Carbon and nitrogen cycling on intertidal mudflats of a temperate Australian estuary. I. Benthic metabolism

The light and dark inundated fluxes of O2 and total CO2 (TCO2), as well as the concentrations of chlorophyll a and phaeopigments, were measured (ex situ) on the upper and lower portions of 2 intertidal mudflats—1 in the upper Huon Estuary (salinity 4 to 32) and 1 in a marine side-arm of the estuary (salinity 17 to 34)—over 4 seasons. Dark-exposed fluxes of O2 and CO2 were also measured on the upper and lower mudflats of both sites over 2 seasons. Exposed fluxes of O2 were generally not significantly different to the fluxes measured during inundation. Exposed fluxes of CO2 were generally 3 to 5 times lower than inundated fluxes of TCO2. At the more sheltered site in the upper estuary, significantly greater rates of primary production were measured on the upper mudflat. In contrast, the more marine site had lower rates of primary production, and no significant difference in rates of primary production were observed across the inundation gradient. It is proposed that a greater exposure to wave energy (as indicated by sediment grain size) at the marine site was the cause of the lower rates of primary production. Rates of TCO2 consumption in the light were generally greater than those of O2 production. It is suggested that O2 effluxes are greatly reduced in the light as a consequence of the re-oxidation of sulphides within the sediments.