Role of Bioroughness, Bioirrigation, and Turbulence on Oxygen Dynamics at the Sediment-Water Interface

Models that have been developed to quantify the oxygen flux at the sediment-water interface (SWI) generally do not explicitly consider the influence of bioroughness (mounds and burrows) and bioirrigation. We performed a numerical study of the influence of overlying water velocity, bioroughness, and bioirrigation on the oxygen flux across the SWI. We found that compared with a flat bed, bioroughness significantly increases O₂ transport at the SWI as a result of enhanced turbulence and pressure differences across the roughness. Bioirrigation can also enhance O₂ transport across the SWI by a factor of up to 10 when the roughness Reynolds number (Re_*) is low, but the influence of bioirrigation decreases with increasing Re_*. Burrows increase O₂ penetration depth, and bioirrigation causes asymmetric distributions of O₂ along burrows. Despite the complexity of O₂ distribution in sediments, the net exchange across the SWI can be described by the relationship of O'Connor and Harvey (2008, https://doi.org/10.1029/2008WR007160) (Formula presented.) when the shape is two-dimensional or when the burrow density is low. When the burrow density is large, flow is three-dimensional and flow interactions between burrows become important. Under these conditions the net exchange across the SWI increases by up to a factor of 4. A burrow number is introduced, Bu = [burrow density]^1/2 [burrow height], to correct the coefficient in O'Connor and Harvey's relationship, that is, a = 0.005 for Bu < 0.05 and a = 0.02 for Bu ≫ 0.1.