Nocturnal methane accumulation in the pycnocline of an anoxic estuarine basin

Diel studies of vertical methane distributions were made on seven dates during five field expeditions over a period of 3 yr to the permanently anoxic southern basin of the Pettaquamscutt River estuary. On five occasions, methane concentrations were constant over a diel cycle, indicating balanced production and consumption. On two other occasions, methane concentrations varied dramatically between sunset and sunrise, increasing (and then decreasing) by up to several µmoles per liter between sunset and sunrise in the chlorophyll-rich pycnocline. Our observations on these two dates require a nocturnal imbalance between methane production and methane oxidation. Data from 1992 suggest that production exceeds oxidation in the early part of the evening and that oxidation exceeds production after around midnight. On other dates, production and consumption are balanced, although turnover rates could still be high. Methane concentration variations that occurred nocturnally did not seem to be associated with tidal or advective effects, as comparisons were made on density horizons, and very little lateral variability in methane concentrations was found along a longitudinal transect of the basin. Methane profiles taken during the day were essentially identical during each 2-week experiment. In all cases, methane concentrations in the surface waters above the pycnocline were ∼0.1–0.2 µM and remained constant over the sampling period.