Exploring reversibility and contrasting patterns in temperature-size relationships across spatial and temporal scales using subfossil chironomids

The extent to which different magnitudes and directions of temperature
fluctuations explain long-term trends in aquatic invertebrate body size in
nature is largely unknown. Using elevation gradients and paleolimnological
reconstructions, we tested the hypotheses that variations in subfossil
chironomid (non-biting midges) head capsules (HC) will covary with
temperature changes, with opposite morphometric changes occurring during
warming and cooling phases, and that body size variation can be modified
by other environmental conditions unassociated with temperature
variations. Results indicated that the effects of increasing temperatures
on chironomid HC size were reversed when temperatures decreased, with both
warming and cooling producing similar effect sizes, corresponding to a
change in HC length of ~3% per 1°C. Additionally, our results showed that
other environmental drivers can mask temperature effects on chironomid HC
sizes. Specifically, we found that bottom water oxygen concentration was
negatively associated with HC lengths of Chironomus anthracinus-type. We
hypothesize that this pattern is driven by prolonged larval development in
oxygen-depleted lakes and/or changes in basal food sources used by
chironomid larvae, ultimately affecting their final body size. To gain a
better understanding of aquatic insect size variation over long
timescales, it is essential to uncover the underlying mechanisms that
control their body size, as these factors may either enhance or compound
the temperature-size relationship.