Thermal regimes of aquatic ecosystems are predicted to change as climate warming progresses over the next century, and disproportionately so in regions of high latitude and high elevation. Temperature is a major factor controlling organism development, especially among ectotherms where temperature dictates metabolism, gene expression, development of life stages, rate and degree of growth, and fecundity.
Cumulative degree days (CDDs) provide a measure of temperature accumulated over time and can be useful for inferring potential growth in organisms. While CDDs in aquatic environments accumulate in step with those in terrestrial environments, lags in heat transfer and the ability of water to buffer temperature changes leads to differences in the rate and magnitude of CDDs between the two environments, which may lead to disjuncts in the timing of aquatic-terrestrial ecological interactions.
We modelled spatial and temporal patterns in water temperature within a catchment in the Australian alpine region. Our models were used to predict average daily water temperatures under current and two future climate scenarios (rcp 4.5 and rcp 8.5). From these, we calculated average daily CDDs at each site and explored how patterns in water and air temperature CDDs are likely to change under future climate scenarios.
Our findings show that under future climate scenarios, alpine water temperatures will increase, but the magnitude of increase will vary with season. Degree Days will accumulate faster and reach much higher maximums than currently experienced, but at different rates in aquatic and terrestrial ecosystems. Consequently, insects that currently emerge into terrestrial environments during autumn may, in the future, do so in summer, when air temperatures are 12°C warmer than they currently experience. Our findings suggest that, in alpine regions, a few degrees warming may cause a substantial disjunct in the timing of ecological events between aquatic and terrestrial environments.