This study was conducted in the Talladega research watershed (outlet location: 33.76219799, -85.59550775) in the Talladega National Forest (Cleburne County, AL, USA) as part of the Aquatic Intermittency effects on Microbiomes in Streams (AIMS) project, an NSF EPSCoR funded project (OIA 2019603). The project sought to explore the impacts of stream drying on downstream water quality across Kansas, Oklahoma, Alabama, and Idaho, integrating datasets on hydrology, microbiomes, macroinvertebrates, and biogeochemistry. The Talladega watershed drains a non-perennial unnamed tributary of Pendergrass Creek and contains 0.92 km2 of mixed coniferous and deciduous forest in the Piedmont Upland physiographic section. Located near Anniston, AL, the watershed spans an elevation range from 345 to 456 m above sea level and is a tributary to the Coosa River (within the larger Mobile-Tombigbee basin). The region has a humid subtropical climate, with mean daily January and July air temperatures of 5.3°C and 25.3°C respectively, and mean annual precipitation of 1,400 mm/yr.
We collected samples every ~3 weeks from October 7, 2021, to October 3, 2024, at the outlet of our watershed (TLM01, approach 1), seasonally at seven distributed sites (approach 2) and across 39 spatially distributed sites on June 9, 2022- June 10, 2022 (approach 3). We collected triplicate water samples for soluble reactive phosphorus (SRP; µg/L) and ammonia (NH4-N, µg/L) on each sampling occasion with a subset of samples also analyzed for nitrate/nitrite (NO3-N, NO2-N, µg/L). We collected samples at the specified location when water was present using a syringe in a well-mixed area of the stream. We then filtered water through sterile PVDF 0.45 µm syringe filters (VWR) into clean bottles following the AIMS Surface Water Chemistry SOP (Burgin 2024). For all analytes, we froze samples until analysis in the lab using colorimetric methods (APHA 2017) on an AQ300 Discrete Analyzer (SEAL Analytical, Mequon, Wisconsin, USA). We prepared and applied reagents to samples and standards of known concentrations. For SRP, reagents react to form a blue complex, the absorbance of which is measured at 880nm. Known standards were used to create calibration curves ranging from 0 μg/L to 75 μg/L with check standards run every 10. Detection limits were set at 6 μg/L. We used the phenol method for NH4-N with known standards used to create calibration curves ranging from 0 μg/L to 500 μg/L and check standards run every 10 samples. Detection limits were set at 11 μg/L. To determine NOx-N, we analyzed samples with and without cadmium reduction with known standards used to create calibration curves ranging from 0 µg/L to 1000 µg/L for NOx-N and 0 µg/L to 750 µg/L for NO2-N with detection limits set at 20 µg/L NO3-N. Differences between NOx-N and NO2-N were used to calculate NO3-N. Triplicates were inspected for outliers with mean and standard deviation reported in data.