The nitrogen and phosphorus cycle in waters

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.

The forms in one place - what each one tells you

The forms are not interchangeable: the same mass of nitrogen in a different form means something entirely different in the water.

FormWhat it isWhat its predominance suggestsAvailability to algae
N-NH₄ (ammonium)fresh input of matter, product of decompositionfresh pollution or anaerobic conditionsyes; at high pH it turns into toxic NH₃
N-NO₂ (nitrite)intermediate stage of nitrificationunfinished oxidation - often a trace of a recent loadyes, but unstable
N-NO₃ (nitrate)end product of oxidationinput from fields and groundwater, an older loadyes, highly soluble
Organic Nnitrogen bound in organic matterplenty of biomass or domestic sewageonly after mineralisation
Phosphate (PO₄)dissolved ionsa load immediately available to algaeyes, at once
Bound phosphorusin suspension and sedimentsa store that returns when the bottom goes anoxiconly after release

⚠️ Phosphorus returns from the sediments without any new discharge. With no oxygen at the bottom, iron compounds release the accumulated phosphate - so-called internal loading, through which a lake can “fertilise itself” for years after the catchment input has been cut. That is why measuring phosphorus in the water column alone does not say where it came from.

Diagram of nitrogen transformations and the phosphorus loop in a water bodyNitrogen passes through ammonium, nitrite and nitrate forms, and part of it leaves as dinitrogen. Phosphorus cycles between the water column and the bottom sediments. from the catchment: agriculture, sewage organic nitrogen N-NH₄ N-NO₂ N-NO₃ mineralisation nitrification nitrification denitrification - N₂ to the atmosphere phosphates PO₄ immediately available to algae phosphorus in sediments a store from earlier years sedimentation release when the bottom is anoxic internal loading - phosphorus returns with no new discharge PHOSPHORUS NITROGEN
Nitrogen runs along a one-way chain; phosphorus runs in a loop - once the bottom goes anoxic it returns to the water with no new discharge at all.

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.

Sources

  • Phosphorus as the limiting factor - the whole-lake experiment: Schindler D.W. (1974), Eutrophication and recovery in experimental lakes: implications for lake management, Science 184(4139), 897-899 - DOI.
  • Phosphorus loading as a management basis (the OECD loading concept): Vollenweider R.A., Kerekes J. (1980), The loading concept as basis for controlling eutrophication, in: Eutrophication of Deep Lakes - DOI.
  • Internal loading - phosphorus released from sediments under anoxic conditions: Mortimer C.H. (1941, 1942), The exchange of dissolved substances between mud and water in lakes, Journal of Ecology 29, p. 280 and 30, p. 147 - DOI part I · DOI part II.

(Addresses checked on 18 August 2026.)

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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