Phosphorus

PHOSPHORUS (P) • a common growth limiting element in fresh water. 4 main reasons: ¤ rock breakdown in the watershed releases little biologically available P to streams & lakes ¤ the root zone on land intercepts & retains most soluble P compounds ¤ no gaseous phase in the P-cycle & thus rainwater contains little P ¤ any soluble PO4 released into water is rapidly adsorbed onto particles or precipitated with other compounds & is not readily available Sources and nature of P • igneous rocks, apatite (Ca5(PO4)3+) & subsequent weathering • P accumulated in the biota • human waste & anthropogenic sources (human activities) • decay & mineralization of plant & animal corpses

The distribution of organic & inorganic P •

orthophosphate (PO4 3- ): the only significant form of inorganic P

•

> 90% is bound organically in organic phosphate & cellular constituents in the living particulate matter, or associated with or adsorbed to inorganic & dead particulate organic materials

• Total P content of unfiltered water: ¤ P suspension (t’ampai) in particulate matter ¤ P in dissolved form • Particulate P: ¤ P in organisms - nucleic acids DNA & RNA, which are not involved in rapid cycling of P - Low molecular weight esters of enzymes, vitamins - Nucleotide phosphates (ADP & ATP)

¤ Mineral phases of rock & soil ¤ P adsorbed onto dead particulate organic matter or in macro- organic aggregations ii. Dissolved inorganic P in the filtrate: ¤ orthophosphate (PO4 3- ) ¤ polyphosphate ¤ organic colloids or P combined with adsorptive colloids Measurement • P is measured spectrophotometrically by the blue colour PO4 produces with acid molybdate solution • organic P requires a stronger acid digestion prior to assay •

• the reaction of acidified ammonium molybdate with inorganic phosphate (orthophosphate) forms a molybdophosphoric acid that turns blue when reduced • the reducing agent (stannous agent) causes the development of molybdenum blue when soluble orthophosphate is in the sample • organic phosphate (meta phosphate or poly phosphate) are revealed after being hydrolyzed to orthophosphate by heat & acid • the molybdate test on a hydrolyzed sample gives total phosphate (TP) TP – orthophosphate = organic phosphate • The reactive soluble orthophosphate is the fraction that is immediately useful for autotrophic plants

1) Fe 3+PO4 (insoluble)

Fe 2+ (PO4)2 (soluble)

3Fe 2+ + 2PO43- (free)

2) Fe3+ (OH)3 + PO4 3-

Fe 2+(OH)2 + PO43- (free)

(sorbed)

• phosphorus-holding capacity of clay (AlO3.2SiO2.2H2O) Inorganic P (orthophosphate)

Dissolved organic P (excretion & decomposition)

Organic P particle (suspended in live/dead protoplasma)

P CYCLE GEOLOGY

Erosion of P from rocks

Runoff

AQUATIC

ANIMAL P in soil

Dissolved phosphate

DECOMPOSER SEDIMENT

1. Geochemical cycle of P

• geochemical cycle of P: - one-way transport - depend on patterns of erosion & sediment transport - in natural waters: only 5-10% of P is carried in soluble form; the rest moves as sediment particles - sewage & agricultural runoff disturb this balance - phosphate buffer system: equilibrium b/ween phosphate ions (PO43-) & mineral particles

Erosion Cities Sewage Rivers 5-10% PO4, 90-95% inorganic, mineral-P, PO4 sorbed or bound to particles

95% PO4-P

P-recycle Lake P-recycle

P-sink in sediments

The geochemical cycle of phosphorus

Ocean

P-cycle in pelagic waters: Erosion

Streams, rivers, windblown & aerial deposition

Sewage

Bacterioplankton

Particulate-P in organic detritus

Phytoplankton

PO4 (BAP)

Soluble organic-P

Anoxia

Zooplankton

Inorganic sediment eg. Ca3(PO4)2, Fe3(PO4)2

Decay Fish

Organic sediment

In pelagic waters: •organic P: living or dead biomass • recycling comprises much of the activity in the pelagic P cycle • organisms excrete soluble & organic P • decomposition of dead organisms release DOP & phosphate • shallow lakes remain eutrophic because of internal loading (sediment area is exposed to wave mixing) • internal loading in deep lakes is restricted – recover more rapidly from P pollution • the major loss of P from open water: - sedimentation of the biota - chemically formed precipitates • inorganic solid phosphate phases are formed by direct precipitation with Ca, Al, Fe

In the sediment • sediments consist of solid particles separated by liquid-filled interstitial spaces containing pore water • they are mixed by larger benthic organisms & wave action • the sediment-water interface or microzone is the barrier to free interchange of P between sediments & the lake water • if the interface is anoxic: phosphate ions can pass across sediment pore waters & water • oxygenated interface: phosphate ions are precipitated & do not pass freely to the lake water • anoxic sediments release phosphate 1000x faster than releases from oxygenated sediments

The role of zooplankton & fish: • zooplankton excrete 10% of their body P daily depending on: - feeding rate - temperature - type of food - time of day - larval stage • excretion is the main source of readily available phosphate in the epilimnion • the phenomenon of ‘bloom & crash’ of phytoplankton population

The role of rooted macrophytes & algal decomposition

PO43(85%)