Methane dynamics across a tidally flooded riverbank margin

Abstract

The cycling of methane across the soils of the tidally flooded bank margins in the tidal freshwater portion of the White Oak River estuary, North Carolina, was investigated from October 1990 to November 1991. A comparison between these bank edges and adjacent submerged stations was made. The bank stations showed large seasonal variations in methane production rates, with very little being produced in winter and large amounts (up to 1,000 mg m−2 d−1) during summer. This seasonality seems to be linked to the cycle of growth and senescence of vascular plants growing in these soils. In contrast, the submerged stations produced smaller amounts of methane year‐round, with moderate increases during summer. Vascular plants are not rooted in these submerged sediments.Seasonal variations in methane production rates at the tidally flooded bank stations led to significant changes in the dissolved methane pool sizes. Although production rates and pore‐water concentrations increased with seasonal increases in soil temperatures, the flux of methane to the atmosphere remained fairly low and variable. Methane flux varied with stage of the semidiurnal tidal cycle and exhibited a bimodal distribution, with fluxes greatest when the water level was nearest the soil surface. During summer months, methane production was up to 10 times greater than its flux to the atmosphere. This large difference between production and flux is attributed mainly to bacterially mediated methane oxidation at the soil surface during low‐tide exposure and(or) in the root zone of plants. However, in midsummer, lateral export of dissolved methane from the bank soils, driven by tidal flushing, may account for almost 30% of this discrepancy between production and flux.