Exploring upwelling phenology and high-frequency hydrographic variability in the Humboldt Archipelago during winter-spring transition and La Niña conditions

The upwelling system off Coquimbo in central-northern Chile (29–30°S) provides multiple ecosystem services, including the high biodiversity in the Humboldt Archipelago, which benefits human well-being. The influence of upwelling in the archipelago, along with other high-frequency processes that lead to changes in temporal hydrographic conditions (temperature, pH, and oxygen), may have profound effects on the functioning of this ecosystem. In this study, we explored the upwelling phenology (frequency, magnitude, and duration of upwelling events), together with high-frequency variability analyses (semidiurnal, daily, and synoptic), and the vertical structure of the water column in the archipelago from August to November 2022. This period corresponded to the austral winter-spring transition of 2022, which also marked the end of a prolonged La Niña event, during which four months of continuous surface wind and surface (10 m) and subsurface (30 m) hydrographic measurements were analyzed along with four hydrographic surveys. Upwelling events observed throughout the study period promoted negative hydrographic anomalies in temperature (−1.1 °C), oxygen (−79 μmol kg⁻¹), and pH (−0.18 pH_NBS), sometimes lasting up to a week. The synoptic scale dominated the hydrographic variability during the winter (48 % of the variance), whereas the diurnal component became more significant during the spring (37 % of the variance), likely due to higher solar radiation and wider pH/oxygen cycles. Moderated by phytoplankton biomass, low pH (< 7.8 pH_NBS) and hypoxic conditions (< 60 μmol kg⁻¹) prevailed below 30 m depth in the archipelago throughout the study period. Further studies may address the influence of this environmental variability on biological processes related to the productivity of the Humboldt Archipelago.