2.1. Study area and sampling
The study region is located in the lower Kolyma watershed (Figure 1) in a transitional zone between boreal forest (northern taiga) in the south and tundra ecosystems in the northern Yakutia. Kolyma is one of the largest pan-Arctic watersheds with a total area of 653,000 km2, a total river length of 2,129 km, and annual water discharge of 109 ± 7 km3 (Holmes et al. , 2012), fully underlined by continuous permafrost (Mann et al. , 2012). The Arctic coastal lowlands of northeastern Yakutia constitute vast accumulative plains formed mostly by the Late Pleistocene loess-like Yedoma Ice Complex, characterized by a high ice and organic matter content (Abramov et al. , 2021). Permafrost thickness in the area reached 500-600 m, and mean annual ground temperatures (MAGT) before the 1990s were of −8 to −11°C within the tundra and −7°C to −8°C in the taiga (Yershov et al. , 1991). The very high ice content (up to 95% by volume) of Yedoma deposits in the Kolyma lowland (Strausset al. , 2021) makes them extremely vulnerable to climate warming. In 2000-2016 (Ran et al. , 2022), MAGT in the studied region increased significantly against the former century due to climate warming, and now ranged from −8°C to −4°C. Mean annual air temperature (MAAT) in Cherskii (68.75°N, 161.28°E, 28 m asl) increased from −12 °C in the 1960s to −10 °C in the 2000s, with a positive trend of 0.0472 °C per year (Sakai et al. , 2016).
MAAT in Cherskii in the year of sampling (2021) amounted to -9.8°C and was lower than the MAAT in the years 2006-2021 (−8.9°C) (Suppl. Mat. 1, Table S2 ). However, the maximum air temperature in 2021 was the highest in the years 2006-2021 (tmax = 33.4°C) and it occurred directly before our sampling (Suppl. Mat. 1, Table S2 ,Figure S1 ). In the study area, the minimum air temperature ranged from −40.2°C to −49.7°C and in 2006-2021 was observed in January, February or December. The maximum air temperature was observed in June-August, ranging 25.9°C – 33.4°C (Suppl. Mat. 1, Table S2 ). The mean monthly air temperature was higher than 0°C from May to September (in 2006-2021) (Suppl. Mat. 1, Figure S2 ).
Samples have been collected between 19th and 22nd July 2001 (28 in total). River waters were collected in the four cross sections of Kolyma (Cherskii, samples K11-K13; Yermolovo, K21-K23; Duvanny Yar, K31-K33, K31b-K33b; Kolymskoye, K4), and in the mouth sections of the Anyui (A1, A2) and Omolon (O1) tributaries. In addition, a meltwater creek and ice were sampled from a permafrost cliff located at Duvanny Yar (P1-P3), and lake waters were collected from two thermokarst lakes (L11-L15, L21-L24) (Figure 1, a detailed description in Suppl. Mat. 1, Table S1 ). The Kolyma river samples were taken at the middle depth of three vertical profiles in the channel cross sections, except of Kolyma upstream of the permafrost cliff, where only one sample (from the middle of the channel) was collected. Water samples were pumped out with a filterless submersible 12V pump (details in Chalov et al. , 2022). In both lakes, samples were taken along a vertical profile located above the deepest point of the lake. Among the sample sites, direct permafrost influence was expected at the permafrost cliff, in Lakes 1 and 2 and in Kolyma cross section K3 (directly below permafrost cliff).