Abstract: The variable nature of free-flowing rivers allows for the biological diversity of aquatic macroinvertebrates to increase across environmental and temporal gradients. A major environmental filter in river ecosystems is the split between lotic (riffle and run) and lentic (pool) macrohabitats. Stream organisms, such as macroinvertebrates, can be limited to specific habitats because of the physiology required to persist in the different physicochemical environments present in lotic and lentic habitats. Seasonal variation and disturbance related to high and low flow events can also alter the physicochemical properties of rivers and thus change macroinvertebrate communities. Our research aims to understand how aquatic macroinvertebrate diversity is related to differences in environmental conditions across different macrohabitats and temporally across seasons.
We sampled three different river reaches on a free-flowing section of the Verde River in Arizona during five sample events in fall and spring from fall 2017 to fall 2019, for a total of 135 samples. Within each site, three macroinvertebrate samples were taken in three different macrohabitats (riffles, runs, and pools) that were defined by depth and flow characteristics. We also measured physical and chemical variables associated with each macrohabitat.
We identified 100 unique taxa representing 84 genera, 63 families, and 13 orders. We found that macrohabitat type accounted for most of the variance explained in macroinvertebrate community dissimilarity (PERMANOVA; R2=0.3953) followed by sample event (R2=0.1431). NMDS ordination and indicator species analysis revealed that each macrohabitat harbored a unique community composition, with the greatest contrast being between riffle and pool communities. Community dissimilarity differed significantly between fall and spring sample events (PERMANOVA; p< 0.05), but not between different fall sample events. This indicated that a shift in community composition was occurring in spring and then shifting back in fall. One potential explanation is disturbance from spring floods. Following an unusually large flood ( >50,000 cubic meters/s) before the spring 2019 sample event, we found that some taxa, particularly blackflies, had a large increase in abundance, while other taxa, like freshwater clams and net-spinning caddisflies, had a large decrease in abundance. Our results reinforce the idea that the dynamic free-flowing condition of rivers maintains high biodiversity both spatially and temporally, and that seasonal dynamics and disturbance play an important role in shifting the composition of biological communities.
