HABITAT SELECTION BY THE STREAM MAYFLY PARALEPTOPHLEBIA-GUTTATA

The causes and consequences of habitat distribution of larvae of the m ayfly Paraleptophlebia guttata were analyzed in a 2nd-order stream in west-central Kentucky. Among macrohabitats, larvae were typically sign ificantly more dense in runs and riffles than in pools. Densities were positively related to the percentage of coarse substrata in macrohabi tats, but were not correlated with current velocity, predatory fish de nsities, or the quantity of fine particulate organic matter (food) in macrohabitats. This relationship between density and substratum coarse ness was further evidenced in an in-situ colonization experiment in wh ich larvae selected gravel and pebble substrata over sand. Among micro habitats, larvae were more abundant in leaf litter and in gravel/pebbl e than in silt/sand. Laboratory manipulations showed that predation ra tes by fish were significantly lower on mayflies in leaf litter and gr avel than in fine substrata. The combined effects of substratum partic le size and fish predation determine habitat distributions of larvae. Larger larvae were more abundant in runs than in riffles, but larval g rowth rates in artificial stream channels were similar between current velocities simulating those in riffles and runs. These results, coupl ed with data from drift samples, suggest that larvae shifted macrohabi tats with size. Size-related differences in macrohabitat use may refle ct size-related changes in microhabitat use; larvae were significantly larger in leaf litter than in other microhabitats, and leaf litter am ounts were greater in runs than in riffles.