Considering multiple anthropogenic threats in the context of natural variability: Ecological processes in a regulated riverine ecosystem

Abstract

AbstractRivers are among the most altered environments globally, but identifying which threats are responsible for observed biotic and abiotic changes is complicated by natural drivers of variation. The Bow River, Canada provides an ideal model to resolve these influences and explore spatial relationships. It originates from pristine Rocky Mountain headwaters and is subsequently impacted by typical human alterations: damming, municipal channelization and effluent release, and agricultural impacts (nutrient enrichment and water withdrawal for irrigation). By coordinating studies of the Bow River's biota, we demonstrate how threat–driver interactions depend on season and the abiotic factor and biotic community or species of interest. We conclude that impact severity and riverine recovery depend on the threat magnitude, its longitudinal position and proximity to other threats and natural drivers. We found that river regulation, water extraction and bank armouring interact to limit geomorphic processes resulting in depleted riparian woodlands and numbers of fish species, though a large, undammed tributary nearby allows quick recovery downstream. We highlight the implications of the longitudinal position of the threats because cold‐water fish species are disproportionately impacted through the area where the human impacts on the Bow River overlap. We illustrate how the interactions between flow, nutrients and temperature lead to macrophyte‐ or algae‐dominated communities and associated shifts in fish composition and biomass. Finally, we applied our increased understanding of ecological riverine processes to conclude that management techniques such as flushing flows or functional environmental flows are likely to have only minimal or conditional success in the Bow River.