Mineralization of Organic Matter in Intertidal Sediments of a Tropical Semi-enclosed Delta

Abstract

Rates and pathways of organic matter decomposition in sediments were examined during winter and summer in two mangrove forests and two accreting mud flats in Hinchinbrook Channel, north-eastern Australia. Rates of O2consumption (range: 2·8–61·0mmolm−2day−1) and CO2release (range: 1·8–21·9mmolm−2day−1) were faster in winter than in summer. Low respiratory quotients (CO2/O2range: 0·24–1·08) and a comparison of other metabolic pathways with total carbon oxidation (Tcox) in summer suggests that most O2was used in oxidizing reduced solutes. Rates of total carbon oxidation were greater in the mangrove sediments than in the mud flats. In winter, sulphate reduction in the mangroves (range: 4·9–8·3mmolSm−2day−1) accounted for a much greater proportion (45–78%) of Tcoxthan in the mud flats where sulphate reduction (range: 0·8–1·0mmolSm−2day−1) was a minor (12–14% of Tcox) metabolic pathway. In summer, sulphate reduction accounted for a greater (62–>100%) proportion of Tcoxin both mangrove (range: 7·9–10·1mmolSm−2day−1) and mud flat (range: 0·8–2·3mmolSm−2day−1) sediments. Rates of denitrification in summer were rapid but highly variable at all four sites (range: 2·9–6·9mmolN2m−2day−1). Rates of Fe and Mn reduction were slow at all four sites (range: 0·0–0·27mmolCm−2day−1) suggesting that metal reduction was a minor decomposition pathway. No methane was detected in the porewater or released from sediments at any of the sites. There was no net microalgal primary production at any of the sites. Mineralization efficiency of organic carbon was low (9–14%) at the most sheltered mud flat, but efficiencies were greater (44–60%) and nearly equivalent at less sheltered mud flat and mangrove habitats, respectively. Imbalances of mineralized carbon between the sum of the various pathways and Tcox, particularly at the mangrove sites, suggest unaccounted losses of dissolved carbon. Large seasonal changes in porewater Cl−concentrations and lack of solute concentration differences with overlying tidal water, imply dilution and tidal advection and/or drainage of interstitial water by intense summer rains.