Methane cycling and groundwater sources in mangrove-dominated coastal lagoons, Yucatan Peninsula, Mexico

Tropical wetlands are known to be a major source of methane to the atmosphere, yet very little is known about methane cycling and flux from tropical coastal areas, including mangrove ecosystems. In this study, methane distributions in the sediment and water column, along with calculations and direct measurements of atmospheric methane flux, were conducted in three mangrove-dominated coastal lagoons on the Yucatan Peninsula, which range from nearly pristine to heavily impacted by domestic and industrial wastes. Despite high concentrations of sulfate in the overlying water column, large amounts of methane are produced in the shallow lagoon sediments, resulting in significant methane flux to the atmosphere. In the most pristine lagoon, surface water methane concentrations varied predictably along the length of the lagoon, showing a strong negative correlation with salinity. Surface water methane concentrations and measured atmospheric flux from the most polluted lagoon showed some of the highest measured values in this study, along with large variability over small spatial scales, resulting in greater uncertainty in estimated methane flux. The high flux values also indicate that the contribution of coastal wetlands to the global methane budget may significantly increase as a larger portion of the coastal wetland area is affected by pollution.; Due to the lack of other freshwater sources such as rivers and surface run-off, submarine groundwater discharge plays a critical role in the delivery of nutrients and low-salinity water to the coastal areas of the Yucatan Peninsula. In this study, radium isotope budgets and direct measurement of water flows were used to identify distinct groundwater sources, calculate discharge rates of high-nutrient brackish groundwater to Celestun Lagoon and the adjacent coast, and to compare the delivery of organic carbon and nutrients to the coastal ocean from both the lagoon and submarine groundwater discharge. The results of this study show that both lagoon export and direct groundwater discharge along the coast contribute significant amounts of carbon, nitrogen, and silica to the coastal ocean, and that alterations in either the groundwater quality or biogeochemical cycling within the lagoon would have large impacts on the adjacent coastal area.