The cycling and oxidation pathways of organic carbon in a shallow estuary along the Texas Gulf Coast

Abstract

The cycling and oxidation pathways of organic carbon were investigated at a single shallow water estuarine site in Trinity Bay, Texas, the uppermost lobe of Galveston Bay, during November 2000. Radio-isotopes were used to estimate sediment mixing and accumulation rates, and benthic chamber and pore water measurements were used to determine sediment-water exchange fluxes of oxygen, nutrients and metals, and infer carbon oxidation rates. Using 7Be and 234Th XS, the sediment-mixing coefficient ( D b) was 4.3 ± 1.8 cm 2 y −1, a value that lies at the lower limit for marine environments, indicating that mixing was not important in these sediments at this time. Sediment accumulation rates ( S a), estimated using 137Cs and 210Pb XS, were 0.16 ± 0.02 g cm −2 y −1. The supply rate of organic carbon to the sediment-water interface was 30 ± 3.9 mmol C m −2 d −1, of which ∼10% or 2.9 ± 0.44 mmol C m −2 d −1was lost from the system through burial below the 1-cm thick surface mixed layer. Measured fluxes of O 2 were 26 ± 3.8 mmol m −2 d −1 and equated to a carbon oxidation rate of 20 ± 3.3 mmol C m −2 d −1, which is an upper limit due to the potential for oxidation of additional reduced species. Using organic carbon gradients in the surface mixed layer, carbon oxidation was estimated at 2.6 ± 1.1 mmol C m −2 d −1. Independent estimates made using pore water concentration gradients of ammonium and C:N stoichiometry, equaled 2.8 ± 0.46 mmol C m −2 d −1. The flux of DOC out of the sediments (DOC efflux) was 5.6 ± 1.3 mmol C m −2 d −1. In general, while mass balance was achieved indicating the sediments were at steady state during this time, changes in environmental conditions within the bay and the surrounding area, mean this conclusion might not always hold. These results show that the majority of carbon oxidation occurred at the sediment-water interface, via O 2 reduction. This likely results from the high frequency of sediment resuspension events combined with the shallow sediment mixing zone, leaving anaerobic oxidants responsible for only ∼10–15% of the carbon oxidized in these sediments.