Seiche-induced turbulence and the vertical distribution of dissolved oxygen above and within the sediment were analyzed to evaluate the sediment oxygen uptake rate (Jo2), diffusive boundary layer thickness (δDBL), and sediment oxic zone depth (zmax) 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 Jo2from both sides of the sediment-water interface. Turbulence (characterized by the energy dissipation rate, ε), the vertical distribution of dissolved oxygen across the sediment-water interface (characterized by δDBLand zmax), JO2) and the sediment oxygen consumption rate (Ro2) are all strongly correlated in our freshwater system. Seicheinduced turbulence shifted from relatively active (ε = 1.2 × 10-8W kg-1) to inactive (ε = 7.8 × 10-12W kg-1). In response to this dynamic forcing, δDBLincreased from 1.0 mm to the point of becoming undefined, zmaxdecreased from 2.2 to 0.3 mm as oxygen was depleted from the sediment, and Jo2decreased from 7.0 to 1.1 mmol m-2d-1over a time span of hours. Jo2and oxygen consumption were found to be almost equivalent (within ∼ 5% and thus close to steady state), with Ro2adjusting rapidly to changes in Jo2. Our results reveal the transient nature of sediment oxygen uptake and the importance of accurately characterizing turbulence when estimating Jo2. © 2010, by the American Society of Limnology and Oceanography, Inc.

The signal of Clark-type oxygen sensors is controlled not only by the oxygen concentration but also by sensor dimensions, temperature, and salinity. We have developed a mathematical model that predicts the O2 microsensor signal as a function of these parameters. The only model variable, membrane permeability, can be determined by a one-point calibration; hence, the model can be used for calibrating the signal measured under any temperature and salinity. The model describes the oxygen concentration profile through the sensor and thus allows optimization of sensor construction with respect to stirring sensitivity, signal-to-noise ratio, and response time. The model is demonstrated using oxygen water column profiles off the coast of central Chile.

Here we present, to the best of our knowledge, the first balanced light energy budget for a benthic microbial mat ecosystem, and show how the budget and the spatial distribution of the local photosynthetic efficiencies within the euphotic zone depend on the absorbed irradiance (Jabs). Our approach uses microscale measurements of the rates of heat dissipation, gross photosynthesis and light absorption in the system, and a model describing light propagation and conversion in a scattering-absorbing medium. The energy budget was dominated by heat dissipation on the expense of photosynthesis: in light-limiting conditions, 95.5% of the absorbed light energy dissipated as heat and 4.5% was channeled into photosynthesis. This energy disproportionation changed in favor of heat dissipation at increasing irradiance, with >99% of the absorbed light energy being dissipated as heat and <1% used by photosynthesis at Jabs >700 μmol photon m-2 s -1 (>150 J m-2 s-1). Maximum photosynthetic efficiencies varied with depth in the euphotic zone between 0.014-0.047 O 2 per photon. Owing to steep light gradients, photosynthetic efficiencies varied differently with increasing irradiances at different depths in the euphotic zone; for example, at Jabs >700 μmol photon m-2 s-1, they reached around 10% of the maximum values at depths 0-0.3 mm and progressively increased toward 100% below 0.3 mm. This study provides the base for addressing, in much more detail, the photobiology of densely populated photosynthetic systems with intense absorption and scattering. Furthermore, our analysis has promising applications in other areas of photosynthesis research, such as plant biology and biotechnology. © 2010 International Society for Microbial Ecology All rights reserved.