Hydrologic context and macroinvertebrate community response to floods in 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 Experimental Forest in West Virginia. Floods with recurrence intervals of 18 and 12 yr occurred in February and May 1994. Benthic organic matter and macroinvertebrate abundance and community structure were affected by the floods. Most of the common macroinvertebrate taxa decreased in abundance 70-95% immediately following the February flood. For most physical and biotic variables, recovery to preflood levels was complete before the May flood (42 days) based on preflood levels. Shifts in community structure following floods were small compared to seasonal variation. By August following the May flood, few differences could be detected from August of the previous year. Taxa associated with organic matter accumulations such as leaf packs and organic debris dams were affected less by the February flood than were most riffle taxa; one common leaf pack taxa, Pteronarcys, increased in abundance in riffles following the flood. I attribute the rapid recovery of the benthic community to abundant refugia, including organic debris dams, deep interstitial habitat, and 1st-order tributaries. Analysis of the long-term flow record for the Fernow Experimental Forest suggests that a probability-based hydrologic criterion for a flow disturbance is inadequate for Appalachian headwater streams.