Karst 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 high‐resolution (~2.5‐day) temporal records of dissolved methane concentrations and its stable isotopic content (δ13C) 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 (4,361 ± 89 nM) than during the dry season (5,949 ± 132 nM), suggesting that the wet season hydrologic regime enhances mixing of methane and other constituents into the underlying brackish water. The δ13C of the methane (−38.1 ± 1.7‰) in the brackish water was consistently more 13C‐enriched than fresh water methane (−65.4 ± 0.4‰), implying persistent methane oxidation in the cave. Using a hydrologically based mass balance model, we calculate that methane consumption in the KSE was 21–28 mg CH4·m−2·year−1 during the 6‐month dry period, which equates to ~1.4 t of methane consumed within the 102‐ to 138‐km2 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.

The relatively high cost of acoustic current meters has placed practical limits on the use of these meters in marine research projects. One approach to reducing the cost of current measurements has been the re-invention of the Tilt Current Meter (TCM). Recent developments in accelerometers, magnetometers and low-power non-volatile memory have made it possible to make small, accurate and inexpensive TCMs. Here we describe the design and calibration of the Lowell Instruments TCM and validate its performance in Vineyard Sound, Massachusetts. The performance of four TCMs was compared to an acoustic Doppler current profiler reference meter. The TCMs exhibited an R squared correlation of 0.97-0.98 as well as low slope and offset errors. Based on this demonstrated accuracy we believe the TCM would be useful for a variety of research projects especially where there is a risk of instrument loss or where increased spatial density of measurements is desired.