Chironomid-climate continentality conundrum

PLOS One | https://doi.org/10.1371/journal.pone.0327780 August 5, 2025 7 / 18 Results Climatic and environmental setting of the dataset PCA of the environmental and climate data indicated that GDD5 and ice-cover had the longest gradients (Fig 2A). Most tested variables, except for ATR, KOI, January and October mean temperatures, aligned with the first PCA axis, hence the explana- tory power of the axis was high (25.8%). Among the climatic variables, GDD5, KOI, ATR, October and July air temperatures were significantly correlated with one another and with most of the remaining climatic variables. April air temperatures were correlated with only two climatic variables: KOI and ice-cover. January air temperatures explicit no significant correlations with other variables (Fig 2B). Environmental variables (water depth, pH, soil base saturation, sand and clay content) generally had correlation values of <± 0.7 with climatic variables (GDD5, ice-cover, July, April, and October mean air temperatures, ATR, KOI) except for bedrock which was highly correlated (>0.8) with ice-cover, ATR, and October mean air temperature. Chironomidae assemblage composition and distribution The harmonized dataset includes on average 63 head capsules per sample with counts ranging from 42 head capsules to 139 head capsules. The dataset includes 51 lakes and 73 morphotypes (Fig 3; S1 File). The most abundant morphotypes are Chironomus plumosus-type (0.8–49.1% per sample), Psectrocladius sordidellus-type (0.7–26.9%), Dicrotendipens

(0.7–22.2%), and Tanytarsus pallidicornis-type (0.7–12.8%). The presence of Chironomus anthracinus-type, Heteratany- tarsus, Heterotrissocladius marcidus-type, Sergentia coracina-type, and Zalutschia characterises the transitional and oce- anic parts of the dataset. These taxa are either absent or present in lower quantities in more continental lakes. Transitional lakes are further differentiated from oceanic ones by the presence of Psecrocladius penicillatus-type and Pseudortho- cladius, and higher abundances of Heterotrissocladius marcidus-type, Tanytarsus pallidicornis-type and Microtendipes pedellus-type. Fig 2. (A) Principal component analysis (PCA) with total variation of 27.6% and (B) Spearman correlation matrix of the climatic and environ- mental variables in the dataset: bedrock type; lake-water depth (m); longitude; lake-water pH; lake ice-cover (days); growing degree days with base temperature 5°C (GDD5); October (Oct T), January (Jan T), April (Apr T) and July (Jul T) mean air temperatures (°C); Kerner Oceanity Index (KOI); and annual temperature range (ATR); soil base saturation (soil BS); soil clay content (soil clay); soil sand content (soil sand). Continental sites (orange) are KOI −10–0, transitional (green) are KOI = 0–10, and oceanic (blue) are KOI 10–20. https://doi.org/10.1371/journal.pone.0327780.g002