OXYGEN AND CARBON-DIOXIDE FLUXES AT THE WATER-SEDIMENT INTERFACE OF A TROPICAL LAGOON

Oxygen demand, carbon dioxide release and total alkalinity shift were calculated from changes in oxygen, pH and total alkalinity produced by bottom dark incubations at the water-sediment interface of the 3 bottom types identified in a southwest lagoon in New Caledonia. Total sediment oxygen demand (DELTAO2) was corrected from nonbiological oxygen demand (NBOD) in order to obtain the apparent biological activity (DELTAO2-degrees). Total carbon dioxide flux (DELTACO2) was corrected from total alkalinity shift in order to estimate organic carbon processes. The resulting mean carbon dioxide flux (DELTACO2-degrees = 2.58 mmol m-2 h-1, SE = 0.12) exceeded biological oxygen demand (DELTAO2-degrees 1.60 mmol m-2 h-1, SE = 0.08). The highly significant ratio estimates from functional regression lines of DELTACO2-degrees on DELTAO2 and DELTACO2-degrees on DELTAO2-degrees gave a total community respiratory quotient (CRQ) of 1. 17 (SE = 0.06) and an organic respiratory quotient (CRQ-degrees), involving only direct biological processes, of 1.42 (SE = 0.07) for the lagoon. The significance and the use of these ratios are discussed in order to calculate the anaerobic metabolism proportion (40.3 % of total metabolism for an aerobic respiratory quotient of 0.85). Thus, the simultaneous measurement in the field of O2 fluxes and CO2 fluxes, corrected from appropriate alkalinity changes, allows a rapid approach for estimating carbon production at the water-sediment interface of undisturbed communities.