Fundamental timescales of benthic biolayer activity predict whole-stream chemical transformations

Abstract:

An important contributor to whole stream chemical transformations is the benthic biolayer, a zone just below the sediment-water interface where microbial activity is high. Steep gradients in microbial biomass and oxygen concentrations cause reaction rates to vary with depth through this region of the streambed, and there is a growing recognition that these vertical gradients control whole stream degradation rates. Nevertheless, we lack a modeling framework that explicitly relates the vertical variation of streambed reactivity to predictions of solute retention and transformation at the stream scale. This knowledge gap limits our ability to interpret results from field studies and anticipate how changes to streambed reactivity will alter chemical fate in rivers.

Here, we use numerical simulations and a reactive mobile-immobile approach to relate spatial variations in streambed reactivity to whole-stream predictions of chemical fate. To this end, we conceptualize the streambed as a zone with zero streamwise velocity. Solute migrates vertically through the streambed via diffusion, and it degrades via a first-order reaction whose rate varies with depth in the streambed.

Reactant degradation is determined by three characteristic timescales associated with: (1) solute transport through the benthic biolayer, (2) reaction within the benthic biolayer, and (3) solute retention within the entire streambed. The coupling between the residence time distribution and reactivity in the streambed is quantified by a novel upscaled reactive mobile-immobile model. We compare model predictions to numerical pulse injection experiments in streams with different shapes of the benthic biolayer, as well as different ratios of the three characteristic timescales described above. The upscaled model quantifies and elucidates how the benthic biolayer controls chemical fate at the whole stream scale.