The nitrogen and phosphorus cycle in waters - where nutrients come from

Knowledge base · July 25, 2026

Nitrogen and phosphorus - nutrients - are elements essential to life and at the same time the main culprits of eutrophication. Understanding where they come from and in what forms they circulate in water helps you read test results accurately and design water protection.

Where they come from

  • Agriculture - run-off of fertilisers and animal droppings (the main source of nitrogen and phosphorus in many catchments),
  • Sewage - domestic and industrial,
  • Development and run-off from sealed surfaces,
  • Natural sources - the weathering of rocks (phosphorus), atmospheric deposition (nitrogen).

Forms of nitrogen

Nitrogen circulates in several forms that turn into one another:

  • ammonium nitrogen (N-NH₄) - from fresh sewage and decomposition, among others; depending on pH and temperature it can be toxic,
  • nitrites (N-NO₂) - an intermediate, unstable form,
  • nitrates (N-NO₃) - the final product of oxidation; highly soluble, they readily run off fields,
  • organic nitrogen - bound in organic matter.

The sum of these forms makes up the total nitrogen.

Forms of phosphorus

  • phosphates (orthophosphates) - the form directly available to algae,
  • bound phosphorus - in organic matter and suspension, partly released from bottom sediments (especially under anaerobic conditions near the bottom).

The sum is the total phosphorus - often the deciding factor in lake eutrophication.

Why it matters

An excess of nutrients drives algal production → blooms, a fall in transparency and oxygen deficits. That is why limiting the inflow of nitrogen and (especially) phosphorus is the basis of water protection - see Eutrophication and trophic status.

In practice

Nutrients are determined by a laboratory, and the key is correct sampling (sediment suspension can distort phosphorus). It is worth keeping the results - the different forms of nitrogen and phosphorus - as separate indicators and observing them over time; in LimnoLog you record them in one session and see their trend on a shared chart.

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