Aquatic insect larvae in streams live under the constant influence of unidirectional flow of water, which exposes their populations to continual depletion through drifting. Replenishment of upstream populations by oviposition of flying adults has been a logical explanation for maintenance of upstream populations. Little quantification of this phenomenon exists, however. Upstream flight of adult Baetis mayflies was directly observed in the Kuparuk River, AK. A stable isotope tracer release (15N-labelled ammonium) showed that adult migration extended upstream for up to ca. 2 km. Applicability of these results to other taxa and to other systems is uncertain, however, due to the dwarfed riparian trees, lack of topographic relief, and homogenous landscape at the Arctic Tundra LTER. Anecdotal observations suggest that emergent adults can travel long distances (kilometers), but except for the study by Hershey et al., there are no accurate measurements of migration distances. In addition, no data exist describing the rates of between-stream migration across the landscape.

I propose a study of the migration patterns of emergent adult stream insects that have been labelled while in larval form in the reach below a 15N release. Adult insects will be sampled at the end of a 6-wk 15N-enriched ammonium release, using 14 battery- operated light traps. Traps will be placed in the riparian zone of the stream at various points and at nearby streams depending on local topography and land-use patterns of the site. This will allow evaluation of migration patterns longitudinally along the stream corridor and drainage network, as well as laterally across topographic features and land-use patches (pasture, crops, forest, wetland). Trapped insects will then be sorted to species and analyzed for their delta 15N values to detect isotopic enrichment.

Distances of migration will affect recolonization rates and sucession following disturbance, as well as gene flow between populations in neighboring watersheds. Insects are not expected to migrate across topographic ridges, landscape patches with open canopies, or for long distances laterally through woodlands. Fragmentation of terrestrial woodlands including riparian zones may reduce migration distances. Quantification of the migration patterns of emergent stream insects is thus essential not only to understand basic questions of community ecology, behavior, and evolution, but also applied questions of the effects of habitat fragmentation on the restoration and management of stream ecosystems.