Climate-modulated upwelling drives phytoplankton variability and biomass connectivity in the Humboldt Archipelago coastal system

Abstract

Abstract Marine productivity driven by phytoplankton biomass (chlorophyll-a, Chl) sustains biodiversity, fisheries, and ecosystem services in the Humboldt Archipelago (29°S), an arid coastal upwelling region within the Humboldt Current System. The archipelago is characterized by strong bathymetric gradients and a submarine canyon that generate complex circulation patterns, potentially affecting phytoplankton aggregation. Although upwelling occurs year-round, its efficiency varies seasonally and interannually under the influence of regional atmospheric forcing and large-scale climate modes, including the El Niño–Southern Oscillation (ENSO) and the Pacific Meridional Mode (PMM). We evaluated how local hydrographic structure, upwelling dynamics, and climate variability interact to regulate Chl variability and biomass connectivity within the Coquimbo–Humboldt upwelling system. To address this, CTD-fluorescence profiles collected at a fixed site during 21 near-monthly surveys between November 2022 and December 2024 were combined with satellite observations and atmospheric data. Chlorophyll variability was analyzed using hierarchical Generalized Additive Models, of which the baseline physical model explained 80% of Chl deviance, while inclusion of the PMM increased it to ~ 84% and substantially improved model performance. Seasonal Chl variability was primarily associated with mixed-layer depth shoaling and intermediate Ekman transport, indicating strong control by short-term physical forcing. In contrast, the PMM-Ekman transport interaction revealed that upwelling efficiency depended on the large-scale climatic background. Satellite observations further suggested the episodic export of phytoplankton biomass from the Coquimbo Bay system into the archipelago. These results demonstrate that climate modes modulate local upwelling efficiency, shaping phytoplankton dynamics in one of the most biodiverse coastal regions of the Southeastern Pacific.