NOCTURNAL METHANE ACCUMULATION IN THE PYCNOCLINE OF AN ANOXIC ESTUARINE BASIN

Diel studies of vertical methane distributions were made on seven date s during five field expeditions over a period of 3 yr to the permanent ly anoxic southern basin of the Pettaquamscutt River estuary. On five occasions, methane concentrations were constant over a diel cycle, ind icating balanced production and consumption. On two other occasions, m ethane concentrations varied dramatically between sunset and sunrise, increasing (and then decreasing) by up to several mu moles per liter b etween sunset and sunrise in the chlorophyll-rich pycnocline. Our obse rvations on these two dates require a nocturnal imbalance between meth ane production and methane oxidation. Data from 1992 suggest that prod uction exceeds oxidation in the early part of the evening and that oxi dation exceeds production after around midnight. On other dates, produ ction and consumption are balanced, although turnover rates could stil l be high. Methane concentration variations that occurred nocturnally did not seem to be associated with tidal or advective effects, as comp arisons were made on density horizons, and very little lateral variabi lity 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 concent rations in the surface waters above the pycnocline were similar to 0.1 -0.2 mu M and remained constant over the sampling period.