Evaluating oxygen fluxes using microprofiles from both sides of the sediment–water interface

Lee D. Bryant, Daniel F. McGinnis, Claudia Lorrai, Andreas Brand, John C. Little, Alfred Wüest

Limnol. Oceanogr. Methods 8:610-627 (2010) | DOI: 10.4319/lom.2010.8.0610

ABSTRACT: Sediment–water fluxes are influenced by both hydrodynamics and sediment biogeochemical processes. However, fluxes at the sediment–water interface (SWI) are almost always analyzed from either a water- or sediment-side perspective. This study expands on previous work by comparing water-side (hydrodynamics and resulting diffusive boundary layer thickness, δ_DBL) and sediment-side (oxygen consumption and resulting sediment oxic zone) approaches for evaluating diffusive sediment oxygen uptake rate (J_O2) and δ_DBL from microprofiles. Dissolved oxygen microprofile and current velocity data were analyzed using five common methods to estimate J_O2 and δ_DBL and to assess the robustness of the approaches. Comparable values for J_O2 and δ_DBL were obtained (agreement within 20%), and turbulence-induced variations in these parameters were uniformly characterized with the five methods. J_O2 estimates based on water-side data were consistently higher (+1.8 mmol m^–2 d^–1 or 25% on average) and DBL estimates correspondingly lower (-0.4 mm or 35% on average) than those obtained using sediment-side data. This deviation may be attributed to definition of the sediment–water interface location, artifacts of the methods themselves, assumptions made on sediment properties, and/or variability in sediment oxygen-uptake processes. Our work emphasizes that sediment-side microprofile data may more accurately describe oxygen uptake at a particular location, whereas water-side data are representative of oxygen uptake over a broader sediment area. Regardless, our overall results show clearly that estimates of J_O2 and δ_DBL are not strongly dependent on the method chosen for analysis.