EFFECTS OF SUBSTRATE AND PREDATOR TYPE ON MICRODISTRIBUTIONS AND DRIFT OF A LOTIC MAYFLY

This study examines the effects of substrate and predator type on micr odistributions and drift responses of nymphs of the mayfly Heptagenia hebe in a warmwater sandy stream in northern Michigan. Nymphs were sig nificantly more abundant on cobble/boulder substrates than on gravel/p ebble or woody debris. Mayfly densities among these substrates were un related to densities of predaceous stoneflies (perlids) and hellgrammi tes (Nigronia serricornus). Nocturnal drift, measured instream from en closed substrates, was significantly lower from cobble/boulder substra tes (0.1%) than from gravel/pebble and woody debris. Drift rates were unrelated to stonefly, crayfish, or fish (mottled sculpin and hornyhea d chubs) densities, but were positively correlated with hellgrammite d ensities. However, experiments in artificial stream channels revealed that propensity to drift was strongly linked to substrate type, not pr edator type (hellgrammites and/or stoneflies). Cobble/boulder substrat es apparently function as sinks (where immigration > emigration) for d ispersing H. hebe nymphs in sandy streams with limited suitable habita t.