Macroinvertebrate Assemblages Differ in Lakes with and without Gizzard Shad

Gizzard shad Dorosoma cepedianum are a common omnivorous species that comprise significant biomass in lakes and reservoirs throughout the U.S.  By feeding on detritus, they can alter environmental conditions by increasing sedimentation and nutrient cycling in the water column, and their effects on zooplankton have been well-documented.  To examine the effects of gizzard shad on benthic macroinvertebrates, we sampled 28 lakes in Illinois over a 13 year period (204 lake-years): 13 lakes with and 13 without gizzard shad, and two lakes in which gizzard shad became established during the study period.  Secchi depths were lower whereas chlorophyll a concentrations were higher in lakes with gizzard shad.  Macroinvertebrate taxa differed in their response to gizzard shad: some taxa (including Hirudinea, Hydracarina, Megaloptera, and Tabanidae) were present in lower densities in lakes with gizzard shad whereas others (such as Chironomidae, Trichoptera, and Ephemeroptera) were not different.  No taxa were more abundant in lakes with gizzard shad, and there were no differences in overall macroinvertebrate density.  Whereas adult gizzard shad may consume macroinvertebrates incidentally while feeding on detritus, lower macroinvertebrate densities are likely associated with environmental changes mediated by gizzard shad.