Samples were collected in the lower reaches of Von Guerard stream from below the playa to about 200 m upstream of the F6 gage. Samples were taken on lateral transects extending from the channel center to the wetted margin. Full transect locations are included as a csv file.

Porewater samplers were flexible polyethylene tubing (3/16” inner diameter) encased in a rigid plastic tube and screened at the bottom with fine metal mesh. We installed samplers to depths of both 15 and 30 cm at locations A-D on each transect. Nutrient and DOC samples were filtered through pre-combusted (4 hr at 450ºC) Whatman GF/C glass fiber filters. Nutrient samples were stored frozen in 60 mL HDPE bottles, while DOC and isotope samples were kept chilled (+4ºC) in the dark prior to analysis. Silica and DIN were measured colorimetrically immediately after thawing at room temperature by the Arikaree Environmental Laboratory, University of Colorado Boulder, using a Lachat (USA) QuikChem 8500 Flow Injection Autoanalyzer. Nitrate (as NO₃^– + NO₂^–) was analyzed by standard method 4500-NO₃ I (cadmium reduction flow injection) with a detection limit of 0.004 mg NO₃^––N L^–1. Samples were analyzed for NH₄⁺ by standard method 4500-NH₃ H (phenolate flow injection, detection limit of 0.005 mg NH₄⁺–N L^–1). For Si, the detection limit was 0.004 mg Si L^–1 by standard method 4500-SiO₂ F. We measured water isotopes (δ18O and δD) using a cavity ring-down spectrometer (Picarro, USA) and calculated deuterium excess as δD – 8*δ18O.

For the nitrification potential assay we used sediment from two pilot transects during January 2018. We first removed the top 5 cm of the streambed to avoid contamination from benthic algal mats, then scooped sediment from approximately 5–10 cm depth into sterile Whirlpaks. These sediment samples were stored frozen and shipped to the University of Colorado Boulder for analysis. After thawing the samples at room temperature, we subsampled 20 g of wet sediment from each site in triplicate into sterile 250 mL HDPE bottles and an additional 20 g in triplicate into pre-weighed aluminum weigh boats. We dried the subsamples in the aluminum weigh boats at 105ºC for 24 hours and reweighed to calculate mean volumetric water content for each sampling location. For the samples in 250 mL HDPE bottles, we performed a nitrification potential assay using perchlorate and NH₄⁺ in a phosphate (KH₂PO₄ and K₂HPO₄) buffered solution, which was mixed constantly with the sediment on a shaker table at room temperature. A detailed description of the assay protocol and solution concentrations is provided by Schmidt & Belser [1994]. We took subsamples from each replicate flask at 0, 1 and 8 hours then 1, 2, 3, and 5 days from the start of the assay. All subsamples were immediately filtered through Whatman GF/C glass fiber filters and stored frozen (-20ºC) until thawed for analysis. As perchlorate in the incubation solution inhibits the final oxidation of nitrite (NO₂^–) to NO₃^–, we analyzed the filtered aliquots for NO₂^– colorimetrically using a Lachat QuikChem 8500 Flow Injection Autoanalyzer. Accumulated NO₂^– concentrations were converted to mass NO₂^––N normalized by the calculated dry weight of sediment in each replicate flask.