Diatom taxon abundances at river sites in the Cordillera Blanca region of the Peruvian Andes, 2020

At each site, benthic diatoms were collected from five submerged cobbles. Using a sterile toothbrush, the benthic biofilm on the upper surface of each cobble was scrubbed from a 9 cm2 area using a miniature quadrat. Organic material was removed using the hot hydrogen peroxide (H2O2) method and replicate samples mounted on individual slides. Diatom valves were identified and counted using light microscopy at x 1000 magnification using phase-contrast illumination (Olympus CX43 microscope). A minimum of 500 diatom valves were identified from each replicate. Where less than 500 valves were present, all valves were counted. To determine estimates of valve density, the number of valves identified within coverslip transects was used to estimate the total number present on the whole coverslip (0.5 mL) and then multiplied to m-2 based on sample volume and rock area (9 cm2) sampled. Abundances were then averaged across the five replicates to mean diatom density (valve m-2) at each site.

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