Long-term patterns in phenotypic plasticity of chironomid head capsules in small high-elevation subarctic lakes: exploring possible mechanisms using multi-proxy paleolimnological records

Abstract We quantified temporal variation in the head capsule (HC) length of subfossil chironomids from two high-elevation subarctic lakes in northern Sweden and explored its potential drivers using chironomid assemblages, sedimentary pigments, and stable isotope analyses. Although establishing a reliable chronology was challenging because of the macrofossil-poor sediment context, radiocarbon dating indicates that the cores span approximately the last 5000 years, corresponding to the long-term cooling trend of the Late Holocene in this area. Temporal changes in the scores of the first principal component axis derived from chironomid assemblage data were strongly correlated with variations in the HC length of Micropsectra insignilobus -type in both lakes, with smaller HC reported when Micropsectra insignilobus -type occurred in high relative abundances. These results suggest that long-term changes in chironomid HC size closely tracked shifts in environmental conditions reflected in chironomid taxonomic composition. The observed increase in HC size over time is also consistent with the niche concept, whereby deviations from species-specific optima can alter fitness and promote size-related responses. Furthermore, no significant relationship was detected between the carbon stable isotope composition of chironomid HC and morphometric characteristics, suggesting that dietary changes were likely not a primary driver of the observed HC size variation. As a result, the observed increase in HC size over time could be driven by Late Holocene cooling favoring larger HC sizes in Micropsectra insignilobus -type. Future studies incorporating more robust chronologies are, however, needed to better resolve the links between climate-driven changes, body size dynamics, and community responses in these ecosystems. Overall, our results demonstrate the potential of paleolimnological records to reveal mechanisms underlying long-term body size variation in aquatic invertebrates.