Hydropower induced supersaturation in freshwaters: effects on ecosystems, mitigation and solutions

Rivers are crucial ecosystems providing ecosystem services and biodiversity hotspots. Yet, rivers face multiple human impacts, including those from hydropower. While hydropower offers clean energy, it also causes impacts, like total dissolved gas supersaturation. In hydropower plants, supersaturation occurs when air becomes saturated under high pressure, potentially harming aquatic life in downstream river stretches through bubble formation in the tissue, similar to decompression sickness in humans. The SUPERSAT project aimed to comprehensively study gas supersaturation, especially in the context of hydropower. The project identified that supersaturation is most common in run-of-the-river hydropower plants, particularly those with creek intakes and Francis turbines. In one Norwegian plant, supersaturation levels as high as 230% were measured downstream. Factors like aeration of turbine chambers, blocked intake screens, or air being sucked into deep river dams also contribute to supersaturation. Based on this information, SUPERSAT developed a predictive model to identify hydropower plants at risk of producing supersaturation. The model classified 473 plants in Norway (28% of all plants in the model) to high resk of producing gas supersaturation, and significant portions in Germany and Austria as high or moderate risk. Control measurements in the impacted rivers validated the model's accuracy. A main aim in SUPERSAT was to find biological effects caused by gas supersaturation. Our studies in the lab indicated varying sensitivity among species. Of nine species of benthic macroinvertebrates, seven experienced increased buoyancy under supersaturation and floated on the water surface. Four species had increased mortality. This indicates potential ecological impacts, such as reduced density and altered species composition in river settings. For fish, lab trials ranked sensitivity to gas supersaturation across several life stages of Atlantic salmon and other species. In Atlantic salmon, egg yolk larvae were least sensitive towards supersaturation and presmolt was most sensitive. Among the species, European minnow was least sensitive to gas supersaturation. After the exposure trial, brown trout showed recovery from gas bubble disease symptoms after a week in normal water. SUPERSAT also studied the interaction between acidification and gas supersaturation in Atlantic salmon in the lab, finding no additive stress effect. Field studies of fish and benthic macroinvertebrates confirmed the results from the lab trials on fish and benthic macroinvertebrates. Also, that fish likely cannot detect supersaturation but naturally tend to use deeper river areas with lower supersaturation, which reduces supersaturation-induced mortality. Addressing concerns about interacting effects of supersaturation and aquatic plants, we found that photosynthesis and oxygen production do not significantly increase total dissolved gas supersaturation events and that enhanced CO2 availability from supersaturation will not enhance nuisance growth of aquatic plants. Technical solutions proposed by SUPERSAT include design considerations for new power plants and site-specific adaptations for existing ones. Numerical simulations suggested effective modifications to reduce bubbling zones where supersaturation occurs. In conclusion, SUPERSAT's findings provide extensive knowledge on the environmental risks of gas supersaturation and support the need for regulations to protect freshwater ecosystems. The project emphasizes the importance of implementing mitigation measures in hydropower plants and offers insights into how to execute these measures effectively.

We are confident that the results from SUPERSAT will have a lasting impact. The project has emphasized scientific quality throughout and provided many new insights into the prevalence, biological effects, and dynamics of gas supersaturation. Many of the results, first discovered in SUPERSAT, such as ecosystem effects, fill knowledge gaps and will be highly noticeable in the scientific community. At least 12 scientific papers are expected to emerge from the project, with four already published and five submitted. We have established protocols for managing supersaturation and have invited users (policy makers, management authorities, and industry) to an end-of-project workshop in 2024. Here, we will present results and invite users to provide comments and feedback. The results and the workshop will form the basis for a white paper detailing project background, key findings, and suggestions on how to address gas supersaturation in connection with hydropower plants, including known cases, causes, biological thresholds, and abatement measures. Regarding the anticipated significance for society and sustainability in the industry, the results are highly valuable. They support the need for implementing mitigation measures in hydropower plants and provide insights into technical solutions for the industry and guidance for management on how such measures can be implemented. We have raised awareness of the problem through a series of talks to researchers, the industry, management, and the general public on results from SUPERSAT. As a result, we are now collaborating closely with the hydropower industry across Norway to measure gas supersaturation and reduce its biological effects. Throughout the project, we have provided training to personnel, who have developed profound knowledge on this issue in particular and on sustainability in freshwaters in general. The project has also involved one post-doc and two master's students who conducted their projects and training within SUPERSAT. Both popular and scientific dissemination of the project results will continue in the coming years to maintain public awareness.

Hydropower is the main source of energy supply in Norway and is predicted to increase by 73 % worldwide in the next 10-20 years. Adverse environmental impacts from hydropower production, such as fish migration barriers, changes in sediment dynamics and altered discharge regimes, are known and addressed in Norway. However, impact caused by supersaturation of dissolved gasses (TDGS) is mostly overlooked. It occurs when entrained air dissolve in water under high pressure in the tunnel system of hydropower plants. The water becomes supersaturated when the hydrostatic pressure decreases as the water exists the tunnel. Exposure to TDGS negatively affects aquatic biota primarily through the formation of bubbles (gas bubble disease). This causes acute fish kills or sub-lethal effects, such as increased susceptibility to diseases and changes in behaviour and habitat use. During the most recent years, TDGS has been detected in several Norwegian rivers and preliminary studies indicate adverse effects on the fauna. However, the extent and biological effects in Norway and most other parts of the world are unknown. Especially, little ittle is known about the effects and the sensitivity involved for species in European rivers. The overall aim of SUPERSAT is to reveal biological impacts of gas supersaturation at the individual and community scales and identify effective mitigation measures. We will find the extent of TDGS in Europe and identify hydrological events causing TDGS in hydropower plants with the aim of improving technical solutions to avoid TDGS. Hence, the project will deliver the basis for mitigation and improved solutions supporting a more sustainable and environment-friendly development of the hydropower industry.