Effect of exported mangrove litter on bacterial productivity and dissolved organic carbon fluxes in adjacent tropical nearshore sediments

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Abstract
Mangrove litter is exported from tidal forests to the adjacent subtidal seabed in tropical North Queensland, Australia. Its effect on sediment characteristics and on the dynamics of benthic bacterial communities and dissolved organic carbon (DOC) fluxes was examined. Averaged over stations and seasons, mean bacterial densities in surface (0 to 1 cm) sediments ranged from 0.5 to 20.8 .times. 1010 cells g-1 dry weight; bacterial productivity and daily specific growth rates ranged from 0.02 to 1.86 g C m-2 d-1 and from 0.004 to 0.79 d-1, respectively. Interstitial DOC concentrations ranged from 2.4 to 6.5 mg C l-1 and varied with season but not among stations. Unless sediments were poisoned with mercury, significant net efflux of DOC across the sediment-water interface was not detected despite high concentration gradients of DOC between overlying water and porewaters. Poisoning of the benthic fauna with mercury resulted in generally high rates of DOC release (range: 0 to 2.4 g C m-2 d-1). On average, DOC fluxes across the sediment-water interface provided 51 % (range: 11 to 83%) of bacterial productivity requirements in surface sediments. In laboratory experiments, addition to outwelled mangrove litter (C:N = 52:1) to incubated boxcore samples did not significantly affect bacterial densities, production or growth, and DOC fluxes. Our results indicate that mangrove litter exported to the adjacent nearshore of this region is highly refractory and of ppor nutritional quality, with enhancement of bacterial activity and DOC fluxes apparent only in a semi-enclosed area of highest litter deposition. Rates of litter deposition appear to be low, but sufficient to significantly influence sediment characteristics, e.g. C:N ratios and POC-tannin concentrations. However, subsurface burial and accumulation of litter may, via long-term degradation, support the highly abundant and productive bacterial communities and efficient bacteria-DOC recycling observed in this region.