Flood Pulse and Regional Hydroclimatic Extremes, Not Global Teleconnections, Regulate Phytoplankton Dynamics Across the World's Largest Tropical Floodplain

Abstract Phytoplankton dynamics influence biodiversity, water quality, and carbon cycling across Amazonian aquatic ecosystems, yet basin-scale chlorophyll- a (Chl- a ) variability remains poorly understood due to the hydrological complexity and optical heterogeneity of Amazon floodplains. Here, we investigated how seasonal hydrology and hydroclimatic extremes regulate Chl-a variability across the Pan-Amazon using two decades of MODIS observations (2001 to 2024), an Optuna-optimized support vector regression model, and hydroclimatic diagnostics. The retrieval framework reproduced observed Chl- a with robust predictive performance and uncertainty estimates derived from bootstrap-based 95% prediction intervals. Analyses revealed a dual hydroecological regime composed of predictable seasonal variability synchronized with the flood pulse and superimposed interannual shifts associated with droughts and floods. Long-term trajectories were spatially heterogeneous, with increasing Chl- a concentrations along major floodplain corridors, particularly the Solimões and Madeira systems, and declining trends across smaller tributaries. Drought conditions were generally associated with increased Chl- a concentrations, likely through enhanced residence time and reduced dilution, whereas wet extremes suppressed biomass through flushing and increased hydrological connectivity. In contrast, large-scale climate oscillations exerted relatively weak influence, indicating that Amazon floodplain productivity is primarily regulated by regional hydroclimatic variability rather than broad-scale climate teleconnections. These findings indicate that Amazonian aquatic ecosystems are highly sensitive to hydrological extremes and may undergo substantial ecological reorganization under intensifying drought and flood conditions associated with ongoing climate change.