Water quality, periphyton, and benthic invertebrate data were gathered during the summer of 2024 from South Boulder Creek, a heavily modified stream, to assess the impact of D. geminata on aquatic ecosystem health. Four sites along South Boulder Creek were selected for analysis: El Dorado Canyon, South Mesa Trailhead, Van Vleet North, and Bobolink. El Dorado Canyon, the upstream site located within El Dorado Canyon State Park, is a mountain stream with a relatively undisturbed habitat and low urban land use. This site historically has high coverage of D. geminata during the late summer. The other sites were included as a part of a Boulder Open Space and Mountain Parks (OSMP) research award to study the coverage of D. geminata throughout the lower reaches of South Boulder Creek.
Stream temperature, specific conductivity, pH, and DO were measured using a handheld YSI probe. Flow at each site was assessed using a USGS Pygmy flow meter. Velocity measurements were taken at consistent distances at flat riffles with relatively uniform flow across all channels. A USGS rating curve was used to convert from clicks to velocities, and average flow was calculated from velocities x cross-sectional area. Samples for nutrients and total suspended solids (TSS) were collected in 1L amber glass bottles and transported to CU Boulder laboratory for filtration, preservation, and storage. A grab sample for cations were collected in an acid washed polyethylene bottle and transported to the University of Colorado Boulder Sustainability, Energy and Environment Laboratory (SEEL) for filtration, preservation, and storage. Amber bottle samples were filtered through a GF/C filter, and cation samples were filtered through a Nuclepore filter. After nitrate concentrations have been measured, nutrient samples were sent for analysis to a stable isotope laboratory in SEEC.
Periphyton samples were examined by microscopy and chlorophyll-a was measured using USGS methods. D. geminata cell abundance, ash free dry mass (AFDM) and chlorophyll-a were determined by removing material from a 5 cm x 5 cm area of the surface of five rocks at each site, and an aliquot was preserved with Lugol’s solution. Chlorophyll-a was determined using buffered acetone extraction following Wetzel and Likens (2000) and corrected for phaeophytin by monochromatic acid correction. Absorption was measured using a Thermo Scientific Genesys 20 spectrophotometer. The remaining material was placed into a 50 mL centrifuge tube with 10-20 mL of 30% hydrogen peroxide, allowed to react overnight, and filled up to 50 mL with de-ionized water (DI). Between 1-10 mL of the solution was settled in an Utermöhl settling chamber for 24 hours. Cell counts of D. geminata were made using a Nikon TS100 inverted microscope at 200x magnification. To estimate cells per cm2, ≥300 cells were counted randomly.
Following US EPA protocol, benthic invertebrates were collected from riffles within 100 ft of the site coordinates. Main channel riffles were selected as the habitat for surface benthic invertebrate collections. Samples were collected during summer to allow for comparison between sites, historical data, and nearby streams. Samples were collected monthly, though July was sampled twice to capture peak snowmelt flows. Invertebrates were identified to the family, genus, or species level using a microscopy and the 5th edition of An Introduction to the Aquatic Insects of North America, 2019. Samples were placed into a 15- cell container. Benthic invertebrates were sorted and enumerated up to ≥300 individuals. The number of cells it took to reach the 300 count was noted, and total abundances were estimated by dividing by the fraction of cells counted. Community metrics were quantified with Shannon Diversity Index, abundance, taxa richness, habit, functional feeding groups (FFGs), percent invasive species, Hilsenhoff Biotic Index (HBI), Ephemeroptera (mayflies)/Plecoptera (stoneflies)/Trichoptera (caddisflies) (EPT) index, and ratio of chironomids to EPT organisms. The site Hilsenhoff Biotic Tolerance Index (HBTI) was calculated by finding the average HBI value of all organisms found at a site.