Freshwater streams and their associated riparian and floodplain environments are crucial components to global biodiversity. Aquatic macroinvertebrates within streams are often adapted to and dependent on multiple physicochemical characteristics of these systems, but anthropogenic actions may disrupt these conditions and thus macroinvertebrates and other organisms. Stream impoundment, dam construction, fragment streams longitudinally; the effects of such may take effect through the stream's flow regime, thermal regime, and substrate composition and transport. These and other consequences may then further impact biological metrics such as macroinvertebrate diversity; changes to substrate size and distribution pose immediate challenges to physical habitat conditions. With an increasing awareness of the ecological harm impoundment may cause, mitigation efforts are similarly increasingly considered. Substrate augmentation is one such method which aims to restore pre-impoundment substrate conditions downstream of a dam. Here, we sought to further understand the influence of a dam on downstream substrate conditions and to determine what substrate and other physicochemical conditions yielded relatively high macroinvertebrate biodiversity. We utilized multiple lines of evidence to assess the macroinvertebrate community among various substrate and other habitat conditions in Missouri's East Fork Black River, an impounded Ozark Stream. The downstream waters of this river are unique in their flow and thermal regimes being relatively unaffected due to the presence of the Lower Taum Sauk Dam, leaving substrate as the sole primary stream parameter impacted. Ultimately, we determined how macroinvertebrate diversity was related to the various substrate conditions found in five sites downstream of the dam across four sampling events spanning three years. As expected, we observed that median substrate size decreased with downstream distance of the dam, while substrate size distribution increased. On average, median particle size ranged from 112.5 mm in our most-upstream site (R1) to 36.2 mm in our most-downstream site (R5), and the coefficient of variation of particle size ranged from 70.3 percent to 101.5 percent across the same longitudinal gradient. Substrate itself was not enough to predict macroinvertebrate abundance or diversity, however, as our most-downstream site was substantially deeper and slower-flowing than the sites upstream (R2, R3, R4); corresponding mean abundance and diversity in R1 were, respectively, 139.4 and 1.39, while those values in R5 were, respectively, 57.18 and 1.4. Looking to R4 instead, we observed a macroinvertebrate community more in line with expectations, with mean abundance of 268.6 and mean diversity of 1.92, showing improvement with downstream distance from the dam. Partial Mantel tests indicated that substrate size and size distribution were important drivers of difference in macroinvertebrate abundance and Shannon diversity between sites, but so too were seasonal and hydrologic parameters, such as water temperature, dissolved oxygen concentration, conductivity, and turbidity. Analyzing heatmaps, which allowed for utilizing both substrate parameters as input variables against macroinvertebrate metrics, we concluded that median abundance and Shannon diversity were maximized around median substrate particle sizes of 45 mm to 64 mm and size coefficients of variation of 65 percent to 75 percent, underscoring the importance of a heterogeneous mixture of substrate centered about gravels and cobbles. Future substrate augmentation projects in naturally gravelly streams may consider utilizing mixtures of these conditions to benefit macroinvertebrate communities.Includes bibliographical references
