Hydrochemistry and hydrodynamics of a Mexican Caribbean Lagoon: Nichupté Lagoon System

Abstract The study results describe the hydrogeochemical and the hydrodynamic characterization, and the evolution compounds in the water in the Nichupte Lagoon System (NLS). This is the most representative lagoon of Quintana Roo State and hosts the Natural Protected Area “Manglares de Nichupte” and borders the Punta Cancun-Punta Nizuc National Park. Sampling was performed during rainy and cold-front seasons to asses the distribution and variability of temperature, pH, dissolved axygen salinity, nutrients and major elements. Significant differences were detected for temperature and salinity but not for dissolved oxygen between the seasons. The dissolved oxygen values have considerably diminished in a decade. NO x and PO 4 concentrations were high, and did not show significant differences between seasons. NH 4 + concentration was an order of magnitude higher in the rainy season. Two types of water occur in the study area (Cl) and (Cl-SO 4 -Na-Ca-Mg). A strong predominance of Na-Cl, classifies the lagoon system as a brackish lake. Evaporation was the dominant process and the lagoon behaves like a salty lake, with an evolution in the chemical composition of the water, which became enriched in calcium during the cold-front season, and in sulfates during the rainy season. No precipitation of calcite or gypsum was identified, but this could occur during the cold fronts due to the geochemical evolution observed in the Spencer diagram. Nutrient concentrations have increased in ten years, but some are still below Mexican water quality criteria (CE-CCA-001/89). Some sites present high nutrients concentrations like Bojorquez lagoon sites. The results obtained underline: a) high temperature records vis-a-vis of previous studies measurements (historical records), b) decreasing dissolved oxygen concentrations, c) increasing NO 3 − and PO 4 3− concentrations. These factors combination shows eutrophication signs that should already be taken as an alert. The circulation of the water in the two analyzed periods was qualitatively similar but the magnitudes of the currents were remarkably different between each other. The rainy season (August 2015) showed near-coastal mean currents of magnitude ∼2–3 cm s −1 , while the cold-front season (March 2016) showed mean currents about twice stronger (∼5–6 cm s −1 ). The circulation pattern result in relatively short residence times of the water bodies of the Lagoon System, with values from 3 to 6 weeks in the rainy season; these values can be decreased by 30–50% in the cold-front season.