populations of Mesodinium do not guarantee Dinophysis blooms if spatial co-occurrence is not accompanied by water column structure, which leads to thin layer formation, as in Lasker’s stable ocean hypothesis or if the predator growth season is over. Tracking the frequency of vacuolate Dinophysis cells, irrefutable signal of prey acquisition, with advanced in situ fluid-imaging instruments, is envisaged as a next-generation tool to predict rising Dinophysis populations. 

At the end of summer 2020, a moderate (~105 cells L−1) bloom of potential fish-killing Karenia spp. was detected in samples from a 24 h study focused on Dinophysis spp. in the outer reaches of the Pitipalena-Añihue Marine Protected Area. Previous Karenia events with devastating effects on caged salmon and the wild fauna of Chilean Patagonia had been restricted to offshore waters, eventually reaching the southern coasts of Chiloé Island through the channel connecting the Chiloé Inland Sea to the Pacific Ocean. This event occurred at the onset of the COVID-19 lockdown when monitoring activities were slackened. A few salmon mortalities were related to other fish-killing species (e.g., Margalefidinium polykrikoides). As in the major Karenia event in 1999, the austral summer of 2020 was characterised by negative anomalies in rainfall and river outflow and a severe drought in March. Karenia spp. appeared to have been advected in a warm (14–15 °C) surface layer of estuarine saline water (S > 21). A lack of daily vertical migration patterns and cells dispersed through the whole water column suggested a declining population. Satellite images confirmed the decline, but gave evidence of dynamic multifrontal patterns of temperature and chl a distribution. A conceptual circulation model is proposed to explain the hypothetical retention of the Karenia bloom by a coastally generated eddy coupled with the semidiurnal tides at the mouth of Pitipalena Fjord. Thermal fronts generated by (topographically induced) upwelling around the Tic Toc Seamount are proposed as hot spots for the accumulation of swimming dinoflagellates in summer in the southern Chiloé Inland Sea. The results here provide helpful information on the environmental conditions and water column structure favouring Karenia occurrence. Thermohaline properties in the surface layer in summer can be used to develop a risk index (positive if the EFW layer is thin or absent).