Hydrologic Controls of Methane Dynamics in Karst Subterranean Estuaries
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AbstractKarst subterranean estuaries (KSEs) extend into carbonate platforms along 12% of all coastlines. A recent study has shown that microbial methane (CH4) consumption is an important component of the carbon cycle and food web dynamics within flooded caves that permeate KSEs. In this study, we obtained highresolution (~2.5day) temporal records of dissolved methane concentrations and its stable isotopic content (13C) to evaluate how regional meteorology and hydrology control methane dynamics in KSEs. Our records show that less methane was present in the anoxic fresh water during the wet season (4,36189nM) than during the dry season (5,949132nM), suggesting that the wet season hydrologic regime enhances mixing of methane and other constituents into the underlying brackish water. The 13C of the methane (38.11.7) in the brackish water was consistently more 13Cenriched than fresh water methane (65.40.4), implying persistent methane oxidation in the cave. Using a hydrologically based mass balance model, we calculate that methane consumption in the KSE was 2128mgCH4m2year1 during the 6month dry period, which equates to ~1.4t of methane consumed within the 102 to 138km2 catchment basin for the cave. Unless wet season methane consumption is much greater, the magnitude of methane oxidized within KSEs is not likely to affect the global methane budget. However, our estimates constrain the contribution of a critical resource for this widely distributed subterranean ecosystem.
