Sample collection
Surface sediment samples were collected in Nam Co using an Ekman-Birge bottom sampler from water depths ranging between 0.2 m to 56 m (Table S1). A spatula was used to sample the oxygenized layer of sediments, i.e. about top 2 cm. Additional samples from shallow water sites such as rivers and lagoons were collected randomly from sandy substrate by using a hand-shovel and scraping the upper 2 cm. Approximate wet weight of a sample was 200 grams, which was mixed, split in half, and these two batches were then transferred to Whirl-Pak bags for metabarcoding and morphological identification of diatoms. Limnological parameters were measured at each sampling site using a multi parameter probe WTW 3630 (Table S1). Water anions and cations were measured using ion chromatography (IC) and inductively coupled plasma optical emission spectrometry (ICP-OES), respectively, at the Institute of Geographical Sciences, Freie Universität Berlin (Table S1). The research permit was obtained via the Institute of Tibetan Plateau Research, Chinese Academy of Sciences from the Tibet Autonomous Region Government.
In the field, laboratory samples for the morphological identification of diatoms were stored at 4 °C. Samples for metabarcoding analyses were sieved through 2 mm sieves to remove coarser sediment components. Tap water was used for sieving, and therefore also as negative extraction (and PCR) control for metabarcoding analysis. Approximately, 50 g of sediment were divided between three 50 ml tubes and filled with 96% ethanol (4:1 ethanol:sediment ratio). All used equipment was bleached (10% sodium hypochlorite solution) after each step to avoid cross-contamination. All samples were stored and transported in a freezer (-20 ºC). The study design is illustrated in Figure 1.