Assessing Stream Ecosystem Metabolism and Nitrate Utilization at Reduced Nitrate Levels Using a Chamber-Based Approach: Looking Below, Scaling Up, and Thinking Inside the Box

Abstract:

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 (NO₃) loads has many positive effects, we lack a fundamental understanding of how lotic systems respond to changing concentrations and no methods exist to characterize nutrient uptake behavior below ambient levels. Therefore, it is difficult to predict whether nutrient reductions will meet management goals. To fill this knowledge gap, we developed a chamber-based method which allows characterization of NO₃ utilization along the two major uptake pathways at reduced NO₃ levels. The chamber blocks flow by insertion into upper sediments but allows light in and sediment-water-air interactions to occur. At Gum Slough Springs, Florida, high-resolution in-situ sensors measured water quality while NO₃ reduced from ambient levels (1.40 mg N/L) to below regulatory thresholds (ca. 0.20 mg N/L) within one week. Daytime NO₃ uptake, resulting from both plant uptake and denitrification, was consistently greater than nighttime uptake, which is denitrification alone. Using this method, we compared NO₃ uptake rates (U_NO3) and gross primary production (GPP) across three vegetative regimes (i.e. submerged aquatic vegetation (SAV), SAV with epiphytic algae, and algae alone) and related GPP estimates from the chamber to reach scale. Results showed that U_NO3 and GPP were greatest in SAV, GPP was negatively correlated to [NO₃] in algae, denitrification rates did not vary by vegetation type, and chamber GPP (e.g. 6-8 g O₂/m²/day in SAV) was comparable to reach-scale estimates (6-12 g O₂/m²/day). Our results suggest U_NO3 and GPP differ by vegetation regimes, GPP scales from chamber to reach level, algal presence potentially reduces GPP, and a lack of nutrient limitation even at low [NO₃]. Current work includes replicating measurements across systems as well as refining the decoupling of NO₃ removal pathways using the N₂:Ar approach for denitrification. Overall, this 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.