Variable sediment oxygen uptake in response to dynamic forcing
Seiche-induced turbulence and the vertical distribution of dissolved oxygen above and within the sediment were analyzed to evaluate the sediment oxygen uptake rate (J(O2)), diffusive boundary layer thickness (delta(DBL)), and sediment oxic zone depth (z(max)) in situ. High temporal-resolution microprofiles across the sediment-water interface and current velocity data within the bottom boundary layer in a medium-sized mesotrophic lake were obtained during a 12-h field study. We resolved the dynamic forcing of a full 8-h seiche cycle and evaluated J(O2) from both sides of the sediment-water interface. Turbulence (characterized by the energy dissipation rate, epsilon), the vertical distribution of dissolved oxygen across the sediment-water interface (characterized by delta(DBL) and z(max)), J(O2), and the sediment oxygen consumption rate (R(O2)) are all strongly correlated in our freshwater system. Seiche-induced turbulence shifted from relatively active (epsilon = 1.2 x 10(-8) W kg(-1)) to inactive (epsilon = 7.8 x 10(-12) W kg(-1)). In response to this dynamic forcing, delta(DBL) increased from 1.0 mm to the point of becoming undefined, z(max) decreased from 2.2 to 0.3 mm as oxygen was depleted from the sediment, and J(O2) decreased from 7.0 to 1.1 mmol m(-2) d(-1) over a time span of hours. J(O2) and oxygen consumption were found to be almost equivalent (within similar to 5% and thus close to steady state), with R(O2) adjusting rapidly to changes in J(O2). Our results reveal the transient nature of sediment oxygen uptake and the importance of accurately characterizing turbulence when estimating J(O2).