Dam implications on salt-water intrusion and land use within a tropical estuarine environment of the Gulf of Mexico.

Estuaries are highly productive ecosystems, defined by salt-freshwater exchanges that are significantly altered by changes upstream and in adjacent coastal areas. Tropical estuaries are characterized by the periodic advance and retreat of saline intrusion, depending on seasonality, episodic river flows and flooding events. Salt-water intrusion due to the estuarine dynamics might be affected by dam systems, which could modify the hydrological regime of the estuary in relation to other stressors, such as land use changes. For this purpose, field measurements of salinity, temperature, river-discharge and flow velocities were conducted over a year to analyze the current hydrological regime in the upper estuary of the Grijalva River in the southern Gulf of Mexico, part of the Biosphere Reserve "Pantanos de Centla", one of the most biodiverse areas in the world. Analysis of land use and vegetation cover was performed. Historical implications of the hydrological performance of the four-dam system (1957 to 2014) are presented, together with the upstream-induced changes (i.e. discharge and seasonal water volumes variations): before, between and after the full operation of the dam system. A general loss of seasonality in the river discharge was identified (1974-1987), when critical mean annual water discharges were registered (Qmean from 263.56 to 126.49 m3/s). Chronological changes in the estuary and in the surrounding area due to the introduction of large extensions of cultivated grassland (~1020 km2), reduction in mangrove cover (~223 km2) and tular (~1340 km2) were noticed. These modifications mostly occurred before conservation strategies were implemented, such as the designation of the Biosphere Reserve (1992). This study contributes to a better understanding of the response of estuarine systems to anthropic perturbations and the development of long-term management plans that could take into account climate change and the increase of hydropower development.