MEPS 292:61-74 (2005)  -  doi:10.3354/meps292061

Control of the diffusive boundary layer on benthic fluxes: a model study

B. A. Kelly-Gerreyn*, D. J. Hydes, J. J. Waniek

National Oceanography Centre, Southampton, University of Southampton, Waterfront Campus, European Way,Southampton SO14 3ZH, UK

ABSTRACT : A simple, steady state, reaction-diffusion diagenesis model is used to quantify the possible error associated with benthic flux measurements which neglect the presence of the diffusive boundary layer (DBL). Model application is restricted to non-bioturbated, fine-grained sediments in which oxygen consumption is dominated (~65% of the consumption budget) by organic carbon degradation, oxygen penetration depths are low (<0.5 cm) and solute exchange across the sediment-water interface (SWI) is diffusive. The effect of different thicknesses of the DBL is tested on sediments with different organic carbon reactivities (k = 1, 5, 10, 20, 40 yr–1). When imposing the range of DBL thicknesses observed in nature (0.01 to 0.1 cm) on the model, the model simulates lower oxygen fluxes (by up to 22% for k = 40 yr–1) across the SWI compared to fluxes simulated in the absence of a DBL. Greater reactivity increases the impact of the DBL by lowering the oxygen penetration depth. Changes in the DBL directly influence oxygen fluxes and aerobic mineralisation by changing the diffusion path length to a relatively thin oxic sediment layer. The changes in anaerobic processes are small (<8% for denitrification and <3% for sulphate reduction) and, together with the associated solute fluxes (nitrate, sulphate, ammonium) across the SWI, occur in response to changes in porewater oxygen concentrations induced by the DBL, rather than by direct interactions with the DBL. Changes included a 380% increase in nitrate influxes and a 90% reduction in nitrate effluxes. Rates of nitrification decreased by up to 18%. Thicker DBLs also decreased the organic carbon degradation rate by a maximum of 22%, implicating the DBL as a factor in organic carbon preservation for highly reactive sediments. Measurements of near-bed currents in a macro-tidal estuary (Southampton Water, UK) suggest that the observed range in DBL thicknesses can exist for up to 31% of the time (sampling period: 3 to 4 mo). The presence of these DBL thicknesses in such a dynamic environment, makes it is reasonable to assume that the establishment of the DBL may be widespread. Consequently, it is only reasonable to neglect the DBL over sediments in which aerobic mineralisation is dominant, when organic reactivity is low.

KEY WORDS: Diffusive boundary layer · Benthic fluxes · Organic carbon reactivity · Earlydiagenesis model

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