DECOUPLED DIEL SOLUTES: LINKING PRIMARY PRODUCTION AND NITRATE UPTAKE IN A MONTANE STREAM

Tight coupling of surface water diel dissolved oxygen (DO) and nitrate-N (NO3-N) signals reflects stoichiometric demand of carbon and nitrogen in stream ecosystems. However, DO and NO3-N can become decoupled due to alternative drivers of diel solutes, resulting in conflicting estimates of stoichiometric and modeled NO3-N uptake. In this study, I measured benthic biomass, hydrology, and dissolved solutes in a montane stream located in western MT over a growing season (June-October 2019). Daily stream metabolism and NO3-N uptake were modeled using a single-station open-channel approach. Timing and amplitude of key diel signals were characterized quantitatively to assess decoupling of DO and NO3-N and investigate diel variation in hydrology. I also analyzed the effect of DO and NO3-N benthic footprint lengths on diel signals. Miller Creek was heterotrophic, with ER ranging from -2.07 ± 0.37 to -5.53 ± 0.11 g O2­ m-2 d-1 and GPP from 0.03 ± 0.04 to 0.82 ± 0.10 g O2­ m-2 d-1. Statistical assessment with generalized additive models (GAMs) tied metabolism to localized channel conditions. Reductions in GPP, water temperature, and hydrology predicted declining ER (R2adj = 0.72, n = 124, 72.8% deviance explained). ER also decreased with benthic standing stocks, as measured with AFDM (r = -0.74, p = 0.034) and chl a (r = -0.92, p = 0.0013), and surface water dissolved organic carbon (DOC) concentration (r = -0.73, p = 0.038). Modeled NO3-N uptake (3.20 ± 1.50 to 12.14 ± 2.16 mg N m-2 d-1) agreed with stoichiometric estimates (0.42 ± 0.05 to 10.10 ± 3.5 mg N m-2 d-1) in magnitude, despite clear decoupling of diel DO and NO3-N signals. GPP, water temperature, surface water flow, and light explained 92.1% of the deviance in modeled NO3-N uptake (R2adj = 0.91, n = 110). Daily benthic footprint of NO3-N ranged from 2.5 to 19.5 km, exceeding that of DO on some sampling days, which varied from 1.2 to 4.1 km. This research provides a model for calculating daily NO3-N uptake comparable to estimates from stoichiometry using a single-station approach, which can be applied in streams and rivers with diel DO and NO3-N decoupling