Effect of the diffusive boundary layer on benthic mineralization and O2 distribution: A theoretical model analysis

On the basis of a dynamic diagenetic model, we evaluate and discuss the effect of the diffusive boundary layer (DBL) on benthic O2 exchange and O2 consuming pathways. The analysis documents that the DBL has only minor importance for the annual O2 uptake of coastal cohesive sediments. Imposing static DBL thicknesses of 300–900 mm decreased the annual O2 uptake by only 2–10% in comparison to a situation without any DBL. Lower O2 availability as imposed by a thicker DBL, however, markedly reduced the aerobic heterotrophic respiration but enhanced aerobic reoxidation of solutes released by the stimulated anaerobic respiration. The 2– 10% decrease in the annual O2 uptake was caused mainly by higher benthic release rates of NH þ 4 and Mn 2+. The overall carbon degradation rate—and thus the carbon preservation—remained unaffected by the DBL thickness. Dynamic modeling revealed that abrupt changes in the DBL thickness caused an instantaneous change in the interstitial O2 distribution and in the benthic O2 uptake rate. However, conditions quickly reversed as the porewater profiles of reduced solutes and distribution of reduced solids readjusted even though full steady state was obtained only after several months. In nature the DBL is constantly changing, and thus in situ O2 microprofiles are transient by nature. Dynamic modeling showed that the benthic O2 concentration and the O2 uptake in Aarhus Bay could vary by 30% or more on time scales of a few hours or days solely because of changes in the DBL thickness. The integrated annual O2 uptake remained unaffected by these fluctuations. Solute transport in the water column is dominated by turbulent mixing. Close to the sediment–water interface, however, eddy transport is impeded by friction and viscous forces. The so-called viscous sublayer is defined as the height at which the turbulent mixing, expressed as eddy diffusivity, becomes smaller than the kinematic viscosity of ,1022 cm2 s21, which usually happens 5–10 mm above the sediment surface (Caldwell and Chriss 1979; Boudreau 2001). In this layer, viscous forces dominate, and in the lower part of this layer, the eddy diffusivity falls below the molecular diffusivity (D0) of ,1025 cm2 s21. For cohesive sediments this shift defines the upper boundary of the diffusive boundary layer (DBL). The DBL thickness typically accounts for one-tenth of the viscous sublayer thickness, but as it is defined by the molecular diffusion coefficient, it is solute dependent. Within the DBL, vertical solute transport is mediated mainly by molecular diffusion. The transport rate across the DBL can influence the rate of important biogeochemical processes such as calcite dissolution, growth of ferromaganese nodules, benthic contaminant release, and benthic O2 uptake (e.g., Boudreau and Scott 1978; Jørgensen and Revsbech 1985). The in situ DBL thickness is regulated by the dissipation rate of turbulent energy in the benthic boundary layer (Higashino et al. 2003; Lorke et al. 2003). In laboratory flow systems using moderate water heights and approximating laminar flow conditions, it has been shown that the DBL thickness of a given solute at a given salinity and temperature relates primarily to the free-flow velocity and the sediment roughness (Jørgensen and Des Marais 1990; Steinberger and Hondzo 1999). On the introduction of O2 microelectrodes in aquatic ecology, the existence of the DBL could be directly documented (Jørgensen and Revsbech 1985; Archer et al. 1989a), and a series of detailed studies of the short-term O2 dynamics within the DBL followed (e.g., Gundersen and Jørgensen 1990; Jørgensen and Des Marais 1990; Røy et al. 2004). From high-resolution O2 microprofiles based on averaged O2 concentrations measured at different positions within the DBL, it is possible to infer the effective DBL 1 Corresponding author ([email protected]). Acknowledgments We thank Anni Glud for her assistance during laboratory measurements and Clare Reimers and Bernard Boudreau who constructively reviewed the manuscript. The study was supported financially by the Danish Research Agency (RNG) and the EU commission (via the project COBO, GOCE-CT-2003-505564). Limnol. Oceanogr., 52(2), 2007, 547–557 E 2007, by the American Society of Limnology and Oceanography, Inc.