Habitat scale hydraulics mediate macroinvertebrate response to water withdrawal among watersheds of different size

The importance of watershed variation on instream responses to water withdrawal has been rarely studied for tropical ecosystems.  Four geographically related tropical streams of differing size (discharge) were chosen to study benthic community response to water withdrawal.  It was hypothesized that stream size and habitat flow velocity would be inversely related to macroinvertebrate densities and changes in native taxa richness.  In August 2007 and May 2008, six riffle benthic samples were collected upstream and downstream of a diversion in each stream.  Velocity profiles were measured above each sample and along reach transects.  For stream discharge, there were significant main effects of watershed (F=1857, df=3, p<0.0001) and site (F=7670, df=3, p<0.0001) in addition to an interaction (F=1612, df=3, p<0.0001), suggesting the effects of diversion were dependent on stream size. Similarly, there were significant main (watershed F=5.254, df=3, p=0.0038; site F=7.131, df=1, p=0.01) and interaction effects (F=4.607, df=3, p=0.0073) for total macroinvertebrate density.  Density and richness were usually lower downstream, suggesting impact on biodiversity; possibly driven by significantly lower riffle habitat velocities (all p<0.001).  For tropical stream management, watershed differences appear important for understanding water withdrawal; effects that are mediated by habitat scale changes in hydraulic factors important to native stream taxa.