ABSTRACT: Carbon and nitrogen mineralization were determined along a transect from a mangrove forest to a seagrass meadow in the Bangrong area, Phuket Island, Thailand. Vertical sediment profiles of carbon oxidation were measured as sulfate reduction rates (SRR) using the ³⁵S technique and by monitoring net TCO₂ and DOC production and Fe(III) reduction using anaerobic sediment incubations ('jar' technique). Nitrogen transformations were measured simultaneously as net NH₄⁺ and DON production. In addition, total benthic metabolism and net nitrogen exchange were determined as fluxes of O₂, TCO₂, DOC, and DIN (NO₃^- and NH₄⁺) across the sediment-water interface. Rates of carbon and nitrogen transformations in this vascular-plant (high C:N)-dominated area were low compared with areas fuelled by detritus of marine origin (low C:N). It appears that the high content of structural biopolymers (e.g. lignocelluloses) hampers microbial activity. Suboxic respiration with Fe(III) as electron acceptor accounted for 70 to 80% of the total carbon oxidation in the rooted mangrove forest sediment, whereas SRR and aerobic respiration were responsible for about 20 and <6%, respectively. The role of SRR decreased to about 10% and aerobic respiration increased to 45-65% in an adjacent bioturbated mudflat, while Fe(III) respiration decreased to 30-40%. At the sand flat and seagrass meadow outside the mangrove forest, Fe(III) respiration only accounted for 15 and ~0%, respectively, whereas SRR was responsible for 20 to 45% of the total carbon oxidation. However, the most important electron acceptor in the area outside was oxygen (55 to 75%). The shift in dominance of electron acceptors along the transect is primarily related to the presence of roots and infauna, but the sediment composition (e.g. grain size, organic content and iron content) is believed to be an important co-factor. The net production of ammonium in the sediment was not balanced by fluxes of DIN across the sediment-water interface. The missing nitrogen was assigned to a rapid and efficient bacterial ammonium assimilation at the sediment surface as indicated by ammonium turnover times of about 1 d.

KEY WORDS: Mangrove forest · Seagrass · Carbon · Nitrogen · Mineralization · Sulfate reduction · Fe(III) reduction · Benthic metabolism

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Published in AME Vol. 22, No. 2 (2000) on September 8
ISSN: 0948-3055. Copyright © Inter-Research, Oldendorf/Luhe, 2000