Longitudinal patterns of diversity and secondary production in a large regulated river

Dams interrupt the longitudinal connectivity of rivers and consequently disrupt the structure and function of downstream communities. The Serial Discontinuity Concept recognizes these disruptions and suggests that the impacts of dams on downstream communities should attenuate with increasing distance from the dam. The impacts of dams on communities immediately downstream are well studied, but less is known about how dams affect longitudinal patterns of community structure and functional attributes. To investigate the impacts of a large hydropower dam on downstream macroinvertebrate assemblage structure and function, we sampled riffle habitats along a 49-km reach on the Green River below Flaming Gorge Dam for 1 year. We collected monthly samples from seven sites, ranging from 0.8 to 49 km below the dam. Macroinvertebrate assemblage composition differed among sites below the dam and exhibited distinct longitudinal patterns. Taxonomic richness increased with increasing distance from the dam, but total macroinvertebrate abundance and annual secondary production decreased with increasing distance from the dam. Furthermore, functional feeding group composition differed among sites and also exhibited longitudinal patterns. Our results contribute to a body of evidence that demonstrates longitudinal effects of dams on downstream macroinvertebrate community structure and function.