The flow regimes and associated ecosystem function of many desert rivers in southwestern North America have been significantly altered from their historic state due to anthropogenic alteration. The Rio Grande in particular has a significantly modified flow regime, and several endemic species have been reduced or eradicated from the loss, or fragmentation, of viable habitat, poor water quality, and spread of invasive species. The primary goal of our study is to determine how the modified hydrologic and sediment regimes of the Rio Grande have impacted the aquatic food web structure and function, with an emphasis on native fish diversity. We use a multi-faceted approach to 1) examine the fish community and structure across varying levels of habitat complexity, 2) quantify and describe aquatic food resources, and 3) determine if and how invasive vegetation impacts the aquatic food web using isotopic signatures from terrestrial invertebrates and leaf litter.
We found fish diversity (evenness based on Shannon’s Diversity Index) was greatest and lowest in reaches we a priori classified as high (‘complex’) and low (‘simple’) complexity, respectively, at the microhabitat scale. At the macrohabitat scale, where we compared diversity between alluvial valleys and canyons, fish diversity was greater in canyon reaches than in alluvial reaches. Further, seston algae and soft sediment periphyton abundance (i.e., potential food resources) were significantly lower within ‘complex’ reaches compared to ‘moderate’ or ‘simple’ reaches, which occur primarily in alluvial sections. Conversely, periphyton abundance on hard substrates was greater and more variable for ‘complex’ reaches, which occur primarily in the canyon reaches. Isotopic signatures appeared to vary between native and nonnative vegetation, suggesting nonnative vegetation is contributing differentially to the base of the aquatic food web. Collectively, our results suggest the modern flow and sediment regimes may limit the available habitat and potential food resources suitable for native fishes within the Rio Grande River. Nonnative vegetation may also alter the base of the aquatic food web, potentially reducing or changing food availability during different seasons and/or important fish developmental stages. Further, nonnative vegetation may have shifted a system previously dependent on continuously-available autochthonous sources of energy to one dependent on seasonal allochthonous sources of energy. As such, native fish recovery and maintenance may depend largely on the effective management of stream flow in the Rio Grande, and concordant changes in lower level productivity and food availability.