Habitat use by Allocapnia rickeri and Allocapnia wrayi in a small North Carolina piedmont stream

Abstract

The purpose of this study has been to evaluate habitat use by Allocapnia rickeri and Allocapnia wrayi in a small piedmont stream in Raleigh, North Carolina. Five different surface habitats (debris dams, riffle mineral, riffle leaf material, pool mineral and pool leaf material) were sampled to determine which were favored by actively growing Allocapnia larvae and to detect changes in preference of habitats during the active growth phase. Larvae preferred habitats associated with leaf material, especially debris dams and riffles prior to emergence. Smaller larvae were more commonly associated with riffle mineral habitats before shifting to leaf material habitats. The active growth phase started in late October after diapause break and ended with emergence in December and January. The hyporheic zone was sampled separately using core samples in order to evaluate length of diapause and characteristics of the hyporheic zone habitat. Larvae entered diapause in March and diapause break occurred in late October. The diapause period was longer than reported for Allocapnia in Canadian studies. Larvae were most dense between 10 and 20 centimeters into the hyporheic zone. Temperature did not decrease with depth during the hottest months and dissolved oxygen dropped rapidly with depth into the hyporheic zone. Larvae were collected where dissolved oxygen was near 0 % saturation. Larvae were most dense when pore space was near 15 percent of the sample layer and average particle size was between two and three millimeters. This is the first ecological study of Allocapnia at the southeastern edge of their distribution and in an intermittent stream.