Grab Sample Data Collection

We repeatedly collected spatial biogeochemical data from the Kuparuk River (KUP), Oksrukuyik Creek (OKS), and Trevor Creek (TC) watersheds, Arctic headwater watersheds on the North Slope of Alaska associated with the Arctic Long-Term Ecological Research (LTER) Toolik Field Station site for three consecutive years (2016-2018) in June and August. Our sampling of the TFS watershed networks was designed to capture 30-50 “nested” subcatchments within the Kuparuk, Oksrukuyik, and Trevor Creek. Site selection was based primarily on (1) presence of flowing surface waters, (2) representation across varying subcatchment drainage areas, and (3) site accessibility. Often, we a priori chose sites located at subcatchment confluences, sampling both upstream locations and then downstream of river mixing. In each of the TFS watersheds, we performed 5 repeated synoptic campaigns, sampling each stream network in August 2016, June 2017, August 2017, June 2018, and August 2018 (exact dates in sheet). We accessed sampling sites either on-foot or by helicopter within a 6-hour period.

During each synoptic campaign, at each site we measured in-situ physiochemical variables (this section) and sampled stream surface water for chemical analysis (section 3.2.2). All physical water samples were “grab” sampled directly from the stream thalweg, or as close to mid-channel as could be safely accessed. We collected samples in acid-washed and triple-rinsed 1-L amber PCTE bottles. We used handheld YSI ProDO multiparameter probes (YSI Instruments Part No: 626281) and YSI ProODO Dissolved Oxygen Meter (YSI Instruments Part No: 6050020) to measure specific conductance (µS/cm), pH, temperature (ºC), and dissolved oxygen (DO, in % saturation and mg O2/L) at each sampling site. We placed the probe placed into the water column where the water sample was taken, and waited for the temperature and DO readings to stabilize before recording the final value.

Upon returning to the lab at TFS, we processed each water sample into aliquots for specific analytes within 8 hours of collection. We lab-filtered samples for dissolved water chemistry and nutrients using handheld 60 mL syringes. We triple-rinsed syringes with unfiltered sample water. Then, we sparged each filter cartridge with ~10 mL of sample water prior to sample filtration; we used the sparge volume as the initial bottle rinse. We filtered samples for DOC/TDN into triple-rinsed amber 60-mL HDPE bottles using a 25 mm 0.2 µm cellulose acetate filter (Sartorius CA membrane, 11107-25-N). We filtered samples for dissolved nutrients, anions, and cations into triple-rinsed clear HDPE 60-mL bottles using a 47 mm 0.7 µm glass fiber filter (Whatman GF/F, 1825-047). Additionally, we placed ~60-mL of unfiltered sample water into a clear HDPE bottle for analysis of turbidity (NTU). After processing, we froze samples at -4 ºC until analysis, with the exception of aliquots for DOC and total dissolved nitrogen (TDN). We stored DOC/TDN samples at 2 ºC until analysis. Samples were shipped express to the University of Vermont (UVM) and Brigham Young University (BYU) for further analysis.

We briefly describe our methods here. We measured DOC (as non-purgeable organic carbon, nPOC) and total dissolved nitrogen (TDN) with a total carbon analyzer (Shimadzu TOC-LCPH with a Total Nitrogen analyzer and ASI-L autosampler). We determined dissolved organic matter (DOM) optical properties including the spectral ratio (Sr, unitless) and specific ultraviolet absorbance at 254 nm (SUVA254) from the TOC/TN dataset (Helms et al. 2008, Hansen et al. 2016). We colorimetrically analyzed SRP, particulate phosphorous (PP), and total dissolved phosphorous (TDP) on a spectrophotometer (Shimadzu UV-2600). We quantified inorganic nitrogen species (nitrate, NO3; ammonium, NH4+) using a flow-through injection analysis (Lachat Quikchem Flow Injection Analysis System ). We measured several cations (Na+, Li+, K+, Mg2+, Ca2+, NH4+), anions (F-, Cl-), oxoanions (NO2-, SO42-, NO3-, PO43-) and organic acids (acetate, CH3COO-; and formate, HCOO-) on an ion chromatrography system (IC, IThermo Fisher Scientific Dionex ICS5000). We quantified other geogenic anions and cations (e.g., Al3+, As3-, B3-, Ba2+, Br+, Ca2+, Cd2+, Co2+, CrO4-, Total Cu, Total Fe, K+, MoO32-, Mg2+, Mn2+, Na+, Ni2+, P, Pb2+, S2-, Se2-, Si4+, Sn2+, Sr2+, Ti, V, Zn2+) on an ion chromatography inductively coupled plasma mass spectrometer (IC-ICP-MS, iCAP 7000 series, Thermo Scientific). To estimate turbidity (NTU), we used a Forest Technology Systems (FTS) DTS-12 digital turbidity sensor. We analyzed all samples at room temperature; we thawed frozen samples on a lab bench for 2-4 h prior to analysis.