Biogeochemical dynamics in aquatic sediments: Novel laboratory and field-based approaches

A slow flow, plug-through reactor was developed for measuring equilibrium and kinetic parameters of biogeochemical reactions on intact sections of sediment cores that apply to in situ conditions. Sediments from lacustrine, estuarine and marine depositional environments were studied.; Longitudinal dispersion coefficients were determined from the breakthrough curves of bromide. As applications, the reactor was operated in once flow-through mode or recirculation mode to retrieve, (1) Equilibrium adsorption coefficients (K) of ammonium, $NH+4$, (0.3–2.3 @ 5°C), with the highest value of K obtained in a fine-grained marine sediment, the lowest in a coarse-grained lake sediment. The dependencies of K on ionic strength (range 0.2 to 0.7 m) and temperature (range 5 to 25°C) in an estuarine sediment confirm that the main sorption mechanism for ammonium is ion exchange. (2) Apparent rate constant (k_app) for dissolution of biogenic silica in Southern Ocean sediment (∼10 −10 mol g−1_SiO2s−1 @ 25°C, pH 7.83 to 7.93). Measured k_app values are site specific and the reactivity of biogenic silica decreases almost exponentially with depth below the sediment-water interface. (3) Half saturation constant (K_s) for microbial sulfate reduction (240 μM). The value of K_s is comparable to those reported for a pure culture of sulfate reducing bacteria, but is significantly smaller than the only other K_s value reported in the literature for sulfate utilization in a natural marine sediment. The rate of microbial sulfate reduction was found to be nearly independent of the dissolved sulfate concentration in the range 2.2 to 1 mM.; A sequential extraction technique was used to study the spatial and temporal trends of solid bound iron and sulfur in saltmarsh sediments from Sapelo Island (GA). Ascorbate extractable iron constitutes the reactive iron(III) phase. Readsorption of released iron(II) onto abundantly available organic matter in the sediment suggests a need for site specific calibration of extraction times using natural sediments instead of pure mineral phases. Results from a field study based on solid phase extraction of iron oxyhydroxides and sulfides show their dynamic seasonal cycling in saltmarsh sediment. Their distinct temporal and spatial distribution pattern with depth, and along a transect across a saltmarsh field, is a function of microbial activity, vegetation pattern, and pore water irrigation.