Abstract: Streamflows in the arid southwestern United States, particularly in high elevation catchments, are expected to decline in the coming decades due to increasing water extraction and drought. Aquatic macroinvertebrates, which are a critical part of the aquatic and riparian food web and are used as bioindicators of stream health, are one of many groups of organisms that will be affected by reduced streamflows. Low-order streams are often dominated by repeating pool-riffle sequences, alternating between deep, slow-flowing and shallow, fast-flowing reaches. Riffles, which are often areas of higher aquatic macroinvertebrate diversity and abundance, are typically shallower than pools so they are reaches of the stream that tend to dry first during reduced streamflows.
Our goal is to quantify the aquatic macroinvertebrate communities found within riffle and pool macrohabitats and show how these communities might shift under possible future streamflow scenarios to inform conservation and management decisions. Aquatic macroinvertebrates were sampled in spring 2022 from four streams in designated Wilderness areas in the Central Arizona Highlands, an ecoregion known for geological and ecological diversity. A hierarchical sampling method was employed, with replicate samples taken from both riffles and pools, at two reaches, from each of the four streams, for a total of 48 samples. Physical habitat data, including depth, water velocity, substrate size, water chemistry, and canopy cover, were collected from each macrohabitat.
Preliminary results show riffles contained a higher proportion of taxa with complex lifecycles that are bioavailable to terrestrial consumers, such as birds, bats, reptiles, amphibians, and spiders. Taxonomic richness did not differ between riffles and pools, despite each macrohabitat harboring distinct communities. Nonmetric Multidimensional Scaling (NMDS) ordinations will be utilized to examine how environmental variables influence community structure, as well as how communities compare between macrohabitats. Additionally, linked hydrologic and hydraulic models will be used to determine how proportions of riffles and pools shift under reduced streamflow scenarios. Better understanding how different stream macrohabitats influence aquatic macroinvertebrate abundance and diversity allows us to quantify how these communities will change in response to declining streamflows in a drought-influenced environment and provide guidance for the protection of these Wilderness streams.
