Field Methods
During January 2019, we collected hyporheic sediments nine transects across pools, meanders, and riffles along Von Guerard Stream in Taylor Valley, Antarctica. Each transect was composed of five sampling sites at different lateral positions across the stream channel: two sampling sites were located at the margins of the visible wetted zone, one sampling site was directly under the thalweg, and two sampling sites were located halfway between the wetted margin and the thalweg. Transects ranged from 6.6 m to 13.6 m in total length across the stream channel. At each sampling site, we removed the surface sediment and mat material in order to focus our sampling on the underlying hyporheic sediments. To characterize the diatom assemblages, we collected a sample in a plastic scintillation vial that was preserved in formalin directly after sample collection.

Laboratory Analyses
Diatom assemblages were characterized at six of the nine transects (two transects for pool, meander, and riffle) by counting a total of 300 diatom valves from preserved samples. First, a subset of sediment was transferred to 50 mL centrifuge tubes. We cleaned samples of organic content by digesting with hydrogen peroxide (30 mL of 30% H₂O₂) for 48 hours at 100ºC. After digestions were complete, samples were repeatedly rinsed with distilled water until a neutral pH was reached. We isolated diatoms from sand grains and other mineral aggregates through repeated resuspension and centrifugation to carefully retain the diatoms in the supernatant. Aliquots of supernatant were dried on coverslips and mounted to glass microscope slides using a highly refractive adhesive (Zrax, W. P. Dailey, Philadelphia, USA). We counted diatoms at 1250x magnification on an Olympus BX50 DIC microscope and identified taxa using descriptions listed in the Antarctic Freshwater Diatoms Database. Fragmented diatom frustules were identified to the highest taxon-level possible (mostly genus level). All relative abundance composition data was standardized to the total number of diatom valves counted per sample.