Chironomid-climate continentality conundrum

PLOS One | https://doi.org/10.1371/journal.pone.0327780 August 5, 2025 5 / 18 Thus, the final dataset consists of 53 lake sediment surface samples along the longitudinal gradient 7–61.7 oE, between 54.5 and 59.5 oN latitude (Fig 1, Table 1). The dataset covers a broad range of environmental and climatic gradients. Most of the lakes are situated in low-elevation areas with fully vegetated catchments (Table 1). The surrounding biomes range from temperate steppe in the east to hemiboreal and temperate mixed in the central regions, and coniferous forests in the west. Bedrock includes sandstone in the eastern part of the dataset, limestone in the middle, and gneiss/granitoid in the western part (southern Norway and Sweden). Environmental data The basic environmental variables (lake-water depth and pH, sediment loss-of-ignition (LOI), and catchment soil and bedrock composition) were included to assess their influence on the Chironomidae assemblages and to examine potential confounding effects with the climatic variables. In connection with the surface-sediment sampling, lake-water depth and pH (at 30–40 cm above the sediment surface) were measured in the field. LOI was measured only for Russian and Latvian samples using the standard procedure [52]. The underlying bedrock type was identified using a bedrock map of Europe (Commission of the Geological Map of the World Subcommission for Europe; CGMW). Soil composition data (sand, clay, and soil base saturation) were extracted from the

FAO Digital Soil Map of the World (2003). While a multitude of other environmental parameters could affect the chironomid assemblages, we selected those that are most commonly used and consistently available across the entire dataset. Sediment sample collection and laboratory processing Surface-sediment samples for all parts of the dataset were collected using a gravity corer from the deepest part of each lake. The upper 2 cm of lake sediment were taken for analysis. Sampling did not involve endangered or protected spe- cies and was done following legal acts of the corresponding countries. In the laboratory, sediment samples of 5 cm3 were water-sieved with a 100-μm mesh to remove fine sediment. Each sample was then transferred to a Petri dish from which chironomid head capsules were extracted with fine forceps under a stereomicroscope at 25x magnification. The head capsules were air-dried and mounted in Aquatex® or Euparal® mounting medium. Taxonomic identification was conducted under a light microscope at 100–400X magnification. Taxonomic identification and dataset harmonisation For all four parts of the dataset (Norwegian, Swedish, Latvian, Russian), identification of the chironomid head capsules was done following the taxonomic approach of Brooks et al. [53]. Chironomid assemblages from the Latvian, Russian Table 1. Environmental and climate gradients covered by the new combined dataset. Growing degree days and ice-cover represents sum annual variables. Dataset origin Norway Sweden Latvia Russia Number of sites 6 9 7 31

Latitude (oN) 58.02–58.25 57.2–59.5 56.4–57.3 54.5–56.2 Longitude (oE) 7.0–8.2 11.5–18.3 21.7–27.1 28.2–61.7 Distance to sea (km) 3.2–20.2 2–110 41.2–176 255–1568 Elevation (m, above the sea level) 24–251 28–238 51–108 57–391 July air temperature (°C) 14.6–16.8 16.0–17.3 18.0–20.0 17.8–20.2 Annual temperature range (°C) 14.4–16.7 17–19.4 19.8–22.7 23.0–33.3 Kerner Oceanity Index 15.8–27.5 6–11.3 −2.4–8.4 −4.5 – −1.6 Growing degree days 5 (°C) 1200–1511 939–1088 1599–1729 1497–1880 Ice-cover period (days) 35–83 43–65 91–126 126–196 Sampling water depth (m) 5.2–19.5 6.5–45 2.8–20 0.7–12.5 Lake-water pH 5.1–7 6–7.2 6.8–8.6 7.5–10.8 https://doi.org/10.1371/journal.pone.0327780.t001