Repopulation dynamics of the macrobenthos in a riffle area of the Little Brazos River, Texas

Abstract

Reestablishment mechanisms were investigated after a riffle microbenthic community had been killed. Kills were found to be correlated with rains and run-off from cultivated fields at the time of increased chemical applications to the crops. These rains followed a summer dry period. Most of the pre-kill taxa were found to have reestablished in the riffle within 3 weeks following the kill. Repopulating organisms were located upstream of the biocide source point at a minimum distance of 8 to 12 miles from the sample riffle. The reestablishing organisms were translocated on the backside of spates to the sample site by a combination of drifting and rafting. Rafting is described here as movement downstream while attached to or contained inside organic detritus and aquatic vegetation. Apparently, rafting has never been described in relation to fresh water invertebrate translocation. Many of the riffle microbenthic organisms were found to be associated with floating materials entering the riffle. Chemical factors had no apparent effect on drifting or rafting during reestablishment. Physical controlling factors of diel periodicities had no measurable effect on rafting organisms emphasizing the importance of rafting in the reestablishment of the riffle microbenthic community.