AME 20:203-212 (1999)  -  doi:10.3354/ame020203

Transformation and exchange processes in the Bangrong mangrove forest-seagrass bed system, Thailand. Seasonal and spatial variations in benthic metabolism and sulfur biogeochemistry

M. Holmer1,*, F. Ø. Andersen1, N. Holmboe1, E. Kristensen1, N. Thongtham2

1Institute of Biology, Odense University, SDU, Campusvej 55, 5230 Odense M, Denmark
2Phuket Marine Biological Center, PO Box 60, 83000 Phuket, Thailand

ABSTRACT: Sediment cores from 6 stations along a mangrove forest-seagrass bed transect in the Bangrong area, Thailand, were studied for sediment metabolism (TCO2 production and O2 uptake) and sulfur cycling (sulfate reduction rates and inorganic sulfur pools) in wet and dry seasons over a 2 yr period. Total sediment metabolism, measured as TCO2 production, was highest at the mangrove forest sites and decreased along the transect. Sulfate reduction, however, increased along the transect and attained highest values in the seagrass sediments. Sulfate reduction was generally less important in terminal degradation of organic matter (<2 to 44%) in mangrove forests compared to subtidal sediments in general, and redox potentials revealed a relatively high oxidation level of the sediments along the transect (>0 mV), favoring mineralization processes that employed more oxidized electron acceptors than sulfate. The very low sulfate reduction rates at high and mid-intertidal stations in the mangrove forest were probably caused by oxidation of the sediments due to prolonged air exposure during low tide, an abundant crab population with deep burrows and release of oxygen from roots. There were no major differences among seasons in sediment metabolism, whereas the vegetated sites experienced sulfate reduction activity up to 4 times higher during the wet compared to dry season. Sulfur pools were low in the Bangrong mangrove forest compared to other mangrove sites; lowest pools were coincident with high tidal elevation and presence of rooted vegetation, suggesting that the oxidation processes controlled the pool sizes rather than the sulfate reduction rates. There were no major seasonal effects on the sulfur pools along the transect.

KEY WORDS: Sediments · Mangrove forest · Seagrass beds · Sulfur cycling · Sulfate reduction

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