Long-term thermokarst lake development and internal ecological feedbacks: A new reconstruction from Lake Satagay (Yakutia, Siberia)

The permafrost-shaped landscape of Central Yakutia is particularly rich in thermokarst lakes, which provide important cultural and ecosystem services to the local population. Climate warming and an intensification of agriculture in alaas systems (i.e. mostly drained basins of large thaw lakes formed during the early Holocene under warm climatic conditions) in the Central Yakutian Lowlands may lead to pronounced changes in water resources, water quality, nutrient loading and biodiversity. This could in turn threaten the livelihoods of affected communities, who depend on functional alaas ecosystems. To better foresee potential future impacts of environmental changes on internal lake ecological processes, it is important to gain a better understanding of how thermokarst lakes reacted to such changes in the past. Here, we present a new paleoenvironmental reconstruction of ecological changes within Lake Satagay (N 63.078, E 117.998, Nyurbinsky District), covering the last ca. 10,800 years. We use sedimentological and XRF-derived geochemical parameters, in addition to the metabarcoding of sedimentary ancient DNA (sedDNA) for diatoms and aquatic plants, and microscopic diatom analyses, to evaluate sedimentological and biodiversity shifts throughout the Holocene. Our study revealed 53 diatom DNA sequence types and 53 species morphologically. High distributions of Stephanodiscus and Fragilaria, among multiple other diatom genera in the early Holocene, indicate that initial formation of this typical alaas lake occurred earlier than expected (i.e. before 10,800 BP). In recent millennia diatom abundance decreased and their community is almost exclusively composed of Pseudostaurosira and Fragilaria. Composition of aquatic plants show an overall dominance of Ceratophyllaceae and strong fluctuations in Potamogetonaceae likely related to lake level and water chemical changes. All proxies investigated support that lake conditions and biotic composition has been resilient since 4,000 BP, but youngest samples since 47 BP indicate that land use influence has been crucial for the lake quality. This study represents a step towards a better understanding of climate and human-impacted alaas lake development and its consequences for their ecosystem services in eastern Siberia in the near future

Fires and forests: A reconstruction of Holocene fire-vegetation relationships in Central Yakutia, Siberia

The year 2021 set new records for wildfire extent in the Republic of Sakha (Yakutia) in eastern Siberia, Russia. Wildfire seasons in this unique region, characterized by its deciduous boreal forest and permafrost landforms, are becoming more intense. Some fires are threatening local communities, while their smoke covers vast stretches of land every summer, posing health risks to people even in the distance. At the same time, the larch trees of the eastern Siberian boreal forest stabilize the permafrost soils below as guardians of a continental-scale storage of terrestrial carbon. It is still largely unknown how the current trend of wildfire intensification will develop in the future, and how it will modify the structure of the boreal forests within the next decades to centuries. However, even though needed for a well-founded evaluation of long-term impacts of changing fire regimes, data on past trends of wildfire activity still remains scarce in eastern Siberia. Here, we present a new reconstruction of boreal fire and vegetation dynamics, spanning the last ca. 10.8 ka. Continuously analyzed macroscopic charcoal particles and a REVEALS-transformed pollen record from a sediment core from Lake Satagay (N 63.078, E 117.998) give insight into long-term trends and relationships between changes in fire regime and vegetation composition and coverage. The data indicates that modern larch-dominated forests co-exist with a lower severity fire regime, whereas early Holocene open larch-birch woodlands enabled increased charcoal accumulation and thus supported a higher severity fire regime. Considering the expected increase in tree mortality caused by wildfires and insect damage, likely to thin out currently denser tree stands, this fire-vegetation relationship suggests a potential upcoming positive feedback on intensifying fire regimes

Holocene wildfire and vegetation dynamics in Central Yakutia, Siberia, reconstructed from lake-sediment proxies

Wildfires play an essential role in the ecology of boreal forests. In eastern Siberia, fire activity has been increasing in recent years, challenging the livelihoods of local communities. Intensifying fire regimes also increase disturbance pressure on the boreal forests, which currently protect the permafrost beneath from accelerated degradation. However, long-term relationships between changes in fire regime and forest structure remain largely unknown. We assess past fire-vegetation feedbacks using sedimentary proxy records from Lake Satagay, Central Yakutia, Siberia, covering the past c. 10,800 years. Results from macroscopic and microscopic charcoal analyses indicate high amounts of burnt biomass during the Early Holocene, and that the present-day, low-severity surface fire regime has been in place since c. 4,500 years before present. A pollen-based quantitative reconstruction of vegetation cover and a terrestrial plant record based on sedimentary ancient DNA metabarcoding suggest a pronounced shift in forest structure toward the Late Holocene. Whereas the Early Holocene was characterized by postglacial open larch-birch woodlands, forest structure changed toward the modern, mixed larch-dominated closed-canopy forest during the Mid-Holocene. We propose a potential relationship between open woodlands and high amounts of burnt biomass, as well as a mediating effect of dense larch forest on the climate-driven intensification of fire regimes. Considering the anticipated increase in forest disturbances (droughts, insect invasions, and wildfires), higher tree mortality may force the modern state of the forest to shift toward an open woodland state comparable to the Early Holocene. Such a shift in forest structure may result in a positive feedback on currently intensifying wildfires. These new long-term data improve our understanding of millennial-scale fire regime changes and their relationships to changes of vegetation in Central Yakutia, where the local population is already being confronted with intensifying wildfire seasons

Late Glacial and Holocene vegetation and lake changes in SW Yakutia, Siberia, inferred from sedaDNA, pollen, and XRF data

Only a few palaeo-records extend beyond the Holocene in Yakutia, eastern Siberia, since most of the lakes in the region are of Holocene thermokarst origin. Thus, we have a poor understanding of the long-term interactions between terrestrial and aquatic ecosystems and their response to climate change. The Lake Khamra region in southwestern Yakutia is of particular interest because it is in the transition zones from discontinuous to sporadic permafrost and from summergreen to evergreen boreal forests. Our multiproxy study of Lake Khamra sediments reaching back to the Last Glacial Maximum 21 cal ka BP, includes analyses of organic carbon, nitrogen, XRF-derived elements, sedimentary ancient DNA amplicon sequencing of aquatic and terrestrial plants and diatoms, as well as classical counting of pollen and non-pollen palynomorphs (NPP). The palaeogenetic approach revealed 45 diatom, 191 terrestrial plant, and 65 aquatic macrophyte taxa. Pollen analyses identified 34 pollen taxa and 28 NPP taxa. The inferred terrestrial ecosystem of the Last Glacial comprises tundra vegetation dominated by forbs and grasses, likely inhabited by megaherbivores. By 18.4 cal ka BP a lake had developed with a high abundance of macrophytes and dominant fragilarioid diatoms, while shrubs expanded around the lake. In the Bølling-Allerød at 14.7 cal ka BP both the terrestrial and aquatic systems reflect climate amelioration, alongside lake water-level rise and woodland establishment, which was curbed by the Younger Dryas cooling. In the Early Holocene warmer and wetter climate led to taiga development and lake water-level rise, reflected by diatom composition turnover from only epiphytic to planktonic diatoms. In the Mid-Holocene the lake water level decreased at ca. 8.2 cal ka BP and increased again at ca. 6.5 cal ka BP. At the same time mixed evergreen-summergreen forest expanded. In the Late Holocene, at ca. 4 cal ka BP, vegetation cover similar to modern conditions established. This study reveals the long-term shifts in aquatic and terrestrial ecosystems and a comprehensive understanding of lake development and catchment history of the Lake Khamra region.</jats:p

Carbon and nitrogen composition of the sediment core EN18224-4 from Lake Satagay, Central Yakutia, Siberia

This dataset comprises total organic carbon (TOC), total inorganic carbon (TIC) and total nitrogen (TN) from sediment core EN18224-4, retrieved from Lake Satagay in Nyurbinsky District, Central Yakutia. TOC and TIC were observed from every cm of the core except 93-94 cm. TN was observed from 17 samples. The sedimentological analytics were performed in the Sediment laboratory at AWI Potsdam, Germany

Permafrost-thaw lake development in Central Yakutia: sedimentary ancient DNA and element analyses from a Holocene sediment record

In Central Yakutia (Siberia) livelihoods of local communities depend on alaas (thermokarst depression) landscapes and the lakes within. Development and dynamics of these alaas lakes are closely connected to climate change, permafrost thawing, catchment conditions, and land use. To reconstruct lake development throughout the Holocene we analyze sedimentary ancient DNA (sedaDNA) and biogeochemistry from a sediment core from Lake Satagay, spanning the last c. 10,800 calibrated years before present (cal yrs BP). SedaDNA of diatoms and macrophytes and microfossil diatom analysis reveal lake formation earlier than 10,700 cal yrs BP. The sedaDNA approach detected 42 amplicon sequence variants (ASVs) of diatom taxa, one ASV of Eustigmatophyceae (Nannochloropsis), and 12 ASVs of macrophytes. We relate diatom and macrophyte community changes to climate-driven shifts in water level and mineral and organic input, which result in variable water conductivity, in-lake productivity, and sediment deposition. We detect a higher lake level and water conductivity in the Early Holocene (c. 10,700–7000 cal yrs BP) compared to other periods, supported by the dominance of Stephanodiscus sp. and Stuckenia pectinata. Further climate warming towards the Mid-Holocene (7000–4700 cal yrs BP) led to a shallowing of Lake Satagay, an increase of the submerged macrophyte Ceratophyllum, and a decline of planktonic diatoms. In the Late Holocene (c. 4700 cal yrs BP–present) stable shallow water conditions are confirmed by small fragilarioid and staurosiroid diatoms dominating the lake. Lake Satagay has not yet reached the final stage of alaas development, but satellite imagery shows an intensification of anthropogenic land use, which in combination with future warming will likely result in a rapid desiccation of the lake

Pollen records from Lake Satagay, Yakutia, Russia

These datasets of sediment core EN18224-4 from Lake Satagay (63°04'40.8N 117°59'52.8E), Central Yakutia, Russia, comprise the following data: (I) Radiocarbon age dating results by AWI MICADAS; (II) continuous macroscopic charcoal counts (> 150 µm), including the sum of particles per sample, their respective distributions in size classes (>150 µm, 300-500 µm, > 500 µm) and morphotype groups, as well as a low-resolution record of microscopic charcoal concentration from pollen slides (III) pollen and non-pollen palynomorph counts. The datasets have been described in more detail in the accompanying research paper. Metabarcoding results of sedimentary ancient DNA of terrestrial plants, also featured in the research paper, will be uploaded to the Dryad database (datadryad.org)

Macroscopic charcoal record from Lake Satagay, Yakutia, Russia

These datasets of sediment core EN18224-4 from Lake Satagay (63°04'40.8N 117°59'52.8E), Central Yakutia, Russia, comprise the following data: (I) Radiocarbon age dating results by AWI MICADAS; (II) continuous macroscopic charcoal counts (> 150 µm), including the sum of particles per sample, their respective distributions in size classes (>150 µm, 300-500 µm, > 500 µm) and morphotype groups, as well as a low-resolution record of microscopic charcoal concentration from pollen slides (III) pollen and non-pollen palynomorph counts. The datasets have been described in more detail in the accompanying research paper. Metabarcoding results of sedimentary ancient DNA of terrestrial plants, also featured in the research paper, will be uploaded to the Dryad database (datadryad.org)

53 orthomosaics processed from Unoccupied Aerial Vehicle (UAV) image data of lake shorelines sampled during a field campaign in Central and Eastern Yakutia, Siberia in 2021 (RU-Land_2021_Yakutia)

During the RU-Land_2021_Yakutia summer field campaign in August and September 2021 in the Verkhoyansk Mountain Range in Eastern Yakutia and in the Central Yakutian Lowland, multispectral drone-based images were acquired over 53 selected lakes to analyse the vegetation and shallow lake waters along shores and to record the current lake shorelines. The images were taken in the course of further investigations of the lakes during that summer expedition. Baisheva et al. (2022) gives an overview of the lakes studied and the corresponding hydrochemistry. In addition, we published datasets including water isotope data of the lake (Stieg et al. 2022) and vegetation surveys of the lakeshores (Stieg et al. 2022). This dataset includes the orthomosaics (in raw data format (DN), and for the good-quality acquisitions normalised to surface reflectance) and the processing reports of the 53 sampled lakes. The event list gives an overview of the relevant lake information, which can be found here: https://doi.pangaea.de/10.1594/PANGAEA.955723 We used a consumer-grade, lightweight Unoccupied Aerial Vehicle (UAV) set up in combination with a D-RTK Station (GNSS antenna). The multispectral images were taken by a DJI Phantom 4 quadcopter UAV including an imaging system with one Red-Green-Blue RGB sensor and five spectral channels, able to capture both colour and narrow band images (5 bands: Blue, Green, Red, Red-edge, Near-infrared). A standardized flight plan was used to capture the shoreline of the lakes whenever possible using the DJI GS Pro app. If a preliminary route planning was not possible due to non-high-resolution map material or external circumstances (limited view, wind, rain), it was flown manually. For both flight procedures, multispectral images were taken automatically every 2 seconds. Speed was set to 4 m/s and altitude above ground level to circa 55 - 60 m. The produced UAV images were processed to construct the orthoimages using the software Agisoft Metashape Professional, version: 1.7.5. build 13229 (64 bit). Orthoimages are geometrically corrected images that are georeferenced to the topography (the relief) and vegetation (the top-of-canopy elevation). The orthomosaics were constructed from the multiple overlapping pictures from different camera viewpoints which make it possible to create a photogrammetric point cloud reconstruction and constructing the orthomosaics using structure from motion/multi-view stereo (SfM-MVS) techniques. There was no preselection of the images before the processing. Regular settings of the software were used, the processing parameters are listed in the individual processing report of each orthomosaic. The Micasense DLS2 illumination-sensor data, measured in parallel during each acquisition, was used to normalize the Digital Number (DN) of the orthomosaics to surface reflectance