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).