Application of Acoustic Signals to Investigate Macrophyte Growth and Bottom Sediment Stability after Carp Removal in a Small Lake

Abstract

This study combined acoustic-signal based devices including subbottom profiler (SBP) and side-scan sonar (SSS) and in-situ sediment data to investigate macrophyte growth and stability of bottom sediments before and after carp removal in Lake Wingra, Wisconsin, U.S.A. In 2007, i.e. before carp removal, carp deteriorated water quality, and macrophytes only grew to the water depth of 2 m. Meanwhile, SBP data showed a loose and fluffy sediment layer in unvegetated region, which was easily affected by wave or current motions. In 2009, i.e., after more than half (51%) of carp was removed, SBP data presented disappearance of the fluffy sediment layer, and both SBP and SSS detected that submerged vegetation grew in deeper water depth. In-situ sediment data performed that bulk sediment density and critical shear stress collected in 2009 became greater than those in 2007, which means bottom sediments turned into more consolidated and were harder to be resuspended. Secchi depth obtained during 2008 to 2010 greater than that in the previous ten years indicated that water became clearer. In short, the fluffy sediment layer may be the main source for suspended sediments to deteriorate water quality, and removal of carp is crucial for stabilizing bottom sediment and improving water clarity in this small lake. By using several acoustic features, lake managers can monitor environmental changes more effectively and efficiently, and provide appropriate management efforts in short time.