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.