Assessing the impacts of artificial inlet opening and freshwater extraction and on salinity and temperature gradients in a tropical coastal lagoon

The coastal lagoon of La Mancha, in Veracruz, Mexico, is a site of great biological, ecological, and economic importance because of its high biodiversity, tourism and fishing activities. Two current anthropogenic actions that could be altering this system are the artificial opening of the inlet by fishermen, to increase the inflow of marine organisms, and the extraction of fresh water to supply recent housing developments. To assess the impacts of these actions, a two-dimensional numerical model (Delft3D) was calibrated and validated, and the horizontal salinity and temperature gradients were simulated by varying the amounts of freshwater extraction for opened and closed inlet conditions in rainy and dry seasons. The results show that in open inlet conditions, even when the water discharged from the streams is varied due to the season, a very marked salinity gradient forms from north to south, with oligohaline, polyhaline, and euhaline conditions. When the inlet is open, in the northern part of the lagoon, most of the water body has high salinity values (35 ppt), and when the aperture closed, salinity dilutes throughout the system to mesohaline and polyhaline conditions (5−30). The freshwater demand associated with current developments in the surroundings of La Mancha, did not significantly modify the temporal and spatial distribution of temperature and salinity in the lagoon. However, the increasingly constant artificial opening of the inlet has implications for these gradients. The changes in salinity and temperature gradients are of great importance for the flora and fauna of these ecosystems. The inadequate and frequent opening of the water body to the sea alters salinity and temperature conditions which will greatly affect the biota, many of which are of commercial interest.