SEA LEVEL RISE IMPACTS ON COASTAL KARST AQUIFERS

Coastal karst aquifer systems are threatened by eustatic sea level rise, which currently is around 3 mm/yr, although this rate varies through time and by location. Changing sea level alters head gradients between aquifers and the ocean, causing water and solutes to exchange between the aquifer and ocean at a salt water-fresh water interface referred to as the subterranean estuary. Salt water intrusion to the subterranean estuary may alter biogeochemical reactions and limit submarine groundwater discharge of fresh water and terrestrial solutes to the coastal ocean. These effects could combine to diminish potable water quality and quantity, reduce solute fluxes to surface estuaries, degrade coastal ecosystems, and alter dissolution effects of carbonate aquifers. To evaluate these potential processes, we have measured short-term variations in sea level and effects of these variations on the salinity, pH, dissolved oxygen, and nutrient concentrations over two 2-wk periods at a submarine karst spring in a reef lagoon of northeastern Yucatan Peninsula. This spring discharges brackish water; however, lagoon water intruded into the spring when sea level elevations increased to >0.08 m above the mean elevations measured during our study periods at high spring tides and with wind set up. If eustatic sea level rise accelerates as projected, this elevation may be permanently passed within a few decades, although intrusions may not become permanent if the hydraulic head of the Yucatan aquifer also increases. Intrusion of oxygenated lagoon water into the conduits remineralizes organic matter and oxidizes sulfide thereby lowering pH and causes calcite to dissolve, which releases mineral-bound phosphorous. Additional phosphorous is released along with nitrogen with the oxidation of organic matter. Nutrients flow to the lagoon when discharge resumes and may help limit degradation of coral reefs from discharge of low pH water. These effects may be limited as increased intrusions with sea level rise shifts discharge locations landward and reduces nutrient fluxes to reefs. Our observations illustrate the vulnerability of coastal karst aquifers and ecosystems, including reefs, to sea level rise.