Chironomid-climate continentality conundrum

PLOS One | https://doi.org/10.1371/journal.pone.0327780 August 5, 2025 12 / 18 which may explain why lake-water pH has a higher explanatory power in our dataset compared to other chironomid data- sets [21,42]. The strong negative correlation between ice-cover and bedrock type (r = −0.83) suggests that variations in bedrock may also influence the water pH, in addition to the direct effects of ice-cover. The high explanatory power of bedrock type (16.8% variation in chironomid assemblages) can be explained by the fact that bedrock influences water chemistry, soil, terrestrial vegetation type, productivity of the ecosystem, and catch- ment erosion processes [76–84]. The explanatory power of lake-water depth (8.3%) is considerably stronger than in other studies [21,42,85]. This may be due to the broad depth range in our dataset (1–45 m), whereas the datasets referenced above did not include lakes deeper than 21 m. Water depth influences chironomids via variations in water temperature, oxygen concentration, habitat structure, macrophytes presence and food quality and availability [37,86]. The properties of surrounding soils (base saturation, sand and clay content) most probably have an indirect effect on chironomid assem- blages through changing the limnological conditions by regulating the drainage of organic and inorganic components and during catchment erosion processes [87–93]. The independence of lake-water depth and pH, as well as soil properties from climatic variables makes it possible to separate the effect of local environmental factors from regional climate factors

on the chironomid assemblages. Morphotype-specific relationships with kerner oceanity index Chironomid taxa identified as characteristic of the continental group (Fig. 5) are commonly identified as warm summer- related ones [21,39,42]. Our study indicates that a preference for warm summer temperatures is accompanied by a tolerance for a short growing season in these taxa. The only exception is Neozavrelia, which was previously considered as a cold stenothermic taxon [53]. However, according to recent findings from central, eastern, and northern Europe [21,42] it appears to be a warm-related one. The modern distribution of Neozavrelia taxon includes both extreme oceanic climates (Norway, Russian Far East, Japan) as well as highly continental ones (Eastern Siberia) [53,94]. Glyptotendipes pallens-type has been observed to tolerate severe winter conditions [95], which probably helps it to survive in continental climates. Microchironomus, Polypedilum sordens-type, and Glyptotendipes have been recorded emerging in March and April, even beneath the snow/ice or through ice cracks [96]. Such behaviour can be considered as an adaptation to cold winters [97]. Also, in continental conditions of rapid seasonal change and hot summers, the emergence in early spring helps to avoid extreme air heat during mating. Chironomus plumosus-type, identified as a continental to transitional mor- photype, reveals another survival strategy: during winter diapause nearly all the larvae are in the fourth instar and start the

active emergence in June in Russia [98]. Also, a study by Self et al. [24] found that Chironomus plumosus-type show sig- nificant responses to continentality. From a morphological perspective, a small body size (Microchironomus, Naeozavrelia) and pigmentation (Glyptotendipes pallens-type, Chironomus plumosus-type, Polypedilum sordens-type, Neozavrelia) can help to survive cold winter conditions [97]. Most of the oceanic and oceanic-transitional morphotypes reveal a cool summer-related distribution [21,39,42]. Preference of a longer growing season is suggested by our data. Self et al. [24] identified Heterotrissocladius marcidus-type, Heterotanytarsus, Sergentia coracina-type, Pseudorthocladius, and Thienemannimyia as low continen- tality (oceanity) dependent taxa, which aligns well with our results. Also, these authors explain the distribution of Pseu- dorthocladius and Thienemannimyia in oceanic sites with their terrestrial and splash zones habitats where melting, refreezing, or wind removal can expose the chironomids to cellular damage from repeated freeze-thaw cycles. Ter- restrial chironomids have adapted to such disturbances through behavioral and physiological mechanisms, including hibernation and the ability to lower their body’s freezing point [97]. Terrestrial Limnophyes, which is oceanic climate related in our dataset, is expected to have the same adaptations. Heterotrissocladius marcidus-type [99], Sergentia coracina-type [100], Chironomus anthracinus-type, Procladius [95], and Tanytarsus chinyensis-type [101] emerge in

mid-summer and in autumn (September-October), and are thus adapted to oceanic cool springs, mild summers, and warm autumns.