SAMPLE COLLECTION AND EQUIPMENT
Samples for greenhouse gas (methane, carbon dioxide) analysis were collected from Falling Creek Reservoir and Beaverdam Reservoir during the ice-free period (March - November) for 2015 - 2021 from the deepest site in each reservoir. Most sampling occurred between the hours of 9:00 and 15:00; however, some sampling occurred outside of these hours, including some overnight sampling. For more information about nighttime sampling, see Doubek et. al. 2018. Exact sampling times were included in the DateTime column starting in 2018. After 2018, exact times, when recorded during sample collection, are indicated in the DateTime column with a Flag_DateTime value of 0. If exact times were not recorded during sample collection, time was standardized to 12:00 and Flag_DateTime was set to 1. Prior to 2018, sample timestamps for samples collected between approximately 9:00 and 15:00 were standardized to noon and Flag_DateTime was set to 1.
Samples were collected from several routine depths including: 0.1, 1.6, 3.8, 5.0, 6.2, 8.0 and 9.0 m in Falling Creek Reservoir and 0.1, 3.0, 6.0, 9.0, and 11.0 m for Beaverdam Reservoir. These sampling depths were chosen to approximately match outtake valves in Falling Creek Reservoir and Beaverdam Reservoir. Samples from Falling Creek Reservoir were also collected from a gauged weir on the primary inflow to Falling Creek Reservoir (Site = 100), as surface samples from the wetlands adjacent to the reservoir (Site = 200), and from the reservoir outflow (Site = 1.1). In Beaverdam Reservoir, additional samples were collected from the several sites at the outflow below the dam (Site = 1.1, 1.2, 1.3, 1.4). In summer 2019, additional surface samples were collected along the stream and reservior continuum for both Falling Creek Reservoir and Beaverdam Reservoir as part of the Reservoir Continuum project (Falling Creek Reservoir: Creek Sites = 102, 101, 100, 99, 200; Reservoir Sites = 20, 30, 45, 50; Reservoir Outflow = 01 and Beaverdam Reservoir: Creek Sites = 100, 200; Reservoir Sites = 20, 30, 45, 50; Mid-Reservoir Outflow Pipe = 01). Routine depth samples were collected monthly in March-April and September-November and twice weekly for Falling Creek Reservoir and weekly from Beaverdam Reservoir from May-August. Samples were collected with a 4-L Van Dorn sampler (Wildlife Supply, Yulee, FL, USA) and transferred to into two replicate 20 mL glass vials without exposure to air and crimp sealed. Surface samples were collected via grab samples for all stream and up-stream reservoir locations. Samples were stored on ice and analyzed within 24 hours of collection following McClure et al. (2018).
SAMPLE ANALYSIS AND EQUIPMENT
Dissolved methane and carbon dioxide concentrations were measured in each water sample on a Gas Chromatograph (GC) with a Flame Ionization Detector (FID) and Thermal Conductivity Detector (TCD) following McClure et al. (2018). Briefly, immediately prior to analysis, a 2-mL headspace was created with Helium (He) by displacing 2-mL of sample water. The headspace was equilibrated by shaking each 20-mL vial at 300 rpm for 15 minutes. The 2-mL headspace was then injected into the GC. The oven temperature was set to 35 degrees C with a carrier gas (He) flow rate of 15 mL/min. CH4 had a retention time of 1.3 min on the FID and CO2 had a retention time of 2.8 min on the TCD. Dissolved concentrations of CH4 and CO2 in water were calculated using the observed head-space concentrations and Henry's Law (McClure et al. 2018). From 2015-2018, samples were analyzed on a GC SRI, Model 8010. From 2019-onwards, samples were analyzed on a Shimadzu Nexis GC-2030.
DATA FLAGS
Replicate samples (Rep 1 and 2) were often collected for each Date, Site, and Depth. If a second rep was not collected, the data was flagged as '1' (sample was not collected). If the analyzed sample had a value below the minimum detection limit (MDL) the sample was flagged as '2' (sample below MDL). A running MDL was calculated for each GC separately (2015-2018, GC SRI, Model 8010; 2019-onwards, Shimadzu Nexis GC-2030) from routinely measured air concentrations (MDL for each year is presented; 2015 CH4 = 0.00229, CO2 = 3.92; 2016 CH4 = 0.00256, CO2 = 3.83; 2017 CH4 = 0.00210, CO2 = 3.81; 2018 CH4 = 0.00294, CO2 = 3.74; 2019 CH4 = 0.00103, CO2 = 3.74; 2020 CH4 = 0.00103, CO2 = 3.75 umol/L; 2021 CH4 = 0.00252 umol/L, CO2 = 4.36 umol/L). Replicates of each sample were assessed after sample analysis using calculated percent difference between replicate samples as well as the limit of quantification (LOQ = 3*MDL). If the difference between replicate samples was < LOQ, then the samples were not flagged ('0', samples are good). If the difference between samples was >LOQ and the percent difference between replicates samples was <30%, then the samples were not flagged ('0', samples are good). If the difference between samples was >LOQ and the percent difference between replicates was >30% but <50%, then the samples were flagged as '3' (difference between reps is above LOQ and above 30% difference and below 50% difference). If the difference between samples was >LOQ and the percent difference between replciates was >50%, then the samples were flagged '4' (difference between reps is above the LOQ and above 50% different). For all flags, replicates were retained in the dataset.
REFERENCES
Doubek J.P., Campbell K.L., Doubek K.M., Hamre K.D., Lofton M.E., McClure R.P., Ward N.K., Carey C.C. (2018). The effects of hypolimnetic anoxia on the diel vertical migration of freshwater crustacean zooplankton. Ecosphere, 9(7):02332. https://doi.org/10.1002/ecs2.2332
Hounshell A.G., McClure R.P., Lofton M.E., Carey C.C. (2020). Whole-ecosystem oxygenation experiments reveal substantially greater hypolimnetic methane concentrations in reservoirs during anoxia. Limnology and Oceaography - Letters, 6: 33-42. https://doi.org/10.1002/lol2.10173
McClure R.P., Hamre K.D., Niederlehner B.R., Munger Z.W., Chen S., Lofton M.E., Schreiber M.E., Carey C.C. (2018). Metalimnetic oxygen minima alter the vertical profiles of carbon dioxide and methane in a managed freshwater reservoir. Science of the Total Environment, 636. pp. 610-620. https://doi.org/10.1016/j.scitotenv.2018.04.255