Practice question

Eutrophication is enrichment of water with nutrients, especially phosphorus in many fresh waters and nitrogen in many coastal systems. The added nutrients remove a growth limitation on phytoplankton or cyanobacteria, allowing rapid population increase that appears as an algal bloom. Dense blooms reduce water clarity and shade submerged vegetation. When algal cells die, bacteria decompose the organic matter and consume dissolved oxygen; stratified bottom waters may then become hypoxic or anoxic. Fish kills, loss of oxygen-sensitive species, toxin-producing cyanobacteria, unpleasant odours, and altered food webs may follow. Nutrient enrichment can initially increase biomass or the abundance of a few species, but it does not reliably boost biodiversity and often reduces it through dominance and oxygen stress. Clear water and higher oxygen are therefore opposite to the common late consequences. Natural eutrophication occurs slowly as basins age, whereas fertilizer, manure, sewage, and urban runoff can accelerate it dramatically. The bloom is the immediate visible response because nutrient supply raises photosynthetic growth before decomposition generates oxygen depletion.