# Thermal tolerance of giant salmonfly nymphs (Pteronarcys californica) varies across populations in a regulated river

## TL;DR

Giant salmonfly nymphs from warmer habitats have lower thermal tolerance than expected, suggesting thermal variation, not just high temperatures, affects their survival.

## Contribution

The study reveals that thermal variation, rather than maximum summer temperatures, drives unexpected thermal tolerance differences in salmonfly populations.

## Key findings

- Warmer habitat salmonfly nymphs had a cooler optimal thermal range than cooler habitat nymphs.
- Thermal variation, not just high temperatures, influences thermal tolerance and nymph survival.
- Summer daily maximum temperatures may limit aerobic capacity in warmer habitat nymphs.

## Abstract

Contrary to our prediction, salmonfly (Pteronarcys californica) nymphs originating from the warm extreme of a thermal gradient had a cooler metabolism-based optimal temperature range than nymphs originating from a cooler habitat. We posit that these local adjustments reflected the combination of thermal extremes and thermal variation experienced within the habitats.

Warming of aquatic ecosystems is transforming the distribution, phenology and growth of the organisms dependent upon these ecosystems. Aquatic insects such as stoneflies are especially vulnerable to warming because the aquatic nymph stage of their life cycle depends on cool, well-oxygenated, flowing water habitat. We tracked thermal effects on available aerobic capacity of the aquatic nymph stage of an iconic and vulnerable stonefly species, the giant salmonfly (Pteronarcys californica), to compare habitat thermal regime measurements for two salmonfly populations from habitats separated by a gradient in summer weekly maximum temperatures. Contrary to expectations, the thermal optima range of the warmer habitat population was cooler than for the cooler habitat population. We posit that this unexpected interpopulation variation in thermal response is more strongly driven by diel and seasonal thermal variability than by the highest summer temperatures experienced within respective habitats. Additionally, we show that summer daily maximum temperatures could result in periodic limits in available aerobic capacity to support work of the warmer habitat nymphs and may be the mechanism underlying reduced abundance relative to the upstream cooler habitat population. Our findings provide insight into potential thermal and metabolic mechanisms that could regulate the success of ecological and culturally important aquatic insect species experiencing global change. We conclude that thermal regimes and thermal variation, not just mean and maximum temperatures, are critical drivers of aquatic insect responses to water temperatures.