Variations in sedimentary carbon remineralization rates in the White Oak River estuary, North Carolina

Abstract

Rates of microbially mediated sedimentary organic carbon remineralization vary along the salinity gradient of the White Oak River estuary, N.C. A direct comparison of two sites, one upriver dominated by methanogenesis and one downriver dominated by sulfate reduction, indicates a more rapid rate of remineralization upriver. Measurements of diffusive ∑CO2 flux plus CH4 diffusive (56%) and bubble (44%) fluxes at the upriver, freshwater site from June 1986 to March 1988 yield an average yearly flux of 1.35 ± 0.35 mmol m−2 h−1. At the downriver midestuarine site, the dominant ∑CO2 flux for the same period is 0.46±0.02 mmol m−2 h−1. The uncertainties in these yearly fluxes are calculated from observed ranges in duplicate flux measurements. About 43% of the incoming organic C is remineralized at both sites. The greater remineralization upriver, of which 47% can be attributed to methanogenesis, appears to be supported by rapid decomposition of detritus derived from freshwater, emergent and submersed macrophytes, including Pontederia, Ceratophyllum, and Najas in surficial sediments. The high organic C content of these upriver sediments may, however, result from the long‐term storage of slower degrading plants, such as Typha and Taxodium.