Erik ~xistensen', Marianne ~o l m e r ' , Nipavan ~u s s a r a w i t~Institute of Biology, Odense University. DK-5230 Odense M, Denmark * Phuket Marine Biological Center. PO Box 60. Phuket. 83000 Thailand ABSTRACT: Sediment metabolism (surface O2 uptake and CO2 production) and 35S-S0z-reduction (0 to 10 cm) were measured during January 1990 (dry season) a t 3 stations in the Ao Nam Bor mangrove, Phuket, Thailand. Sulfate reduction as measured by the chromium reduction technique was highest at 6 to 10 cm depth at all stations (600 to 800 nmol S dC1), indicating that significant activity may have occurred below 10 cm. Vertical translocation of metabohzable organic substrates, either due to subsurface root growth or due to downward transport of newly deposited organlc matter by bioturbation appeared responsible for the high subsurface activlty The major reduced sulfur compound at all 3 statlons was FeS2, both as recovered 35S-label In the sulfate reduction assay (82 to 97 O/O) and in the total sediment sulfur pool (86 to 100 %). Very low amounts of reduced sulfur were found as HS-, FeS and So. The general absence of HS-was caused by preclpltatlon reactions controlled by a large sedimentary Iron pool. The estimated area1 based rates of sulfate reductlon (0 to 10 cm, 28 to 50 mm01 m-'d-l) could support around 100 % (65 to 142 O/O) of the measured CO2 flux across the sediment-water interface. This implies a deficient or non-steady state COz flux since the sulfate reductlon est~mate ignored any activity occurring below 10 cm depth. Possible causes for thls deficlt may include CO2 assimilation by roots, advective loss of pore water COz, and excessive CO2 gas loss during alr exposure. In general, the mechanisms underlying carbon and sulfur cycling In thls mangrove swamp appears remarkably similar to those operating in salt marshes.

Rates of sediment O2 uptake, CO2 production, sulfate reduction and acetate turnover were examined during January 1992 in the Ao Nam Bor mangrove, Phuket, Thailand. The impact of air exposure on O2 uptake was most pronounced in the intensely bioturbated high-intertidal zone (6.4 times higher than during water cover), and decreased to almost zero in the low-intertidal zone. This indcates a gradual increased area of sediment-air contact zones with tidal elevation due to changes in surface topography. Based on an average water cover for January, the diurnal O2 uptake -and thus total decay of deposited detritus -was 4 to 5 and 8 times faster in the high-intertidal compared to the mid-and low-intertidal zones, respectively. Sulfate reduction rates were generally low. The depthintegrated (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 acetate uptake were also higher in the midthan in the low-intertidal zone (no data from high-intertidal) However, the depth-integrated acetate uptake was consistently about 2.6 times the rates of CO, release and 5 to 6 times the 0 to 11 cm integrated sulfate reduction, which suggests that pool sizes of acetate and thus uptake rates may have been overestimated. In conclusion, while benthic respiration in the mid-and low-intertidal zones of the Ao Nam Bor mangrove was dominated by sulfate reduction with acetate as carbon source. 'suboxic' conditions related to bioturbation in the active high-intertidal sediment made respiration by other electron acceptors than SO.,-more important.

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