Abrupt shifts of diatom communities driven by nutrient enrichment and extreme flooding events in a large and deep tropical reservoir

Reservoirs combine features of lake and river ecosystems. Diatoms accumulated in lacustrine sediments integrate planktonic and benthic species from lacustrine and riverine zones, respectively, providing long-term signals of ecological and environmental changes across these contrasting habitats. We hypothesized that planktonic and benthic diatoms respond differently to nutrient loading and hydrological changes, and that prolonged nutrient enrichment reduces community resilience, increasing sensitivity to external disturbances, potentially exhibiting abrupt shifts during long-term succession. We analyzed diatoms and paleoenvironmental proxies from a varved sediment core in a large tropical reservoir, using precipitation records to reconstruct diatom succession history. Two abrupt shifts were detected: the first shift triggered by extreme flooding superimposed on ongoing nutrient enrichment, the second from later intensification of nutrient loading. Benthic diatoms strongly reflected hydrological changes through shifts in richness and composition, while planktonic diatoms primarily responded to nutrient enrichment via cell size variation. Co-occurrence networks indicated declining community complexity and stability over time. Our findings demonstrate that hydrological extremes can accelerate ecological transitions under nutrient enrichment conditions, with planktonic and benthic diatoms showing distinct sensitivities to environmental forcing. These results underscore the role of riverine inflows in reservoir management amid rising nutrient enrichment and hydrological variability.