Seasonal variations in residence time and water age in the Mundaú-Manguaba Estuarine Lagoon System, AL (Brazil)

Abstract

The Characteristic Hydraulic Times (CHT), called Residence Time (RT) and Water Age (WA), were analyzed in the Mundaú-Manguaba Estuarine Lagoon System (MMELS – AL, Brazil), using the SisBaHiA® – Environmental Hydrodynamics Base System, for two scenarios: wet season, from May to July 2018; and dry season, from October to December 2018. The hydrodynamic modelings appropriately represented water level variations measured in the lagoons. The analysis of residual currents revealed how hydrodynamic circulation was affected by seasonal variations, and provided essential information to understand the results of the RT and WA simulations. This study deepened the knowledge regarding the RT of the water masses inside the lagoons, and reinforced how river discharges significantly influence their export, which resulted, considering the spatial scope, in much smaller RT in the wet season compared to the dry season. The WA analysis added new and significant knowledge about water renewal, from a different perspective, and as yet little-explored in the study region. Among the results obtained, the most relevant refer to the dry season, in which it was found that the “oldest” waters in the Mundaú L. occurred near the southeast region, while in the Manguaba L., in the vicinity of the central region. Both regions were confirmed as the places that present the most critical trophic state index in each lagoon, showing that WA is a CHT that indicates the regions most conducive to more critical trophic states.
 Keywords: Mundaú-Manguaba lagoons, residence time, water age.