Running waters as ecosystems

Abstract

Abstract Nutrients (mainly nitrogen and phosphorus) are essential for life. They are taken up and transformed by living things, while nutrients are mostly transported through catchments in solution (nitrogen) or mainly as particles (phosphorus). Plants require fixed (not molecular) nitrogen for growth, while transformations of nitrogen (largely N2, nitrous oxide, nitrate, nitrite and ammonium) are linked with the oxidation of organic matter, releasing energy to heterotrophic organisms. Phosphorus is less soluble and in clean waters is usually scarce (plants and microbes absorb it efficiently). Flow displaces nutrients downstream, creating a ‘spiral’ rather than a cycle. Streams reduce fixed nitrogen and release molecular nitrogen to the atmosphere but nutrient pollution increases leakage downstream and promotes the production of NO2. Agriculture is a source of nutrients, leading to a tension between food production and water security/climate change. Biotic vectors (e.g. animal migrations) transport nutrients across systems.