Are nutrients retained by river damming?

Abstract

Are nutrients retained by river damming?Rivers are dammed for hydropower, flood control, navigation or water storage for human use.Globally, about half of the rivers are impacted by damming [1], with major consequences for biodiversity, fish migration, primary production, greenhouse gas emissions and retention of nutrients [2,3].The traditional view is that dam closure leads to an increase in water residence time, settling of suspended particles and improved light conditions.This amplifies primary productivity and thus the stripping of dissolved nutrients from the water while transiting [2].The imported and locally produced organic matter settles to the sediment, where part of it is mineralized to carbon dioxide, methane, phosphate and ammonium, and another part is temporarily or permanently buried.This nutrient transfer from transiting river water to the sediment causes removal of nutrients via burial and dinitrogen gas production by denitrification and Anammox.This retention from dam construction reduces nutrient supply to downstream ecosystems and thus partly alleviates eutrophication.This retention of nutrients scales with water residence time, and most mathematical models for regional and global nutrient transport along the aquatic continuum from streams to estuaries are based on this concept [2,3].However, this simple depiction has its limitations.Nutrients enter reservoirs in various forms (e.g.organic and inorganic, particulate and dissolved, unreactive and reactive) and intense biogeochemical cycling within reservoirs may modify the forms of and ratios between the nutrients with consequences for downstream biodiversity and ecosystem functioning (e.g.replacement of diatoms by green algae or cyanobacteria).In this issue, Chen et al. [4] convincingly show that a series of reservoirs in the upper Mekong river increased rather than decreased the bioavailability of nitrogen and phosphorus downstream.Specifically, non-bioavailable particulate phosphorus was retained while soluble reactive P and bioavailable particulate P increased.Moreover, dissolved inorganic nitrogen entered the reservoir in the form of nitrate, but exited as

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