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Phosphorus - (Phosphorum) - P
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Due to its high reactivity, phosphorus occurs in nature only in the form of phosphorus compounds, to the greater extent in the minerals of Earth crust. According to its average molar concentration of 1,6.10-6 mol/l, phosphorus is the sixteenth most abundant element in seawater. Expressed in mass units its average concentration is 4,96.10-5 g/l (50 μg/l).

Version 2012-III

Phosphorus - (Phosphorum) - P


Due to its high reactivity, phosphorus occurs in nature only in the form of phosphorus compounds, to the greater extent in the minerals of Earth crust. According to its average molar concentration of 1,6.10-6 mol/l, phosphorus is the sixteenth most abundant element in seawater. Expressed in mass units its average concentration is 4,96.10-5 g/l (50 g/l).


Phosphorus occurs in seawater in organic particulate form, as well as in dissolved form (inorganic and organic phosphorus). Organic phosphorus is the product of animal excretion and of the disintegration processes of deceased organisms. At the surface waters it represents a significant - yet unstable - fraction of the total dissolved phosphorus in seawater. The phosphorus cycle in seawater has still not been fully described and understood. Inorganic dissolved phosphorus occurs in seawater mainly in the form of H3PO4 (phosphoric acid) dissociation products.

Influence on seawater chemistry

The dissociation of phosphoric acid leads to the formation of the following phosphate anion forms:

H3PO4 -> H+ + H2PO4-
H2PO4- -> H+ + HPO42-
HPO42- -> H+ + PO43-

Average representation of individual phosphate anion forms in seawater as a function of the pH value.


Phosphoric acid is present as H2PO4- at ca 85% at pH 8,1 in seawater - and at 15% as PO43-. From the reefkeepers point of view and for the purpose of this article of major importance is the low pH value range, in which calcium reactors operate with lower pH values the PO43- form disappears and the HPO42- and H2 fractions become more prevalent. The occurrence rate of H2PO4- / HPO42- / PO43- in seawater is influenced - among other parameters - mainly by the pH value. It is very important to know that calcium (Ca) and magnesium (Mg) have the ability to form ion pairs with the different phosphate ion forms with varying bond force. The occurrence of the different phosphate forms in seawater at 25C and at a pH between 8,1 and 6,0 looks as follows:

pH 8,1 Free In ion pair with: Total
Mg Ca
pH 6,0 Free In ion pair with: Total
Mg Ca

Each ion pair type has different bond force (due to a varying association constant) and may influence certain chemical and biological parameters of calcium and magnesium. At pH 8,1 magnesium and calcium bind significant quantities of inorganic phosphorus in ion pairs, and thus phosphorus appears in its entirely dissociated form at only ca 45%. However when the pH values sink, the number of ion pairs also sinks and free inorganic phosphorus then occurs in free - entirely dissociated form - at 70% at a pH of around 6,0.

Even though - as already mentioned - all aspects of the phosphorus occurrence in seawater in its different forms and concentrations are not fully known and understood, one should bear in mind that different combinations of its absolute concentration and pH values can greatly influence the phosphate reactivity and the bioavailability of calcium and magnesium. As a consequence and under the certain conditions the influence of phosphorus on marine organisms can be devastating.

Different forms of the phosphoric acid anions have the ability to form differently strong ion pairs with the calcium and magnesium cations, possessing different properties.

Biogenous role

Phosphorus is one of the key biogenic elements and is present in all living organisms. Among others it occurs in the DNA and RNA molecules, cellular energy suppliers (ADP, ATP) and certain fats (phospholipids). As far as the marine organisms are concerned, phosphorus is a key nutrient, which enters the food chain through sea autotrophic organisms - organisms, which are able to metabolize inorganic substances into organic ones. These are primarily photosynthesizing phytoplankton - algae (including Zooxanthellae), cyanobacteria and certain protozoa - for which phosphorus is the primary nutrient.

Nevertheless, phosphorus is also the limiting nutrient. All living organisms need phosphorus, but the latter occurs in seawater in relatively small concentrations. The available phosphorus quantities are therefore a limiting growth factor for the living matter in a given water volume.

The life in general is a subject to the presence and to the constant supply of the phosphorus.

Phosphorus and Reefkeeping

From the above mentioned facts it could be deduced, that if the available phosphorus in seawater is lower than the demand for it - which holds true -, then an increased phosphorus concentration in seawater should be advantageous for marine organisms and should improve the situation - which does not hold true. In this case it is necessary to differentiate between primary phosphorus consumers as is phytoplankton and other living organisms, as corals for instance. For phytoplankton an increased phosphorus concentration would be definitely advantageous, while this would have a negative effect on corals, because they are adapted to natural - and therefore a low - phosphorus concentration. This leads to the following apparent paradox for reefkeeping:

It is necessary to guarantee the living organisms in the aquarium a constant phosphorus supply; however the phosphorus concentration in seawater must be kept at low, in essence trace concentrations.

This paradox is not easy to solve. This is, among other factors, due to the fact that the character of the common phosphorus test-kits does not allow precise determination of its concentration in the region around 50 g/l. The only solution is to abide by the following rules:

  • Remove the phosphates from the tank, but not eliminate them
  • Ensure a phosphorus supply with the controlled application of the right nutrition, containing:
    • particulate organic matter (POM)
    • as well as dissolved organic matter (DOM)


The combination of high phosphates concentration and low pH value can have a very negative effect on the seawater chemistry in the reef tank. It is necessary to keep the phosphates concentration at trace levels (undetectable with common tests), while at the same time to maintain a constant phosphorus supply to the tank in particulate and dissolved form using the appropriate food preparation.

Occurrence in seawater:

Organic matter; different phosphate anion forms

Recommended concentration:

50 g/l; 0 when using common tests

Influence on:

All life parameters of organisms

Biogenous role:

All living organisms without exception

CS Reefkeeping Concept

CS DispoPhos



An Introduction to the Chemistry of the Sea, Michael E. Q. Pilson, Pearson Education
Chemical Oceanography, Second Edition, Frank J. Millero, CRC Press

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