Effects of short‐term variations in sea level on dissolved oxygen in a coastal karst aquifer, Quintana Roo, Mexico

Abstract

Sea‐level rise should cause salt‐water intrusion into coastal aquifers and limit fresh submarine groundwater discharge. Pargos Spring offshore of Puerto Morelos, Quintana Roo, Mexico, intermittently discharges brackish water and allows intrusion of lagoon water with seawater salinity to the aquifer. Lagoon water intrusion occurred when sea level was > 0.08 m above mean observed values during the study period. Salt water intrusion will be permanent within a few decades at the current eustatic sea‐level rise rate of ∼ 3 mm/yr. A mixing model demonstrates that oxygen dissolved in the lagoon water is reduced as it intrudes the spring. Dissolved oxygen (DO) reduction is greater at the spring vent than at sensors ∼ 10 m inside the conduit, reflecting rapid reaction kinetics. DO reduction results from organic carbon remineralization, which also releases N and P to the water. Increased frequency of intrusion events or continuous intrusion may alter microbially mediated biogeochemical reactions, thereby increasing aquifer vulnerability to sea‐level rise.

Organic matter quantity and quality across salinity gradients in conduit‐ vs. diffuse flow‐dominated subterranean estuaries

“…Submarine springs provide ~78.5% of the discharge at the Puerto Morelos lagoon while diffuse seepage from the beach shore face provides the remaining discharge (Beddows et al ; Null et al ). Submarine springs can reverse flow during extreme high tides, storm surges, wind set‐up or weakened Yucatan current, allowing surface seawater to intrude into conduits (Parra et al ; Young et al ). These events may occur daily and persist for several hours if tidally driven (Young et al ) or may persist for several days if due to storm surges or wind setup (Parra et al ).…”

“…Submarine springs can reverse flow during extreme high tides, storm surges, wind set‐up or weakened Yucatan current, allowing surface seawater to intrude into conduits (Parra et al ; Young et al ). These events may occur daily and persist for several hours if tidally driven (Young et al ) or may persist for several days if due to storm surges or wind setup (Parra et al ). Intrusion events create a brackish mixing zone between freshwater and salt water that extends approximately 1–4 km inland (Beddows et al ).…”

“…However, shorter residence times of 0.33 d may occur in the upper sediment layers of Indian River Lagoon (< 70 cm) because of bioirrigation (Martin et al ; Table ). In contrast, rapid groundwater flow in Yucatan conduits generates turbulent mixing zones that may extend across 100s of meters within the STE (Parra et al ; Young et al ; Table ). Despite the wide mixing zone compared to Indian River Lagoon, groundwater flow rates at the outlets of submarine springs have been measured at 0.1–0.5 m s −1 (Parra et al ), implying that water residence times may range from < 1 to 20 min within the freshwater–salt water mixing zone.…”

“…These interfaces result from gradients of TEA availability. For example, surface salt water contains reatively high concentrations of oxygen (Young et al ) and sulfate, whereas fresh groundwater may contain high concentrations of nitrate, but little dissolved oxygen or sulfate (Kroeger et al ; Kroeger and Charette ). OM remineralization mostly results from microbial processing; however, microbial metabolism of each TEA has a unique energy yield and leads to distinct reaction kinetics (Froelich et al ).…”

“…Although the extent of STE biogeochemical processes relative to residence times is poorly characterized, we hypothesize that long residence times will increase extents of biogeochemical reactions among water, solids, and microbes. We test this hypothesis through characterization of CDOM in STEs representing two hydrogeologic end members with disparate residence times: one with slow, widely distributed flow through siliciclastic sediments (Indian River Lagoon, Florida; e.g., Martin et al ) and another with rapid, point‐discharge through a carbonate karst aquifer (the Yucatan Peninsula, Mexico; Swarzenski et al ; Null et al ; Young et al ). We first assess changes in the quantity and quality of OM along salinity gradients and use salinity‐based conservative mixing models (CMMs) to evaluate CDOM dynamics in the two STE end members.…”