Nutrients, frequently phosphorus and/or nitrogen, often limit aquatic primary productivity. The availability of nutrients required by phototrophs varies with chemical and biological species, site and season. A rapidly increasing, resource-demanding human population that uses water as a convenient waste-disposable system has caused widespread nutrient pollution leading to ‘eutrophication’. In conjunction with other multiple pressures such as climate change, this has altered the natural communities in an ecosystem, and caused biodiversity loss. It also causes a cascade of undesirable consequences for human use of water, including the growth of potentially toxic microalgal and macroalgal blooms, and deoxygenation leading to fish kills and the release of nutrients from the sediment to the water. Remediation, driven by legislation, is focused on limiting nutrient losses from agricultural systems while maintaining the ability to produce food sustainably and increasing nutrient capture in works treating domestic and industrial waste and the production of a circular economy for nutrients.

Seagrasses in marine systems and freshwater plants (macrophytes) in inland waters are important primary producers that structure their local ecosystems. They comprise the embryophytes: bryophytes, pteridophytes and angiosperms. They have adapted the genetic heritage of their land plant ancestors in response to the opportunities and challenges of life underwater. This has involved a reduction in the structures and processes required to manage: water content, the low physical support in air and the high levels of UV radiation. In contrast, inorganic carbon can restrict photosynthesis underwater but can be minimised by growing in sites with high CO2 concentrations, exploiting CO2 in the air or sediment and by CO2 concentrating mechanisms that rely on bicarbonate uptake, C4 photosynthesis or Crassulacean acid metabolism. Most aquatic embryophytes are, like their terrestrial ancestors, rhizophytic, allowing nutrients to be taken up from both the sediment and water. Some, especially the bryophytes, are haptophytic and only obtain nutrients from the water column.