Evidence of hydrodynamic control of phytoplankton seasonality in a tropical urban estuary

Tropical urban estuaries receive year-round nutrient loads, yet their strongest phytoplankton blooms often occur in the dry season. The proximate control on this seasonality remains unresolved in wastewater-impacted tropical systems. Here we apply a transient one-dimensional Carbon Generic Estuarine Model (C-GEM) to the Saigon River Estuary, southern Vietnam, and evaluate it against 24 months of monitoring data (2017–2018). Simulated dry-season chlorophyll-a at the urban station reached 37 μg L^(-1), matching observations and reproducing the observed approximately sevenfold dry-to-wet contrast. Urban residence time increased from approximately 20 days during the monsoon to approximately 53 days during the dry season, and the chlorophyll–residence-time relationship showed an inflection at 32 days. The corresponding Damköhler number shifted from Da≈33 in the dry season to Da≈1 in the wet season, consistent with a transition from bloom-permissive to bloom-suppressive conditions. The hydrodynamics-related predictors explained 60–62% of simulated phytoplankton variability at the upstream bloom-forming stations, whereas nutrient loads contributed 21–27%; near the urban discharge sources, nutrients accounted for up to 40%. The coupled carbonate module showed persistent urban CO_2 supersaturation (pCO_2 > 10,000 μatm) and mean estuarine evasion of 142.9 Gg C yr^(-1). These results show that hydraulic residence time is the dominant proximate control on bloom expression in this hyper-eutrophic estuary. Wastewater reduction remains the durable management solution, whereas flow augmentation may serve only as a constrained near-term complement in regulated tropical systems. Similar growth–flushing behaviour is most likely in tropical megacity estuaries that combine chronic nutrient excess with strong monsoon-driven retention swings.