Coupled Effects of Hyporheic Flow Structure and Biogeochemical Heterogeneity on Nutrient Dynamics in Rivers

Abstract:

Co-injections of conservative tracers and nutrients are commonly used to assess net reach-scale nutrient transformation rates and benthic/hyporheic uptake parameters. However, little information is available on spatial patterns of metabolism in the benthic and hyporheic regions. We used particle tracking simulations to explore the effects of localized metabolism on estimates of reach-scale nutrient uptake rates. Metabolism locally depletes nutrient concentrations relative to conservative tracers, causing their concentration profiles of injected nutrients and conservative tracers to diverge. At slow rates of hyporheic exchange relative to rates of metabolism, overall hyporheic nutrient uptake is limited by delivery from the stream, and effective reach-scale nutrient uptake parameters will be controlled by the hyporheic exchange rate. At high rates of hyporheic exchange relative to rates of metabolism, the injected tracer can propagate beyond regions of high microbial activity, which commonly occur near the streambed surface. In this case, the injected tracer may not adequately capture timescales of nutrient replenishment in the most bioactive regions. With increasing heterogeneity in local metabolic patterns, effective reach-scale biogeochemical transformation rate constant increases, such that the heavy tail of nutrients breakthrough curve becomes less evident while the curve shifts in. More observations of hyporheic rates and metabolic patterns are needed to understand how flow heterogeneity and reaction heterogeneity interact to control nutrient dynamics at reach-scale.