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As nitrate levels in lotic systems have increased, nutrient reduction strategies have become the centerpiece of water quality standards to protect and restore stream ecosystems. While reducing anthropogenic nitrate loads has many positive effects, we lack a fundamental understanding of how lotic systems respond to changing concentrations, making it difficult to predict whether reductions will meet management goals. In-stream methods exist to characterize nutrient uptake behavior as nutrient concentrations change from ambient to saturation, but no methods exist to estimate these effects at concentrations below ambient levels. To fill this knowledge gap, we developed a chamber-based method, which allows characterization of nitrate utilization along the two major uptake pathways at concentrations below ambient levels. The clear Plexiglass chamber blocks flow by insertion into upper sediments but allows light in, and sediment-water-air interactions to occur. Using in-situ sensors, we measured water quality (nitrate, dissolved oxygen, pH, oxidation-reduction potential, temperature, and light) at high temporal resolution (15 min. intervals) over several days as nitrate was depleted. At Gum Slough Springs, a spring-fed river in Marion County, Florida, nitrate reduced from ambient levels (1.20 mg N/L) to below regulatory thresholds (ca. 0.20 mg N/L) within one week. Daytime nitrate uptake, resulting from both plant uptake and denitrification, was consistently greater than nighttime uptake, which is denitrification alone. Estimates of uptake rates were not significantly correlated with changing nitrate levels for either pathway. Replicating measurements in other systems, refining the decoupling of nitrate removal pathways, determining system responses to reduced nitrate levels across gradients in sediment and vegetation properties, and assessing other drivers of primary production are priorities for future work. The method shows promise as a tool for in-situ ecosystem-scale assessments of nutrient retention below ambient concentrations, and thus may enable future investigations focused on predicting how rivers will respond to enrichment and restoration. |