Seasonal and longitudinal variability of benthic macroinvertebrates collected from a series of dam impoundments in a cold-water tailrace

Abstract

Natural lotic systems follow longitudinal, biophysical gradients as described by the river continuum concept; however, anthropogenic structures such as dams fragment these gradients and alter downstream communities, i.e. the serial discontinuity concept. We investigated the effects of successive impoundments on benthic macroinvertebrate assemblages in the Lower Mountain Fork River (LMFR), a 19-km cold tailwater reach in southeastern Oklahoma fragmented by four dams. Cold releases from Broken Bow Dam maintain year-round trout habitat but substantially modify thermal regimes and flow patterns relative to natural warmwater systems. From June 2016 through May 2017, we sampled 12 sites monthly, which were divided into three zones with 4 sites per zone: below the spillway, below the powerhouse discharge and Old Park dam, and below the re-regulation dam. We quantified abundance, taxa richness, percent Ephemeroptera-Plecoptera-Trichoptera (%EPT), diversity indices (e.g. Shannon and Simpson), functional feeding groups, and community structure using nMDS, ANOSIM, and SIMPER analyses. We collected 19,194 individuals representing 51 taxa. Assemblages were dominated by Trichoptera, Diptera, Isopoda, and Ephemeroptera, with collector/filterers comprising the most abundant functional group. Community structure differed significantly among dam-influenced reaches (ANOSIM global R = 0.521, p < 0.01). Sites nearest the spillway were characterized by high abundance but reduced diversity and low %EPT, driven by dominance of cold-tolerant taxa, including Simulium , Isopoda, and Amphipoda. In contrast, downstream sites exhibited greater taxa richness, higher diversity, and increased %EPT values, suggesting partial recovery along the longitudinal gradient. Seasonal variation was evident, with highest abundance in winter and peak diversity in spring. Our findings support the serial discontinuity concept, demonstrating that hypolimnetic releases and successive impoundments create spatially structured assemblages dominated by cold-tolerant taxa near dams, with gradual biophysical adjustment downstream. Despite substantial alteration, macroinvertebrate communities exhibited resilience and partial convergence toward regional warmwater assemblages at increasing distances from cold-water discharge.