The risk of high-biomass HABs: Triggers and dynamics of a non-toxic bloom of Prorocentrum micans in Chilean Patagonia

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 × 103 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. [Display omitted] •HB-HABs caused by Prorocentrum micans are expanding to higher latitudes, making Chilean Patagonia particularly vulnerable to its blooms in the future.•P. micans blooms in Chilean Patagonia seem to originate from a combination of hydrographic, land-based anthropogenic activities and climate change related factors.•An eddy that persistently appears in the mean flow of the Gulf of Ancud could serve as a hotspot for the accumulation of HAB cells.•P. micans HB-HABs can cause health and economic impacts, due to DMS production and mass mortalities in aquaculture farms, respecti