Minor water-level fluctuations strongly modulate air-water diffusive CH4 flux in a tributary bay of the Three Gorges Reservoir.

Minor water-level fluctuations are common in large, regulated reservoirs, yet their effects on air-water methane (CH4) emission in tributary bays remain unclear. We examined Xiangxi Bay (XXB) of the Three Gorges Reservoir (TGR) during three surveys spanning a drawdown-rise-drawdown sequence (0.19-0.25 m d-1) near the end of the TGR' annual drawdown. Vertical profiling of dissolved CH4 (CCH4), δ13C-CH4, environmental variables, velocity were combined with estimates of diffusive CH4 fluxes (FCH4). Minor fluctuations did not change the downstream-to-upstream pattern of FCH4 in XXB, but mean FCH4 during drawdown (WLDP) reached 55.38 μg m-2 h-1, 17.4 times the value during the water-level rise (WLRP; 3.18 μg m-2 h-1), the ratio reached 31.3 in the upstream. During WLRP, mixing depth (Dm) increased from 2.80 to 13.07 m, stratification stability (Rh) decreased from 8.07 to 4.21 m-1, and CCH4 fell to one-twelfth of the WLDP value. Velocity fields indicated that mainstem intrusion shoaled upstream during WLDP, potentially redistributing CH4-enriched bottom water toward the bay upstream surface. Layered water exchange during WLRP retained this water at depth and favored dilution and oxidation. Less negative surface CH4 stable carbon isotopes (δ13C-CH4) signature was consistent with oxidation, although water mass mixing may also have contributed. Stratification stability (Rh) was the most consistent predictor of FCH4; chlorophyll a (Chl.a) and total phosphorus (TP) provided additional predictive information during WLDP. Minor fluctuations can change diffusive CH4 flux by more than an order of magnitude without altering its longitudinal pattern. CH4 budgets for large riverine reservoirs should resolve short water-level rising and falling stages. Mitigation through water-level regulation still requires tests of fluctuation amplitude, rate, and duration.