Environ. Sci. Technol. 2008, 42, 8611–8612
Response to Comment on “Enhancement and Inhibition of Denitrification by Fluid-Flow and Dissolved Oxygen Flux to Stream Sediments” We appreciate the comments and the analysis of our recent manuscript (1) presented by Petrie and Diplas (2). We agree that the boundary roughness condition of sediments can be used to demonstrate the threshold behavior for denitrification and offer the following comments regarding the interpretation of physical mass transfer effects on biogeochemical processes such as dissolved oxygen (DO) uptake and denitrification. Characterizing the fluid-flow conditions using a “boundary” Reynolds number (Re/ ) u/ks/ν, where u/ is the shear stress velocity, ks is the roughness height, and ν is kinematic viscosity) as suggested by Petrie and Diplas depicts two distinct regions relating to the ratio of DO to nitrate (NO3-) flux, as shown in Figure 1 of ref 2. The ratio of DO to NO3flux was constant for Re/ < 4, and increased for Re* > 4. This representation of the fluid-flow conditions clearly demonstrates the threshold condition where increased DO flux inhibits denitrification rates. For Re/ < 4, the constant flux ratio suggests that the flux of DO and NO3- is limited only by the physical mass transfer from the water column to the sediments. The increasing flux ratio for Re/ > 4 is caused by the kinetic uptake limitation of NO3- imposed by the increasing DO flux. This threshold condition for the inhibition of denitrification occurred at Re/ ≈ 4, which happens to be near the value defining the difference between hydraulically smooth and transitionally rough boundaries. However, one should be cautious in inferring that a hydraulically smooth to a transitionally rough boundary criterion directly predicts the threshold for denitrification inhibition by DO flux. The fluxes of DO and NO3- from the water column to the sediments are impacted by both physical mass transfer and the biogeochemical uptake processes within the sediments, and that resistance to flux (limitation of mass transfer or kinetics) can occur on both the water-side and the sedimentside of the interface. The physical mass transfer of a solute across the sediment-water interface is governed by fluidflow conditions above the interface and porous media transport in the sediments. The experimental conditions of our study did not facilitate resistance to mass transfer within the sediments, which can be caused by low permeability sediments or groundwater discharge. In terms of the potential kinetic limitations for DO and NO3-, it is important to consider the biogeochemical processes involved and the environmental variables that affect these uptake rates. DO uptake in aquatic sediments is a combination of biotic respiration, chemical oxidation, and decomposition of organic material. In general, respiration and decomposition dominate DO uptake in near-surface sediments where microbial populations and organic material concentrations are highest (e.g., 3), whereas chemical oxidation of DO can occur deeper in the sediments where reduced metals are formed (4). Increasing the physical mass transfer of DO from the water column to the sediments stimulates respiration and decomposition resulting in greater DO uptake rates in near-surface sediments as shown in a and b in Figure 5 of ref 1. However, there is an inherent uptake capacity associated with the conditions of the sediment (i.e., microbial populations, organic material, particle size, and geochemistry) that once surpassed, the DO penetrates deeper into the sediments 10.1021/es802816z CCC: $40.75
Published on Web 10/18/2008
2008 American Chemical Society
and the DO concentration at the sediment-water interface increases. Denitrification is carried out by facultative aerobes, so the zone for denitrification tends to occur just below the DO penetration depth. Previous studies have shown that denitrification rates decrease as DO penetrates deeper into the sediments caused by the increased transport length required of NO3- to travel before reaching the denitrification zone (e.g., 5). Also, heterotrophic bactrial populations (as well as denitrifiers, Figure 4 in ref 1) and organic matter concentrations tend to decline with depth in the sediments (3), which can also limit denitrification rates. The threshold condition for DO flux enhancement and inhibition of denitrification is related to the physical mass transfer of solutes across the sediment-water interface. In our study, overcoming mass transfer limitations by increasing the flow velocity in the water column increased both DO and NO3- uptake rates as the increased delivery of DO increased respiration rates, which enhanced denitrification by stimulating the heterotrophs in the nearsurface sediments that can rapidly switch from DO to NO3respiration once the DO is consumed (6). With further increases in the flow velocities of the water column, the DO uptake capacity was surpassed in the near-surface sediments and resulted in greater DO penetration depths and inhibition of denitrification rates by the processes described above. Therefore, the true control on the inhibition of denitrification is the inherent DO uptake capacity, which is partly affected by physical mass transfer, but also by environmental variables of the sediment. In our manuscript, we chose to plot our results as a function of increasing fluid-flow conditions described using u/ and Reynolds number (Re ) UH/ν, where U is the mean velocity and H is the half-water depth), which demonstrated both the enhancement and inhibition of dentrification rates. Although the normalized fluxes and roughness height presentation proposed by Petrie and Diplas does a better job of depicting the threshold conditions where the onset of denitrification inhibition by DO flux begins, it does not show the enhancement of denitrification under lower fluid-flow conditions. It is often assumed that enhanced nutrient uptake results from lower velocities caused by increased contact time between solutes and microbes; however, we show that there is an optimal range in velocities that is needed to overcome mass-transfer limitations as well as initiate heterotrophic activity. In conclusion, we agree with Petrie and Diplas’s assertion that there is a need for further investigation with a variety of sediments and fluid-flow conditions, and that detailed velocity data near the sediments are vital for interpretation of the results. We also point out that continued studies of biogeochemical gradients across the sediment-water interface relating to denitrification will also need to consider the environmental variables affecting the inherent DO uptake capacity. To truly understand the inhibition of denitrification by DO flux will require that we are able to define and quantify the intrinsic DO uptake capacity of sediments.
Literature Cited (1) O’Connor, B. L.; Hondzo, M. Enhancement and inhibition of denitrification by fluid-flow and dissolved VOL. 42, NO. 22, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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oxygen flux to stream sediments. Environ. Sci. Technol. 2008, 42, 119–125. (2) Petrie, J.; Diplas, P. Comment on “Enhancement and inhibition of denitrification by fluid-flow and dissolved oxygen flux to stream sediments”. Environ. Sci. Technol. 2008, 22, 8609–8610. (3) Fischer, H.; Kloep, F.; Wilzcek, S.; Pusch, M. T. A river’s livermicrobial processes within the hyporheic zone of a large lowland river. Biogeochemistry 2005, 76, 349–371. (4) Jørgensen, B. B.; Boudreau, B. P. Diagenesis and sedimentwater exchange. In The Benthic Boundary Layer: Transport Processes and Biogeochemistry; Boudreau, B. P., Jørgensen, B. B., Eds.; Oxford University Press: London, 2001; pp 211244.
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(5) Nielsen, L. P.; Christensen, P. B.; Revsbech, N. P.; Sørensen, J. Denitrification and photosynthesis in stream sediments studied with microsensor and whole core techniques. Limnol. Oceanogr. 1990, 35, 1135–1144. (6) John, P. Aerobic and anaerobic bacterial respiration monitored by electrodes. J. Gen. Microbiol. 1977, 98, 231–238.
Ben L. O’Connor and Miki Hondzo St. Anthony Falls Laboratory, Department of Civil Engineering, University of Minnesota-Twin Cities, 2 Third Avenue Southeast, Minneapolis, Minnesota 55414 ES802816Z
