Methods

Dissolved gas samples were collected at 0.5, 3, 5 and 7 m using the
headspace method. From January to March 2020, water at each discrete depth
was pumped directly into the bottom of a 1-L Nalgene bottle and flushed with
at least three times the volume before being capped with a rubber stopper.
60 mL of ambient air was added while 60 mL of sample water was removed from
the bottle and equilibrated by shaking for 90 seconds. From May 2020
onwards, water was pumped into a closed bottle system, and using syringe,
105mL of water was extracted and 35mL of ambient air was added. The
headspace was then equilibrated for 2 minutes by shaking and 10 mL of
equilibrated gas sample was then removed from the bottle and injected into a
5.9 mL Labco Exetainer vial that had been previously vacuumed. While in the
field, samples were stored in pouches within a survival suit to prevent
extreme temperature change. We analyzed the gas samples for CO2 and CH4 with
a gas chromatograph (GC-2014; Shimadzu Scientific Instruments) equipped with
a methanizer and flame ionization detector. Greenhouse gas concentrations
were calculated according to Henry’s law and corrected by measured ambient
air.<br/>

Methods

Rate of bubble releaseWe deployed 30 inverted funnel-style bubble traps (Molongoski and Klug, 1980; Baulch et al., 2011) on Allequash Creek on 31 May 2013 to measure volumetric bubble release rates. Fifteen traps were placed in two sandy sediment sections and 15 were placed in muck sediments in the wetland portion of the creek (number 8&ndash;22), which sits in-between the two sandy sections. Site 1 was the most downstream sampling site (water flows from East to West). Traps were sampled approximately every other day after 1 June 2013 until 31 October 2013 (we omitted the first samples collected 24 h following trap installation; total of 65 sample events per trap). Our sampling design allowed us to assess both the spatial and temporal variability in ebullition along Allequash Creek and how ebullition related to potential controlling factors such as sediment composition, atmospheric pressure, groundwater CH4, and organic matter content (discussed further below). To characterize our ebullition time series from Allequash Creek in the larger context of the NHLD, we installed an additional 12 traps on three additional creeks (Mann Creek, Stevenson Creek, and North Creek in the Trout Lake drainage; three per site in an even mix of sand and muck sediments) and the headwater spring ponds that drain into Allequash Creek on 23 June 2013 which we sampled approximately every week for the remainder of the study. Bubble traps had a bottom surface area of appr. 503 cm2 which narrowed at the top into a graduated (1 mL resolution) syringe and 3-way stopcock. Traps were attached to steel poles that were pounded into the substrate. Traps were almost completely submerged and contained no headspace at deployment. Water depth below traps averaged 55.7 cm, but we were unable to place traps in locations where water depth was shallower than 15 cm. Water velocity during baseflow at the traps averaged 0.06 m s -1 (range = 0.003&ndash;0.23 m s -1). We sampled traps by carefully approaching them either by boat (muck sites) or by wading (sandy sites) to avoid induced ebullition. Volume of accumulated gas in the trap was based on the graduated syringe, and volumes less than 1 mL were recorded as zero. Traps were reset between sampling events by refilling them completely with water to eliminate all headspace. To assess the hypothesis that declines in atmospheric pressure are related to increased bubble release (Mattson and Likens, 1990; Comas et al., 2011), we compared a 15 min resolution atmospheric pressure time series recorded using a Vaisala BAROCAP barometer deployed near trap number 7 with a subset of the bubble release time series.

Methods

Sampling DesignSampling of gas partial pressures, and gas transfer velocities was performed weekly at five stream sites (Mann Creek, Allequash Lower Creek, Allequash Middle, Stevenson Creek, North Creek) that drain into Trout Lake (Figure 1), one of the regions larger lakes. Sampling began in May 2012 and continued through September 2012. These data were used to establish variability in gas transfer rates for the basin, and to investigate spatiotemporal patterns. To test for the effects of upstream lakes, sampling at 30 additional longitudinal transect sites along 6 streams (5 sites per stream; Figure 1) was conducted approximately every 3 weeks beginning in May 2012. Transect streams were chosen based on the presence or absence of lakes in the upstream watershed. Streams with lakes (Lost, White Sand, Aurora) were sampled starting at the approximate lake outlet (site selection based on aerial photographs), and along a 2000 m transect (0m, 250m, 500m, 1000m, 2000m). Upstream lake chemistry (epilimnion) was also sampled during late July or early August 2012 at the lake center to allow for a direct comparison with streams. Streams without upstream lakes (Stella, Mud, and North) were sampled at an arbitrary upstream location (0m) and followed the same sampling progression as streams with lakes. We analyzed stream chemistry and stream morphology data from a regional stream survey (Lottig and others 2011) which we use to scale fluxes to the NHLD (Figure 1). We also studied groundwater CO2 and CH4 patterns along a hillslope transect at Allequash Creek during 2001. In 2002, we monitored hourly CO2 and O2 dynamics at the four WEBB sites to assess the role of ecosystem metabolism.