Melt-driven freshwater inputs as potential drivers of harmful algal blooms: evidence from a diatom bloom in Northwestern Patagonia

Abstract

Abstract Harmful algal blooms (HABs) in coastal waters are an increasing ecological and socio-economic concern in Northwestern Patagonia (NWP). One widely accepted mechanism proposes that reduced freshwater input weakens water-column stratification and increases residence time, allowing nutrient-rich oceanic waters to intrude and favor HAB development. Here, we demonstrate that riverine controls exerted on HABs extend beyond low-flow conditions to include shifts in the timing and magnitude of high-flow pulses. Using long-term hydro-meteorological records, satellite-based data, and a citizen science water quality time series from the Puelo River, we assess how peak freshwater inputs modulated the conditions leading up to a diatom HAB ( Thalassiosira cf. pseudonana ) in the Reloncaví Fjord (41.5°S) in November 2023. Based on inference from spring 2022 and 2024 monitoring, such discharge peaks provide short-lived windows of enhanced nutrient availability, characterized by elevated reactive silica (> 100 µmol L⁻¹), dissolved iron (> 0.3 µmol L⁻¹) and nitrogen-rich nutrient ratios (N:P > 16). An anomalous high-turbidity pulse, usually dampened by downstream lake regulation, suggests threshold effect of meltwater inputs, which likely gave this taxon a competitive advantage. Antecedent conditions also include a sustained winter runoff effect on stratification and exchange flow, followed by an unusually dry early spring, weakening stratification during the period of increased solar radiation, both preconditioning the system for the HAB.