Spring peak flows control abundance of Cladophora in a Hydropower‐Impacted River

Abstract

AbstractThe lower Deschutes River represents the 161‐km reach downstream of three hydroelectric dams flowing to its confluence at the Columbia River. We examined hydrology and nutrient chemistry from 2015–2017 to 2019 to determine factors affecting periphyton community composition and abundance following a change in the outlet structure and depth of release waters from the major upstream impoundment. The periphyton community composition was abundant and diverse in 2015 and 2016 and lower in 2017 and 2019. Filamentous chlorophytes were dominant in the first 2 years but were sparse in the subsequent years. Cyanophytes were dominant in 2017 and 2019. The expectation entering the study was that altered nutrient chemistry associated with the change in operations resulted in conditions more favorable for periphyton growth. Concentrations of total and soluble phosphorous are independent of reservoir operations, being high in all years because of natural weathering of P‐rich volcanic rocks in the basin. Concentrations of TN and DIN were low all study years, resulting in a system that was N‐limited, but with similar chemistry among the study years. The hydrology during 2015 and 2016, with abundant chlorophytes, exhibited low flow in the spring, whereas high spring flows were observed in the latter years. We conclude that high spring flows were far more influential in affecting the periphyton community composition in the lower Deschutes River compared to nutrient chemistry.