The Tracer Additions for Spiraling Curve Characterization (TASCC) model has been rapidly adopted to interpret in‐stream nutrient spiraling metrics over a range of concentrations from breakthrough curves (BTCs) obtained during pulse solute injection experiments. TASCC analyses often identify hysteresis in the relationship between spiraling metrics and concentration as nutrient concentration in BTCs rises and falls. The mechanisms behind these hysteresis patterns have yet to be determined. We hypothesized that differences in the time a solute is exposed to bioactive environments (i.e., biophysical opportunity) between the rising and falling limbs of BTCs causes hysteresis in TASCCs. We tested this hypothesis using nitrate data from Elkhorn Creek (CO, USA) combined with a process‐based particle‐tracking model representing travel times and transformations along each flow path in the water column and hyporheic zone, from which the bioactive zone comprised only a thin superficial layer. In‐stream nitrate uptake was controlled by hyporheic exchange and the cumulative time nitrate spend in the bioactive layer. This bioactive residence time generally increased from the rising to the falling limb of the BTC, systematically generating hysteresis in the TASCC curves. Hysteresis decreased when nutrient uptake primarily occurred in the water column compared to the hyporheic zone, and with increasing the distance between the injection and sampling points. Hysteresis increased with the depth of the hyporheic bioactive layer. Our results emphasize that good characterization of spatial heterogeneity of surface‐subsurface flow paths and bioactive hot spots within streams is essential to understanding the mechanisms of in‐stream nutrient uptake.

中文翻译：

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疏水性生物活性层中的停留时间说明了溪流中的硝酸盐吸收

螺旋曲线表征的示踪剂加法（TASCC）模型已被快速采用，以解释在脉冲溶质注入实验中获得的突破曲线（BTC）浓度范围内的流内养分螺旋度量。TASCC分析通常会在BTC中养分浓度升高和降低时，在螺旋度量与浓度之间的关系中发现滞后现象。这些迟滞模式背后的机制尚未确定。我们假设溶质在BTC的上升和下降分支之间暴露于生物活性环境（即生物物理机会）的时间差异会导致TASCC的滞后。我们使用Elkhorn Creek（CO，美国）与基于过程的颗粒跟踪模型相结合，该模型代表了沿水柱和低渗区中每个流动路径的传播时间和转换，其中生物活性区仅包含一个薄薄的表层。河床中硝酸盐的吸收是通过交换性交换和硝酸盐在生物活性层中累积的时间来控制的。这种生物活性停留时间通常从BTC的上升到下降四肢增加，从而在TASCC曲线中系统地产生滞后现象。与排液区相比，当主要在水柱中吸收养分时，滞后性降低，并且进样点和采样点之间的距离增加。滞后性随着低生物活性层的深度而增加。