# Multi-Year Monitoring of Ecosystem Metabolism in Two Branches of a Cold-Water Stream
## Materials and Methods
### Physical and Chemical Data  
Water temperature and dissolved oxygen were measured every 10 min from mid-May through early November in 2017, 2018, and 2019 using a HOBO(r) Model U26-001 (Onset Computer Corporation, Bourne, MA, USA) probe. The probes were placed mid-stream in riffie areas and approximately 15 cm above the bottom. Conductivity was measured with a HOBO(r) Model U24-001 (Onset Computer Corporation, Bourne, MA, USA) probe for 2017 in the North Branch and 2017 and 2018 in the South Branch. Water depth was measured with a HOBO(r) Model U20L-04 (Onset Computer Corporation, Bourne, MA, USA) probe. A HOBO(r) weather station was deployed next to the North Branch site in full sunlight. The weather station was outfitted with photosynthetically active radiation (PAR) (HOBO(r) Model S-LIA-M003, Onset Computer Corporation, Bourne, MA, USA) and barometric pressure sensors (HOBO(r) Model BPB-CM50, Onset Computer Corporation, Bourne, MA, USA). For a portion of 2018, the PAR sensor malfunctioned. Fortunately, a PAR sensor (Li-Corr(r) LI-190R sensor, Onset Computer Corporation, Bourne, MA, USA) was functioning at another site 21.8 km away throughout this study. We developed a regression relationship between this more distant site and the Valley Creek site when both were functioning (R2 = 0.86) and an estimate of PAR at Valley Creek was made for the time when the PAR sensor was malfunctioning by taking data from the more distant site. An additional HOBO(r) PAR sensor was placed in a forested area 250 m upstream of the North Branch sampling site and 600 m from the South Branch site. This allowed for the assessment of the potential impact of forest shading on light availability. Discharge measures were provided by the Washington County Minnesota Conservation District (Oakdale, MN). On the North Branch, the discharge station was about 250 m upstream of the sampling location. On the South Branch, the station was about 5 m upstream of the sampling location. Discharge data were not available for the North Branch for 2018 and 2019 until 18 July 2019, due to a malfunction of the gauge. Both of the streams are in a well-defined channel with the North Branch being approximately 2.5 m wide during base flow and the South Branch being about 4 m wide at base flow. The South Branch has coarser substrate (average \(\phi\) size = \(-\)1.14--fine gravel) than the North Branch (average \(\phi\) size = \(-\)0.43--coarse sand) [46]. There are few macrophytes in the channels and with the little biofilm being found on the relatively fine substrates. Minipiezometers were installed in both streams to assess the level of groundwater input or recharge (inside diameter--0.7 cm, screen length--4.3 cm) to a depth of 1.1 m below the sediment-water interface. Stream depth and water depth in the piezometer were measured to calculate the vertical hydraulic gradient, an indication of the amount of groundwater entering or leaving a stream [52]. Weather data, including daily precipitation, and maximum and minimum air temperatures were retrieved from the Utah Climate Center ([[http://climate.usu.edu](http://climate.usu.edu)]([http://climate.usu.edu](http://climate.usu.edu)), accessed on 30 April 2020) for the GHCN:COOP station USC00218039, Stillwater2 SW, MN which is approximately 15 km from the study site. Nitrogen (N) and phosphorus (P) levels were measured by the Washington County Conservation District and the St. Croix Watershed Station (Science Museum of Minnesota, Marine on St. Croix, MN, USA) twice each year in 2017-2019. Differences in the physical and chemical characteristics in the two branches of Valley Creek and between years were assessed using mixed-model ANOVAs. ANOVAs are used to test whether there are significant difference in the means of dependent variables for different values of independent variables. Water temperature, depth, discharge, or conductivity were used as dependent variables and location, year (as nominal variables), and their interactions were used as the independent variables.