Hydropeaking impairs upstream salmonid spawning habitats in a restored Danube tributary

Abstract

Hydropower is considered an important form of renewable energy, often involving hydropeaking. While the effects of hydropeaking on aquatic communities in areas downstream the dam are well understood, there is a lack of studies investigating potential impacts on tributaries located further upstream. In this study, we tested the effects of hydropeaking operations on upstream tributaries in a restored area of the Danube River, with a focus on the periods of backlog and release of water (upramping and down-ramping, respectively) during the filling and release of the reservoir. We used brown trout egg and larval mortality, linked to hydraulic, sedimentary and physiochemical changes in spawning grounds as an indicator. We compared hydropeaking-affected versus non-affected sites in upstream tributaries using HydroEcoSedimentary Tools (HESTs) loaded with clean gravels and brown trout eggs.Egg and larval mortalities were significantly higher in the hydropeaking-affected site with more than 80% egg mortality and almost 100% larval mortality compared to values of 55-63% and 80-85%, respectively, in non-affected sites. Spawning ground quality was significantly altered in the hydropeaking-affected site, where the highest mortalities were observed. Overall, duration of time periods with flow velocities close to zero were a key variable, potentially decreasing oxygen supply for eggs and larvae. Such periods of close to zero flow velocities were driven by backlog periods during the filling of the reservoir, revealing that such events can severely impair ecological integrity of spawning sites in tributaries upstream of dams by slowing the flows in upstream tributaries. Such altered processes can reduce fish population recruitment and need to be considered in future restoration projects.

Innovations in hydropeaking research

“…Overall, the contributions to the SI provide novelties at different levels, but also a valuable compendium on current methods and challenges in hydropeaking research (e.g., Alexandre et al, 2023; Bürgler et al, 2023). With a focus on the novelties on fundamental knowledge, we highlight the following contributions: while most studies focus on downstream impacts, Pander et al (2023) is among the first to investigate the ecological effects of hydropeaking upstream of the storing reservoir. Judes et al (2023) offers new insights regarding how not only current but also past hydraulics affect biotic communities in hydropeaking rivers.…”

“…Ten contributions provided exhaustive analysis of field data, based on either single sites (e.g., Abernethy et al, 2023;Bartoñ et al, 2023;Boavida et al, 2023;Judes et al, 2023;Pander et al, 2023), or comparisons between several sites (e.g., Halleraker et al, 2023;Tena et al, 2023;Tonolla et al, 2023) Different modelling approaches are presented in the SI: from hydraulic, temperature, and habitat modelling (e.g., Antonetti et al, 2023;Jelovica et al, 2023;Judes et al, 2023), to rule-or processbased classification tools (e.g., Bakken et al, 2023;Godinho et al, 2023;Greimel et al, 2023;Reindl et al, 2023), to individual-based models (Hajiesmaeili et al, 2023). Two contributions specifically focus on numer- single contribution examines the use of remote sensed data for the investigation of fish stranding in a large river (Glowa et al, 2023).…”

“…When focusing on the biotic compartment, fish are a primary consideration, with investigations covering different life‐history traits: from spawning (Pander et al, 2023), to juveniles (Antonetti et al, 2023; Auer et al, 2023), and adults (Bakken et al, 2023). Salmonid species, which have received long‐lasting attention probably due to their economic value, are still dominant (Antonetti et al, 2023; Auer et al, 2023; Bakken et al, 2023; Hajiesmaeili et al, 2023; Halleraker et al, 2023; Jelovica et al, 2023; Pander et al, 2023); nevertheless the attention on cyprinid species has increased (Bartoñ et al, 2023; Boavida et al, 2023; Godinho et al, 2023; Judes et al, 2023).…”

“…Ten contributions provided exhaustive analysis of field data, based on either single sites (e.g., Abernethy et al, 2023; Bartoñ et al, 2023; Boavida et al, 2023; Judes et al, 2023; Pander et al, 2023), or comparisons between several sites (e.g., Halleraker et al, 2023; Tena et al, 2023; Tonolla et al, 2023). In addition, Halleraker et al (2023) monitored more than 100 bypass valves in Norwegian hydropower plants to assess their ecological potential in mitigating sharp dewatering events due to turbine shutdowns.…”

“…With a focus on the novelties on fundamental knowledge, we highlight the following contributions: while most studies focus on downstream impacts,Pander et al (2023) is among the first to investigate the ecological effects of hydropeaking upstream of the storing reservoir Judes et al (2023). offers new insights regarding how not only current but also past hydraulics affect biotic communities in hydropeaking rivers Bätz et al (2023).…”