{ Phosphorus

Essential for all living things

Component of DNA, RNA, ADP, ATP, and bone

Usually the most limiting nutrient in phytoplankton productivity

Many forms

Soluble inorganic, soluble organic, particulate organic, and particulate inorganic

Occurs naturally in most soils

Not toxic but can lead to eutrophication

General

Phytic acid, C6H18O24P6, is the storage form in plant tissues

Not readily digestible by animals unless the enzyme phytase is present (mainly microorganisms)

Required in large amounts, but there’s only low concentrations available

Thus, high rates of plant growth (in aquatic ecosystems) require continuous input of phosphorus

Phosphorus in Plants

Not well defined cycle

P has several valence states, -3 to +5, with the most common of +5 in nature

Reactions are mediated by chemotropic bacteria mainly by a chemical cycle, not biological.

P-Cycle

H3PO4 = H+ + H2PO4- K1=10-2.13

H2PO4- = H+ + HPO4

2- K2= 10-7.21

HPO42- = H+ + PO4

3- K3=10-12.36

Polyphosphates such as H6P4O13, has a greater proportion of P than does orthophosphate: PO4 is 25% P which P4O13 is 30.8% P

pH 6-9, H2PO4- and HPO4

2- dominates in natural waters

Example:

Calculate the percentages of H2PO4- and HPO4

2- at a pH of 6

Dissociation of Orthophosphoric Acid

Fig 12.2

Inorganic P reacts with iron and aluminum in acidic sediments:

AlPO4•2H2O + 2H+ = Al3+ + H2PO4- + 2H2O K=10-2.5

FePO4•2H2O + 2H+ = Fe3+ + H2PO4- + 2H2O K=10-6.85

Thus, decreasing pH favors solubility of Fe and Al phosphates

Example: Estimate the solubility of P from variscite (AlPO4•2H2O) for pH 5 and pH 6

Example: Estimate the solubility of P from strengite FePO4•2H2O) for pH 5 and pH 6

Phosphorus-Sediment Reactions

P solubility in aerobic water is controlled by iron and aluminum minerals. Most common are gibbsite, Al(OH)3, and iron (III) hydroxide, Fe(OH)3.

Al(OH)3 + 3H+ = Al3+ + 3H2O K=109

Fe(OH)3 + 3H+ = Fe3+ + 3 H2O K=103.54

Solubilities increase while pH decreases

Al compounds are more soluble than iron compounds

Example: Estimate the solubilities of gibbsite and iron (III) hydroxide at pH 5 and pH 6

More Dissolution

Availability of P from sediment tends to decrease with decreasing pH

Iron and aluminum oxides and hydroxides tend to be more abundant in sediment than aluminum and iron phosphates

When a highly soluble source of P is added to a sediment at pH of 7 or below, the P will react with aluminum and iron and precipitate.

H2PO4- + Al(OH)3 = Al(OH)2H2PO4 + OH-

H2PO4- + FeOOH = FeOH2PO4 + OH-

In soil and sediment, especially the tropics, much of the clay fraction is in these forms. Clays are colloidal and have a large surface area; thus, they can bind to large amounts of P

Silicate clays also can fix P

P absorption by iron and aluminum oxides

Primary P compounds in neutral and basic sediments

Most soluble compound is monocalcium phosphate, Ca(H2PO4)2:

Ca5(PO4)3OH + 7H+ = 5Ca2+ +3H2PO4- +H2O K=1014.46

Apatite is not soluble at pH 7 or above

High Ca2+ and high pH favors formation of hydroxyapatite

Example: Estimate the solubility of P in waters of pH 7 and pH 8 with 15 mg/L calcium (10-3.90M)

Calcium Phosphates

Max. availability of P in aerobic soil occurs between pH 6-7

Here, there is less Al3+ and Fe3+

to react with

Iron P becomes more soluble when the redox potential drops, thus ferric iron reduces to ferrous iron.

P is unavailable to the water column until thermal stratification, but then, it’s only brief

Anaerobic Sediment

Dry matter of plants 0.05-0.5% P

Vertebrate animals (fish) 2-3+% P

Invertebrates 0.5-1 % P

Crustaceans 1% P

P in organic matter is mineralized in the same manner as nitrogen

The Nitrogen:Phosphorus ratio in living organisms and in decaying organic residues varies from 5:1 to 20:1

Organic Phosphorus

TP – P concentration from raw water

SRP – P concentration from filtrated water, amount available to plants

Surface waters <0.5 mg/L TP

(Mostly Particulate) (<0.05 SRP)

Sediment 10-3,000 mg/kg TP

(Mostly bound, ≈85.6% is not removable)

Typically, 10% or less of TP is SRP and readily available for plants

Total Phosphorus (TP) vs Soluble Reactive Phosphorus (SRP)

Phytoplankton absorbs P very quickly. 0.2-0.3 mg/L can be removed within a few hours Macrophytes also remove P quickly, but they can also remove P from anaerobic

zones in the sediment

Most TP is contained in phytoplankton cells

Plants can take up extra P and store it, or they have a luxury consumption

Plant Uptake

Fig 12.4

Sediment is a Phosphorus sink

2 ways that P gets into water:

1) Bioturbidity

2) Diffusion

**Difficult to get into water column

Water – Sediment Interface

Fig 12.7

mg/L TP

Oligotrophic 0.003-0.018

Mesotrophic 0.011-0.096

Eutrophic 0.016-0.386

Hyper-eutrophic 0.75-1.2

Not an exact relationship due to availability of essential nutrients, degrees of turbidity and source of the turbidity.

Hydraulic flushing rate

the percentage of the P input that is retained increases as the hydraulic retention time increases

A lake with high pH and high Ca2+ (high TA and high TH) would require a greater P load to cause eutrophication

Eutrophication

Phosphorus and Nitrogen have a linear relationship:

Typical ratio (5:1 to 20:1): 7:1 N:P

Marine plankton: 106:15: 16:1 C:Si:N:P

P concentration will cause a greater response in plant growth than will an increase in N concentration

Nitrogen is readily recycled in the water system while Phosphorus (limiting recycling) is bound in sediment or plant growth.

Redfield Ratio

Phosphorus is needed for all living organisms

Main source in agriculture and industry is mineral apatite, or calcium/rock phosphate

Municipal and agricultural pollution is a major source of Phosphorus in many water bodies

Forms of Phosphorus in water are dependent on the pH

Sediment is a sink for Phosphorus due to the low solubility of Al, Fe, and Ca phosphates (P is tied up in Al and Fe in acidic soils, and CaPO4 in alkaline soils)

Significance

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