Stable Isotopes Resolve the Drift Paradox for Baetis Mayflies in an Arctic River

The colonization cycle hypothesis states that stream ecosystems would become depleted of insects if flying adults did not compensate for drifting immatures. Using long—term drift and benthic abundance data, we show that a Baetis mayfly nymph population moves downstream during development in the Kuparuk River in arctic Alaska. Baetis relative benthic abundance decreased from early to late season in an upstream unfertilized river section, while simultaneously increasing in the downstream fertilized section. Baetis nymphs drifted significantly more in the upstream unfertilized section, compared to the downstream fertilized section where food was more abundant. Approximately one—third to one—half of the nymph population drifted at least 2.1 km downstream during the arctic summer. A stable isotope tracer experiment and mathematical models show that about one—third to one—half of the adult Baetis population flew 1.6—1.9 km upstream from where they emerged. These results provide a quantitative test of the colonization cycle for the dominant grazer/collector in the Kuparuk River. Quantifying the colonization cycle is essential to understanding stream ecosystem function because offspring of downstream insects are needed for nutrient cycling and carbon processing upstream. Since downstream drift and upstream flight are important components in recovery of streams from disturbances, our results provide a quantitative method for predicting recolonization rates from downstream, essential to estimating recovery.