Differential response of stream periphyton and invertebrate grazers to habitat modification by the emergent macrophyte Justicia americana

An instream experiment was conducted using artificial substrate tiles during summer to examine the relative importance of shading by the emergent macrophyte Justicia americana and top-down control by stream grazers on summer periphyton accrual. Macrophyte treatments included removal of above-ground stems, removal of stems and rhizomes and an unmanipulated control, whereas grazer treatments included snail-accessible and snail-exclusion tiles. Above-ground Justicia structures reduced both sunlight to the stream bed and velocity by almost 50%. Abundance of the dominant snail, Elimia ucheensis (Pleuroceridae), on snail-exclusion tiles was significantly lower than on snail-accessible tiles only during the first week of the experiment; therefore, the barriers were ineffective over the entire experiment. Despite ineffective reductions of Elimia abundance over the entire experiment, periphyton accrual was higher on snail-exclusion than snail-accessible tiles irrespective of macrophyte treatment. Among the macrophyte treatments, periphyton biomass (as ash-free dry mass) was significantly lower on tiles in the unmanipulated control than treatments where Justicia stems were removed. Higher mean differences and relative magnitude of effects were associated with the macrophyte treatments when compared with the grazer treatments. Periphyton appeared to be primarily light-limited by Justicia and secondarily controlled by grazing invertebrates; however, the effects of grazing may have been underestimated because snail-exclusion barriers were ineffective at reducing grazer abundance over the experiment. Removal of Justicia canopy did not result in higher abundance of any invertebrate grazers on tiles; however, the abundance of the freshwater limpet (Ferrissia mcneili) was twice as high on tiles within the more shaded Justicia control sections compared with either of the two open-canopy treatments. Habitat modification by Justicia can negatively affect benthic primary producers and influence the distribution of some, but not all, primary consumers.