HYDROLOGIC CONTEXT AND MACROINVERTEBRATE COMMUNITY RESPONSE TO FLOODSIN AN APPALACHIAN HEADWATER STREAM

The response of aquatic macroinvertebrates to two floods was examined in a 2nd-order stream draining a 132-ha watershed on the Fernow Experi mental Forest in West Virginia. Floods with recurrence intervals of 18 and 12 yr occurred in February and May 1994. Benthic organic matter a nd macroinvertebrate abundance and community structure were affected b y the floods. Most of the common macroinvertebrate taxa decreased in a bundance 70-95% immediately following the February flood. For most phy sical and biotic variables, recovery to preflood levels was complete b efore the May flood (42 days) based on preflood levels. Shifts in comm unity structure following floods were small compared to seasonal varia tion. By August following the May flood, few differences could be dete cted from August of the previous year. Taxa associated with organic ma tter accumulations such as leaf packs and organic debris dams were aff ected less by the February flood than were most riffle taxa; one commo n leaf pack taxa, Pteronarcys, increased in abundance in riffles follo wing the flood. I attribute the rapid recovery of the benthic communit y to abundant refugia, including organic debris dams, deep interstitia l habitat, and 1st-order tributaries. Analysis of the long-term flow r ecord for the Fernow Experimental Forest suggests that a probability-b ased hydrologic criterion for a flow disturbance is inadequate for App alachian headwater streams.