SULFATE REDUCTION, ACETATE TURNOVER AND CARBON METABOLISM IN SEDIMENTS OF THE AO-NAM-BOR MANGROVE, PHUKET, THAILAND

Rates of sediment O2 uptake, CO2 production, sulfate reduction and ace tate turnover were examined during January 1992 in the Ao Nam Bor mang rove, Phuket, Thailand. The impact of air exposure on O2 uptake was mo st pronounced in the intensely bioturbated high-intertidal zone (6.4 t imes higher than during water cover), and decreased to almost zero in the low-intertidal zone. This indicates a gradual increased area of se diment-air contact zones with tidal elevation due to changes in surfac e topography. Based on an average water cover for January, the diurnal O2 uptake - and thus total decay of deposited detritus - was 4 to 5 a nd 8 times faster in the high-intertidal compared to the mid- and low- intertidal zones, respectively. Sulfate reduction rates were generally low. The depth-integrated (0 to 30 cm) sulfate reduction was highest in the mid-intertidal zone, and supported 85 % of the estimated daily CO2 release. In the high- and low-intertidal zones, sulfate reduction supported 11 and 92 %, respectively, of daily CO2 release. Rates of ac etate uptake were also higher in the mid-than in the low-intertidal zo ne (no data from high-intertidal). However, the depth-integrated aceta te uptake was consistently about 2.6 times the rates of CO2 release an d 5 to 6 times the 0 to 11 cm integrated sulfate reduction, which sugg ests that pool sizes of acetate and thus uptake rates may have been ov erestimated. In conclusion, while benthic respiration in the mid- and low-intertidal zones of the Ao Nam Bor mangrove was dominated by sulfa te reduction with acetate as carbon source, 'suboxic' conditions relat ed to bioturbation in the active high-intertidal sediment made respira tion by other electron acceptors than SO42- more important.