The increased use of hydropower is currently driving the greatest surge in global dam construction since the mid-20th century, meaning that most major rivers on Earth are now dammed. Dams impede the flow of essential nutrients, including carbon, phosphorus, nitrogen and silicon, along river networks, leading to enhanced nutrient transformation and elimination. Increased nutrient retention via sedimentation or gaseous elimination in dammed reservoirs influences downstream terrestrial and coastal environments. Reservoirs can also become hotspots for greenhouse gas emission, potentially impacting how ‘green’ hydropower is compared with fossil-fuel burning. In this Review, we discuss how damming changes nutrient biogeochemistry along river networks, as well as its broader environmental consequences. The influences of construction and management practices on nutrient elimination, the emission of greenhouse gases and potential remobilization of legacy nutrients are also examined. We further consider how regulating hydraulic residence time and environmental flows (or e-flows) can be used in planning and operation from dam conception to deconstruction.
Nutrient elimination in dam reservoirs modifies global biogeochemical cycles, with consequences to ecosystem structure and function along river networks.
The global importance of reservoirs as greenhouse gas sources and/or sinks remains heavily debated.
The reservoir hydraulic residence time can be used to develop simple relationships to predict nutrient eliminations, though small reservoirs can have large elimination efficiencies.
Dam-management strategies impact nutrient cycling at all phases of a dam’s life cycle, including removal.
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T.M. was funded through the Natural Sciences and Engineering Research Council of Canada (NSERC), award number PDF-516575-2018. Q.C. was funded through the National Natural Science Foundation of China (no. 91547206). L.B. was funded from the European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement no. 765553, as part of the Euro-FLOW project.
The authors declare no competing interests.
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For nutrients, the net removal of nutrients or nutrient species from the water column in reservoirs via sedimentation and burial or gaseous evasion to the atmosphere.
The over-enrichment of a water body with nutrients, driving high primary production (photosynthesis) and excessive growth of algae, often resulting in harmful algal blooms or toxic cyanobacterial blooms and the development of anaerobic or anoxic conditions.
Biological reduction of nitrate (NO3−) to N2 gas through a series of intermediate reaction steps that can produce nitrite (NO2−), nitric oxide (NO) and nitrous oxide (N2O).
- Redfield–Brzezinski ratio
An extension of the Redfield ratio (C:N:P = 106:16:1), the Redfield–Brzezinski ratio describes the average elemental molar composition of diatoms, defined as C:N:P:Si = 106:16:1:15–20.
- Limiting nutrient
The nutrient that is stoichiometrically in short supply in a system, typically benchmarked in aqueous biogeochemistry using the Redfield or Redfield–Brzezinski ratios.
Primary production that derives carbon from carbon dioxide and energy from sunlight (photosynthesis) or an inorganic chemical.
The formation of methane by methanogenic microorganisms; a form of anaerobic respiration.
Describes a water body characterized by low nutrient concentrations and, thus, low primary productivity.
The biological oxidation of ammonium (NH4+) to nitrate (NO3−). Produces nitrous oxide (N2O) as a by-product.
- Biogeochemical reactivity
In first-order reaction kinetics, biogeochemical reactivity is represented by a rate constant (k) in units of inverse time (T−1) that is multiplied by the nutrient mass or concentration to calculate the rate or flux of a process.
Describes reactive, easily degradable, highly bioavailable chemicals.
A type of flow regulation that produces short-term, high-flow events in river discharge.
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Maavara, T., Chen, Q., Van Meter, K. et al. River dam impacts on biogeochemical cycling. Nat Rev Earth Environ 1, 103–116 (2020). https://doi.org/10.1038/s43017-019-0019-0
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