Chlorophyll-a Dynamics in a Tropical Estuary from Satellite Observations and Coupled Hydrodynamic–Biogeochemical Modeling

In estuarine systems, hydrodynamic processes primarily regulate the spatiotemporal variability of nutrients, and specifically chlorophyll-a (Chl-a), a key indicator of trophic status and water quality. This phenomenon appears particularly pronounced in tropical estuaries such as the Gulf of Urabá. Rivers such as the Atrato (3000 m3/s), León (250 m3/s), and Turbo (100 m3/s) discharge into this gulf. These rivers deliver substantial nutrient loads, including inputs of anthropogenic origin, to the southern and central zones of the gulf, creating strong salinity stratification. In contrast, the northern sector exhibits weaker stratification due to reduced freshwater influence and greater exchange with the Caribbean Sea. Consequently, the Gulf of Urabá displays marked spatial and temporal biogeochemical heterogeneity. The primary motivation for this study is to advance the understanding of surface Chl-a dynamics in a tropical estuary using in situ observations, satellite data, and a coupled hydrodynamic and water quality model. Satellite-derived Chl-a concentrations were initially calibrated against in situ observations and subsequently used, alongside discharge data, to support model calibration and validation. The results demonstrated that simulated and satellite-derived Chl-a concentrations exhibited similar spatial and temporal patterns, revealing a persistent south-to-north Chl-a gradient. The highest concentrations were recorded during May and June, although elevated Chl-a levels persisted in the southern sector throughout the year. Analyses suggest that Chl-a exhibited a remarkably persistent spatial distribution, despite substantial seasonal fluctuations in concentration. This highlights the strong influence of hydrodynamic processes and riverine inputs on primary productivity. Two events characterized by increases in Chl-a concentration in the southern part of the gulf were analyzed in detail. The evolution of concentrations before and after the peak differed substantially between the two events. These findings contribute to a better understanding of the estuarine ecosystem’s functioning and provide valuable information for the management of water quality and fishery resources in this highly productive tropical estuary. A valuable contribution of this study to tropical estuary research is the use of a coupled model alongside comparisons with satellite data and in situ measurements.