Over Two Decades of MODIS-Derived Chlorophyll-a Phenology and Long-Term Trends in Ancient Tropical Lake Poso, Indonesia

Tropical lake ecosystems are vulnerable to degradation due to climate change and anthropogenic pressure. These dual pressures lead to ecological change and alterations in aquatic parameters, including chlorophyll-a (Chl-a). Lake Poso is the third-largest lake in Indonesia; Chl-a variability and phenology in this tropical lake remain understudied due to brief observational records. This study characterizes the spatiotemporal variability and phenology of Chl-a in Lake Poso, Indonesia, over two decades (2003–2025) to establish a multi-decadal baseline and detect early indications of increasing phytoplankton biomass. Chl-a was derived from combined Terra and Aqua MODIS observations processed in Google Earth Engine, followed by trend, phenological, and spatial analyses. Results reveal a significant increase in annual Chl-a and a general upward tendency across the study period. Climatological reconstruction identifies a distinct bimodal phenology, with a primary peak in December and a secondary peak in April. Interannual trends show no significant shift in Start of Season timing; however, primary peak magnitudes tend to increase (+0.079 mg m−3 yr−1), largely influenced by extreme episodic anomalies. Spatial analysis reveals heterogeneous increases in Chl-a, particularly in the central and northern zones. These findings suggest a possible early trajectory toward higher phytoplankton biomass and establish the first satellite-derived multi-decadal Chl-a baseline for Lake Poso.