Variability of the Thermohaline Field in a Large Tropical, Well-Mixed Estuary: the Influence of an Extreme Draught Event

Todos os Santos Bay (BTS) is one of eleven large estuaries along the semi-arid Brazilian northeast coast. It is a positive, well-mixed estuary undergoing progressive fresh water limitations due to natural climatic processes and direct human intervention to the fluvial hydrology. This study investigates the variability of the thermohaline field along three water years (2013–2015) that encompassed close-to-normal hydric conditions and the most severe regional drought in the historical record. Moored temperature and conductivity sensors, monthly CTD profiling along the bay axis, and historical records of meteorology and river discharges showed that the seasonal oscillations of the thermohaline field are 2- to 3-fold larger than higher, tidal frequency oscillations, and that BTS becomes a seasonally negative, hypersaline estuary during a drought. Also, a density plug and inverse estuarine circulation can arise with a delay of the rainy season. Negative ecological impacts have been associated with such hydrographic conditions. Long-term climatic trends and future climatic projections indicate that the BTS and its catchment area may undergo more frequent and acute droughts, suggesting that the bay may often become a seasonally negative estuary.