Chironomid-climate continentality conundrum

PLOS One | https://doi.org/10.1371/journal.pone.0327780 August 5, 2025 2 / 18 Introduction Continentality, a climate parameter that combines information on annual variation in temperature and precipitation, has changed in the past and is predicted to change in the future [1,2]. Continentality of a region depends on the distance from the ocean and the prevailing atmospheric circulation patterns [2]. While the annual temperature range (ATR, calculated as the difference between the coldest and the warmest months) is the simplest and most used metric to estimate continentality, several other indices have been proven relevant to describe the continentality gradient in nature. These include the Gorzynski [3] continentality index, which includes latitude in the index, and the Kerner Oceanity Index (KOI; [4]), which includes October and April air temperatures. Continentality has significantly changed over the last century [2,5], resulting in an increase in Northern Europe, most of North America and East Asia [2], and a decrease in the Eastern Baltic countries (Estonia, Latvia, Lithuania) [6]. These changes are expected to continue in the context of ongoing climate change [6,7]. Continentality variations may occur due to changes in solar radiation, and variations in atmospheric circulation and ocean heat transportation [8]. These changes affect various natural processes, such as permafrost degradation [9], ecosystem productivity [10], biodiver- sity distribution [11], tree bimodality growth [12]. Continentality can affect the aquatic

zoobenthos by inducing variations in the start, duration, and heat accumulation of the growing season [13] as well as the timing of lake turnover [14]. It can also influ- ence the formation and duration of the ice-cover at mid and high latitudes, leading to changes in water pH [15] and dissolved oxygen concentration [16]. Northern and eastern Europe, and the Baltic area in particular, are situated in a transitional zone between continental and oceanic climates, making this region highly suited for stud- ies of long-term changes in continentality. Furthermore, a marked increase in conti- nentality (annual temperature range) has been observed in the eastern Baltic area during recent decades (−1.7 KOI values per decade; [2]), highlighting the urgent need for past continentality-related knowledge, which could assist in making realistic, evidence-based continentality predictions. Understanding long-term continentality is essential for accurate climate modelling, ecosystem management, climate change adaptation, land-use planning, and unravelling the Earth’s geological history. It pro- vides a framework for interpreting both past and future climate dynamics, especially in regions where the influence of landmasses dominates over oceanic moderation. Long-term reconstructions of past changes in climate parameters are often used to determine and predict their impacts on ecosystems (e.g., [17–22]). Only a few reconstructions of palaeocontinentality have been published [23–27]. For instance, increased seasonality has been inferred from cryogenic cave carbonates in Great

Britain during the Younger Dryas period event. [28], and from phosphorus concen- trations changes in stalagmite calcite in western Ireland during the 8.2 ka cooling [29]. Other attempts to develop a continentality reconstruction have been based on tree rings or ice wedges data [25,27]. However, tree-ring data are confounded by too many intertwined environmental and climate factors, while ice wedges are limited to the few regions that historically had permafrost. Thus, not much is known about conti- nentality changes in the past and their impact on ecosystems. the European Union, European Regional Development Fund (ASTRA “TTÜ arengupro- gramm aastateks 2016–2022”). Anneli Poska was supported by Swedish strategic research area BECC (Biodiversity and Ecosystem Services in a Changing Climate); MERGE (ModElling the Regional and Global Earth system at Lund University). Competing interests: The authors have declared that no competing interests exist.