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In order to determine the efficiency of in situ carbon recycling in tropical mangrove sediments, and the contribution of porewater dissolved organic carbon (DOC) to surface-sediment bacterial production, concentrations and fluxes of DOC were measured and compared with simultaneous estimates of bacterial densities, productivity and specific growth rates, in a mangrove forest in Queensland, Australia. The range of interstitial DOC concentrations (4 to 50mgC 1 ' ) was similar to that reported for temperate marine sediments although seasonal variations were much less apparent. Soluble tannins constituted 2 to 51 % of the interstitial DOC pools, which varied significantly with tidal elevation (high > mid > low intertidal) and showed significant variation with sediment depth (to 1 m) only in the mid intertidal. Bacterial densities and activity in surface sediments (to 1 cm) were high (annual range in productivity: 0.9 to 4.1 gC m-* d ' ) , consistent with previous estimates at this site. Surface (1 cm depth) sediment DOC concentrations varied with elevation in the intertidal zone (10 sites from low to high intertidal), and showed no correspondence with seasonal or spatial variations in bacterial parameters, except for a significant correlation (r= 0.71, n = 10) between DOC and bacterial growth rate, in austral autumn, for samples throughout the tidal range. Despite the high concentration gradient of DOC between porewaters and overlying tidal waters, significant efflux of DOC was rarely detected unless sediments were poisoned, to 1 cm depth, with mercury, which resulted in significant and generally high rates of efflux. Efflux rates varied widely with tidal elevation and season (range: 0 to 2 . 4 gC m-' d" ' ) These DOC flux rates provide, on average, 35 % (range: 0 to 100 O h ) of bacterial productivity requirements at the sediment-water interface. This study suggests that the mangrove sediments function as an efficient sink for DOC and that its export from the system is minor.

Dissolved organic carbon-bacteria interactions at sediment-water interface in a tropical mangrove system