Quantifying ambient nitrogen uptake and functional relationships of uptake versus concentration in streams: a comparison of stable isotope, pulse, and plateau approaches

Abstract

Nutrient releases and spiraling metrics are frequently used to quantify the downstream transport of nutrients and to better understand the effects of anthropogenic inputs to downstream waters. Ambient uptake rates in streams can be measured through stable isotope enrichments, while pulse and plateau additions can estimate such rates via extrapolation and modeling techniques, respectively. Data from these releases can be used to estimate ambient uptake rates from nutrient additions and possibly determine the functional relationships between nutrient concentrations and uptake rates. Here, we compared estimated ambient rates calculated from established pulse and plateau approaches, results obtained from new modeling approaches, and rates at ambient concentrations from stable isotope enrichments. Comparative releases of NH 4 Cl and 15 NH 4 Cl were conducted in four experimental reaches across the grassland Kings Creek and urban Campus Creek, KS. Nutrient uptake was predominantly linear with increasing ammonium. Estimated ambient uptake rates varied among sites, release methods, and data analysis approaches.However, plateau ambient rates from new modeling approaches matched closely with measured ambient rates from isotope enrichments at three sites, suggesting that modeled plateau data may be best for a first look at determining nutrient uptake rates at an individual site. Limitations and benefits of each approach vary; however, baseflow discharge may be a key driver when choosing a method. If possible, multiple methods should be attempted at each location and under each novel set of conditions to determine the best approach prior to designing and implementing a more extensive series of measurements.

Nutrient uptake in a simplified stream channel: Experimental manipulation of hydraulic residence time and transient storage

“…For the experiments with weirs, we noted hysteresis in both NH 4 + and PO 4 3− additions and differences in uptake metrics for the rising and falling limbs, as noted by other researchers (Gibson, Reilly, Conine, & Lipshutz, ; Thomas, Valett, Webster, & Mulholland, ; Trentman et al, ). We interpret these patterns to be representative of uptake in different stream compartments.…”

DOC:NO3− ratios and NO3− uptake in forested headwater streams

“…For example, Trentman et al . [] found positive and negative slopes in the rising and falling limbs of their relationship between S w and NH 4 + during TASCC additions. This lack of good fit in the linear regression propagates error into the ambient uptake metrics as well as the calculation of total dynamic uptake.…”

“…Alternatively, biostimulation presents the opposite pattern where S w decreases with increased concentrations because the added nutrient is quickly taken up by the biota thus causing the molecule to travel shorter distances due to greater demand. These same increasing and decreasing patterns would be mirrored in a hysteresis model, depending on the direction of the hysteresis (i.e., clockwise or counterclockwise) as both increasing and decreasing trends can be found in S w for one experiment [ Trentman et al ., ].…”

Increased resource use efficiency amplifies positive response of aquatic primary production to experimental warming

“…We used the TASCC method (Covino, McGlynn, & McNamara, ) to separately measure –N, –N, and SRP uptake rates in both streams each month. The TASCC method allows for a well‐constrained estimate of uptake rates at ambient concentrations, although all nonisotopic addition methods overestimate these rates to some degree (Trentman et al., ). In summary, a slug containing dissolved nutrients (NH 4 Cl, NaNO 3 , or Na 2 HPO 4 ) and Cl − (NaCl) was added far enough above the upstream metabolism station to allow for adequate mixing (~5–10 m).…”