Connecting Groundwater and Surface Water Sources in Groundwater Dependent Coastal Wetlands and Estuaries: Sian Ka’an Biosphere Reserve, Quintana Roo, Mexico

Groundwater and surface water samples were collected in five different regions of the Sian Ka’an Biosphere Reserve (SKBR) along the eastern coast of the Yucatan Peninsula in Quintana Roo, Mexico. Samples were analyzed for major ions, total phosphorus, total nitrogen, δ18O, and δ2H. Chemical modeling and a coupled principal component analysis and end-member mixing model were used to identify three groundwater sources that discharge to the coastal wetlands and estuaries of the SKBR. A sulfate-dominated and a calcium-dominated fresh groundwater source were found to contribute significantly to the headwaters of a southern and northern SKBR estuary, respectively. In the northern part of the Reserve, an elevated road disrupts the flow of freshwater through the estuarine zone creating hypersaline conditions and mangrove dead-zones. In a more pristine estuary to the south, coastal groundwater discharge associated with petens (tree islands) accounted for ∼20 % of the surface water in the mid-estuary. This coastal groundwater discharge from the petens adds a significant amount of phosphorus to the surface water in the estuary relative to the upstream and downstream sources. The lower alkalinity measured in the surface water relative to the high-alkalinity groundwater, despite clear indication of groundwater discharge, suggests that inorganic carbon export through degassing of CO2 could represent important carbon process in mangrove ecosystems. Our results indicate an important groundwater discharge mechanism that may facilitate nutrient delivery to karstic, oligotrophic estuaries when upland and marine nutrient supplies are depleted