Characterizing the functional relationship between invertebrate drift and two primary controls—flow regime and benthic density

Background/Question/Methods

Two fishes of management concern in the Colorado River below Glen Canyon Dam, non-native rainbow trout and endangered humpback chub, are drift feeders.  Long-term monitoring of invertebrates is critical for evaluation of ongoing adaptive management experimentation because invertebrate production in this regulated river is low and dam operations strongly affect invertebrate production; however, the processes that regulate invertebrate drift in large rivers are poorly described, except in qualitative and general terms.  We sampled invertebrate drift concentrations (#·m^-3) and benthic density (#·m^-2) monthly for 1.5 years in the tailwater reach downstream of Glen Canyon Dam.  Benthic invertebrates were sampled during early morning hours when discharge was at daily minima whereas drift was sampled throughout the day and across a range of discharges.  We used mixed effects models to characterize the functional relationships among invertebrate drift concentrations, benthic invertebrate density, and discharge (m³∙s^-1).  Separate models were developed for each of the four taxa that dominate invertebrate production or contribute disproportionately to fueling fish growth—Gammarus lacustris, Potamopyrgus antipodarum, Chironomidae, and Simulium arcticum. 

Results/Conclusions

We found that the concentration of drifting invertebrates is controlled by the magnitude of the discharge, and by the density of benthic invertebrates upstream from the site where drift is measured.  Increases in discharge associated with hydroelectric power production cause exponential increases in the concentrations of Gammarus and Potamopyrgus in suspension (discharge raised to 3.4 and 3.7, respectively).  Concentrations of Chironomidae increase linearly with discharge (discharge raised to 1.0) while concentrations of Simulium decrease with discharge (-3.1).  Drift concentrations for all four taxa are positively related to benthic density, and for all taxa except Simulium the relationship is linear.  Concentrations of Simulium are negatively density-dependent and inversely related to discharge, which highlights the important role that behavior plays in mediating invertebrate drift entry.  The baseline drift density of Chironomidae and Simulium is 10-fold higher than for G. lacustris and P. antipodarum, which means that at a given benthic density, concentrations of these two insects are considerably higher than for the non-insect taxa.  The results of this study provide evidence that discharge and benthic density are primary controls on invertebrate drift, and the modeling framework we present allows these results to be generalized and applied to other rivers.