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Chapter 3.2 The Phosphorus Cycle: Quantitative Aspects and the Role of Man
205 Chapter 3.2
THE PHOSPHORUS CYCLE: QUANTITATIVE ASPECTS AND THE ROLE OF MAN U. PIERROU
*
Valthornsvagen 39, Uppsala,S-572 50 (Sweden)
CONTENTS Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Phosphorus in the atmosphere . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Terrestrial phosphorus . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Aquatic phosphorus . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Concluding remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
205 206 207 207 209 210
INTRODUCTION
Phosphorus is essential for living organisms and is not exchangeable with other elements in biological systems. It is an important constituent of the genetic and information-transfer molecules, deoxyribose- and ribose-nucleic acids, and also of the energy-carrying molecule adenosine triphosphate (ATP) and its di- and monophosphate precursors, ADP and AMP. The special form of AMP called cyclic adenosine monophosphate has a function in controlling different enzymes, Phosphorus is a macro-nutrient but its availability is often in the ng 8-l range. The effects of phosphorus in nature are, therefore, very profound. Phosphorus discharged by a single person in one year (about 2 kg P) is sufficient for the growth of 1Mg of plant material (Vallentyne, 1974), a fact which serves t o illustrate the link between urban communities and eutrophication. Before dealing with the different subcycles, one should perhaps consider a simplified model of the global phosphorus cycle shown in Fig. 3.2.1. The turnover rate of this cycle is regulated by the rate of diagenesis of phosphorus-containing sediments into phosphate rock. This process takes 0.11 Gy (Broecker, 1974) which implies that a period of more than 1 Gy is
* Present address: institute of Limnology, University of Uppsala, Box 557, 5-75122 Uppsala, Sweden.
206
Fig. 3.2.1. Simplified model of phosphorus fluxes within the global phosphorus cycle (from Pierrou, 1976, by permission).
required for one global cycle of phosphorus to be completed. There have been discussions about the formation of phosphorus nodules on the ocean floor ever since these nodules became targets of planned phosphorus mining. Most of these nodules are old, probably more than 100 ky, and are at present being eroded rather than formed. Some phosphorus nodules are forming at the present time under restricted conditions in a few areas of the ocean (Stumm, 1973). Thus, except in terms of the long-term geological record for which limited data are available, the phosphorus “cycle” can be viewed as a unidirectional transport of phosphorus from phosphate rock to marine and, t o some extent, freshwater sediments. PHOSPHORUS IN THE ATMOSPHERE
The role of the atmosphere in the phosphorus cycle seems to be poorly understood. Since it does not exist in the form of stable gaseous compounds, phosphorus in the atmosphere is either adsorbed on particulate matter, e.g. dust (including pollen) and exhaust fumes or dissolved in sea-spray. The fallout of phosphorus, as dry deposition and precipitation, has been estimated to be within the range 3.6-9.2 Tg P y-l for terrestrial ecosystems, 0.0540.140 Tg P y-’ for freshwater ecosystems, and 2.6-3.5 Tg y-’ for the marine ecosystem. This gives a total fallout from the atmosphere of 6.3-12.8 Tg P y-’ (Pierrou, 1976). It should be noted, however, that Emery et al. (1955)
207 estimated the fallout over the marine ecosystem to be zero. The influx of phosphorus to the atmosphere due to high-temperature combustion of organic matter has been estimated to be about 0.08 Tg P y-’ (Pierrou, 1976) of which 0.07 Tg P y-l is the result of the burning of coal (Bertine and Goldberg, 1971). It follows therefore that dust and sea-spray appear to be the major sources of phosphorus in the atmosphere. TERRESTRIAL PHOSPHORUS
The transfer of phosphorus from the terrestrial biomass to soil as dead organic matter has been estimated to be 136.4 Tg P y-l: 133.3 Tg P y-l is derived from plants and 3.1 Tg P y-l from animal material (Pierrou, 1976). The uptake of phosphorus by plants from soil was calculated by Bazilevich (1974) t o be 1 7 8 Tg P y-l, while Stumm (1973) estimated it t o be 236.8 Tg P y-l including that of the freshwater ecosystems. The terrestrial biota has been calculated to absorb 0.065 Tg P y-l from aquatic ecosystems and 0.063 Tg P y-l from industrially made foodstuffs and pharmaceutical products (Pierrou, 1976). An important aspect on which quantitative data are not yet available is the bactefial cycling of phosphorus within soils. This “internal” cycle helps in making phosphorus available for plants. The “natural” influx of phosphorus to soils is hard to assess since no measurements appear to have been made on that proportion of atmosphere fallout of phosphorus (3.6-9.2 Tg P y-’: loc. cit.) which is due to sea spray. According to Hutchinson (1952), the deposition of guano contributes about 0.01 Tg P y-l to terrestrial phosphorus. On 1972 figures, man-made annual contributions t o soil phosphorus were 9.93 Tg in the form of superphosphates (FAO, 1975) and 1.1Tg as human excreta used as a fertilizer (Pierrou, 1976). Much of the phosphorus in the soil is immobilized in the form of complexes with iron, aluminium and calcium, thereby becoming inaccessible to plants. According t o Phillips and Webb (1971) soluble phosphorus rarely migrates more than 2 or 3 cm from a fertilizer granule before being immobilized by reactions with soil components. Some soil components, such as humic acids, increase the solubility of phosphorus compounds. Other processes which diminish the availability of phosphorus t o terrestrial plants are the leaching of soluble phosphorus and the erosion of soils containing phosphorus. The leaching rate has been calculated to be within the range 2.512.3 g P y-l (Pierrou, 1976). The erosion of soil will be discussed in connection with river transport of phosphorus. AQUATIC PHOSPHORUS
The most important flux of the freshwater phosphorus cycle is the large amount of phosphorus transported by river runoff. This flux has been esti-
208 TABLE 3.2.1 Phosphorus inventories (Tg P) (from Pierrou, 1976, by permission) Biomass: Human Terrestrial Marine Fresh water Waters: Fresh Marine Soil: Rocks: Total solid sphere Mineable
<1 1805 128 <1 90 120,000-1 28,000 160,000 1.1 x 1013 6 500-59,000
mated at between 1.9 and 2 Tg P y-’ (Stumm, 1973, Gulbrandsen and Robertson, 1973). Although not specifically defined in these papers, the values appear to refer only t o dissolved and particulate phosphorus, and do not include the suspended sediments that are carried by rivers t o the ocean. Emery e t al. (1955) estimated the total phosphorus transport by rivers, including sediments, t o be 1 4 Tg P y-l. According t o Pierrou (1976) the total phosphorus transport by rivers is in the order of 17.4 Tg P y-l: the dissolved and particulate phosphorus is estimated to be 3.7 Tg P y-l, while the sediment loading is 13.7 Tg P y-l. An important feature of the sediment loading is that only 4.6 Tg P y-l appears to be the result of “natural” erosion. The additional 9.1 Tg P y-l is believed t o be due t o the increase in erosion caused by human activities such as deforestation and extensive agricultural activities. In freshwater ecosystems, the amounts of phosphorus introduced t o sediments are estimated t o be of the order of 1Tg P y-l while the amounts released from sediments annually are estimated t o be less than 1 Tg P (Pierrou, 1976). Emery et al. (1955) calculated that the amount of phosphorus deposited in ocean sediments is 13 Tg P y-l. The amount of phosphorus released from ocean sediments is unknown but is probably relatively small, as the reducing conditions (lack of oxygen) required occur relatively rarely in the ocean. The uptake of phosphorus by phytoplankton in the ocean has been variously calculated as 1300 Tg P y-l (Emery et al., 1955) and 990 Tg P y-l (Stumm, 1973). A similar estimate, about 1000 Tg P y-l, can be made for the amount of phosphorus deposited in oceanic detritus (Pierrou, 1976). The large storage of phosphorus in the ocean (see Table 3.2.1) and the large internal circulation within the ocean can absorb relatively large additions of phosphorus without causing any noticeable effects on the concentration of phosphorus in the water. However, even small increases in phosphorus concentration will increase the fraction of the ocean bottom covered by
209 anoxic waters (Stumm, 1973). According to Broecker (1974), the ocean may balance this oxygen decrease by changing the dynamics of loss and gain of phosphorus between water and sediments. The reaction time is in the order of 10-100 ky. Since the additions of phosphorus to the ocean may multiply over decades, there is a risk of an excessive enrichment of phosphorus in the ocean, especially coastal waters. Because of this, low oxygen conditions may be created which in turn will eliminate the sensitive higher forms of life in these waters. CONCLUDING REMARKS
The main fluxes of the global phosphorus cycle are summarized in Fig. 3.2.2 and the major phosphorus reservoirs in Table 3.2.1. Man’s transport of phosphate rock, fertilizers, meat and cereals may be of importance on a global scale since these transports tend to concentrate phosphorus in geographically limited areas where large problems on a regional and local scale may develop. Indeed, archaeologists have exploited the use of soil phosphorus anomalies for locating ancient settlements (Arrhenius, 1931). In the modern industrialized environment, man’s tendency to accumulate creates a problem of a different magnitude. Regionally man’s impact on the phos-
Atmosphere
26-35
36-9 3
.-. . . A
I
I._._
136
1
10
-
I+
10 Freshwater
w 231
Soil
I
2.-5-123
I,
I&
I
99011300
Marine w a t e r
I
M a r i n e sediment
I
Fig. 3.2.2. A summary of global flows of phosphorus (Tg permission).
(from Pierrou
210
phorus cycle is expressed primarily in aquatic systems. The freshwater ecosystem is very sensitive to additions of phosphorus and reacts to phosphorus additions with increased production of plants and algae. These increased amounts of plant material will use large amounts of oxygen during decay, thereby creating low oxygen conditions in the bottom waters. Foul-tasting and foul-smelling algal products, and toxic compounds from the decaying plants and algae, will diminish the uses of water, for example, for drinking and recreational purposes. Heavily polluted waters may be sources of epidemic infections because of the bacterial content of the phosphorus-containing sewage. Coastal waters react to small-to-moderate additions of phosphorus only if the water exchange with the ocean is impeded, e.g. in fjords and in bays with large inflows of freshwater. In short, the biogeochemical cycling of phosphorus is greatly affected by man, who thereby creates several environmental problems. REFERENCES Arrhenius, O., 1931. Bodenanalyse in der Archaologie. Z. Pflanzenernaehr. Dueng. Bodenkd., Abt. B, 10: 427-439. Bazilevich, N.I., 1974. Energy flow and biogeochemical regularities of the main world ecosystems. In,: A.J. Cave (Editor), Proceeding of the First International Congress of Ecology. Structure, functioning and Management of Ecosystems. Wageningen: Pudoc, pp. 182-186. Bertine, K.K. and Goldberg, E.D., 1971. Fossil fuel combustion and the major sedimentary cycle. Science, 173: 233-235. Broecker, W.S., 1974. Chemical Oceanography. Harcourt Brace Jovanovich, New York, NY, 214 pp. Emery, K.O., Orr, W.L. and Rittenberg, S.C., 1955. Nutrient budgets in the ocean. In: Essays in the Natural Sciences in Honor of Captain Allan Hancock, University of Southern California Press, Los AngeIes, CA, pp. 299-309. FAO, 1975. Production Yearbook, 1974. Vol. 28: 1. Rome: FAO, 328 pp. Gulbrandsen, R.A. and Robertson, C.E., 1973. Inorganic phosphorus in seawater. In: E.J. Griffith, A. Beeton, J.M. Spencer and D.T. Mitchell (Editors), Environmental Phosphorus Handbook, John Wiley, New York, NY, pp. 117-140. Hutchinson, G.E., 1952. The biochemistry of phosphorus. In: L.F. Wolterink (Editor), The Biology of Phosphorus, Michigan State College Press, pp. 1-35. Phillips, A.B. and Webb, J.R., 1971. Phosphorus fertilizers. In: R.A. Olson, T.J. Amy, J.J. Hanway and V.J. Kilmer (Editors), Fertilizer Technology and Use, 2nd edn. Soil. Science Society of America, Inc., Madison, WI, pp. 271-301. Pierrou, U., 1976. The Global Phosphorus Cycle. In: B.H. Svensson and R. Soderlund (Editors). Nitrogen, Phosphorus and Sulphur - Global Cycles. SCOPE Report 7. Ecol. Bull. (Stockholm), 22: 75-88. Stumm, W., 1973. The acceleration of the hydrogeochemical cycling of phosphorus. Water Res., 7: 131-144. VaIlentyne, J.R., 1974. The Algal Bowl Lakes and Man. Miscellanous Special Publication 22. Department of the Environment, Fisheries and Marine Service, Ottawa, Canada, 186 pp.
THE PHOSPHORUS CYCLE: QUANTITATIVE ASPECTS AND THE ROLE OF MAN U. PIERROU
*
Valthornsvagen 39, Uppsala,S-572 50 (Sweden)
CONTENTS Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Phosphorus in the atmosphere . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Terrestrial phosphorus . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Aquatic phosphorus . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Concluding remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
205 206 207 207 209 210
INTRODUCTION
Phosphorus is essential for living organisms and is not exchangeable with other elements in biological systems. It is an important constituent of the genetic and information-transfer molecules, deoxyribose- and ribose-nucleic acids, and also of the energy-carrying molecule adenosine triphosphate (ATP) and its di- and monophosphate precursors, ADP and AMP. The special form of AMP called cyclic adenosine monophosphate has a function in controlling different enzymes, Phosphorus is a macro-nutrient but its availability is often in the ng 8-l range. The effects of phosphorus in nature are, therefore, very profound. Phosphorus discharged by a single person in one year (about 2 kg P) is sufficient for the growth of 1Mg of plant material (Vallentyne, 1974), a fact which serves t o illustrate the link between urban communities and eutrophication. Before dealing with the different subcycles, one should perhaps consider a simplified model of the global phosphorus cycle shown in Fig. 3.2.1. The turnover rate of this cycle is regulated by the rate of diagenesis of phosphorus-containing sediments into phosphate rock. This process takes 0.11 Gy (Broecker, 1974) which implies that a period of more than 1 Gy is
* Present address: institute of Limnology, University of Uppsala, Box 557, 5-75122 Uppsala, Sweden.
206
Fig. 3.2.1. Simplified model of phosphorus fluxes within the global phosphorus cycle (from Pierrou, 1976, by permission).
required for one global cycle of phosphorus to be completed. There have been discussions about the formation of phosphorus nodules on the ocean floor ever since these nodules became targets of planned phosphorus mining. Most of these nodules are old, probably more than 100 ky, and are at present being eroded rather than formed. Some phosphorus nodules are forming at the present time under restricted conditions in a few areas of the ocean (Stumm, 1973). Thus, except in terms of the long-term geological record for which limited data are available, the phosphorus “cycle” can be viewed as a unidirectional transport of phosphorus from phosphate rock to marine and, t o some extent, freshwater sediments. PHOSPHORUS IN THE ATMOSPHERE
The role of the atmosphere in the phosphorus cycle seems to be poorly understood. Since it does not exist in the form of stable gaseous compounds, phosphorus in the atmosphere is either adsorbed on particulate matter, e.g. dust (including pollen) and exhaust fumes or dissolved in sea-spray. The fallout of phosphorus, as dry deposition and precipitation, has been estimated to be within the range 3.6-9.2 Tg P y-l for terrestrial ecosystems, 0.0540.140 Tg P y-’ for freshwater ecosystems, and 2.6-3.5 Tg y-’ for the marine ecosystem. This gives a total fallout from the atmosphere of 6.3-12.8 Tg P y-’ (Pierrou, 1976). It should be noted, however, that Emery et al. (1955)
207 estimated the fallout over the marine ecosystem to be zero. The influx of phosphorus to the atmosphere due to high-temperature combustion of organic matter has been estimated to be about 0.08 Tg P y-’ (Pierrou, 1976) of which 0.07 Tg P y-l is the result of the burning of coal (Bertine and Goldberg, 1971). It follows therefore that dust and sea-spray appear to be the major sources of phosphorus in the atmosphere. TERRESTRIAL PHOSPHORUS
The transfer of phosphorus from the terrestrial biomass to soil as dead organic matter has been estimated to be 136.4 Tg P y-l: 133.3 Tg P y-l is derived from plants and 3.1 Tg P y-l from animal material (Pierrou, 1976). The uptake of phosphorus by plants from soil was calculated by Bazilevich (1974) t o be 1 7 8 Tg P y-l, while Stumm (1973) estimated it t o be 236.8 Tg P y-l including that of the freshwater ecosystems. The terrestrial biota has been calculated to absorb 0.065 Tg P y-l from aquatic ecosystems and 0.063 Tg P y-l from industrially made foodstuffs and pharmaceutical products (Pierrou, 1976). An important aspect on which quantitative data are not yet available is the bactefial cycling of phosphorus within soils. This “internal” cycle helps in making phosphorus available for plants. The “natural” influx of phosphorus to soils is hard to assess since no measurements appear to have been made on that proportion of atmosphere fallout of phosphorus (3.6-9.2 Tg P y-’: loc. cit.) which is due to sea spray. According to Hutchinson (1952), the deposition of guano contributes about 0.01 Tg P y-l to terrestrial phosphorus. On 1972 figures, man-made annual contributions t o soil phosphorus were 9.93 Tg in the form of superphosphates (FAO, 1975) and 1.1Tg as human excreta used as a fertilizer (Pierrou, 1976). Much of the phosphorus in the soil is immobilized in the form of complexes with iron, aluminium and calcium, thereby becoming inaccessible to plants. According t o Phillips and Webb (1971) soluble phosphorus rarely migrates more than 2 or 3 cm from a fertilizer granule before being immobilized by reactions with soil components. Some soil components, such as humic acids, increase the solubility of phosphorus compounds. Other processes which diminish the availability of phosphorus t o terrestrial plants are the leaching of soluble phosphorus and the erosion of soils containing phosphorus. The leaching rate has been calculated to be within the range 2.512.3 g P y-l (Pierrou, 1976). The erosion of soil will be discussed in connection with river transport of phosphorus. AQUATIC PHOSPHORUS
The most important flux of the freshwater phosphorus cycle is the large amount of phosphorus transported by river runoff. This flux has been esti-
208 TABLE 3.2.1 Phosphorus inventories (Tg P) (from Pierrou, 1976, by permission) Biomass: Human Terrestrial Marine Fresh water Waters: Fresh Marine Soil: Rocks: Total solid sphere Mineable
<1 1805 128 <1 90 120,000-1 28,000 160,000 1.1 x 1013 6 500-59,000
mated at between 1.9 and 2 Tg P y-’ (Stumm, 1973, Gulbrandsen and Robertson, 1973). Although not specifically defined in these papers, the values appear to refer only t o dissolved and particulate phosphorus, and do not include the suspended sediments that are carried by rivers t o the ocean. Emery e t al. (1955) estimated the total phosphorus transport by rivers, including sediments, t o be 1 4 Tg P y-l. According t o Pierrou (1976) the total phosphorus transport by rivers is in the order of 17.4 Tg P y-l: the dissolved and particulate phosphorus is estimated to be 3.7 Tg P y-l, while the sediment loading is 13.7 Tg P y-l. An important feature of the sediment loading is that only 4.6 Tg P y-l appears to be the result of “natural” erosion. The additional 9.1 Tg P y-l is believed t o be due t o the increase in erosion caused by human activities such as deforestation and extensive agricultural activities. In freshwater ecosystems, the amounts of phosphorus introduced t o sediments are estimated t o be of the order of 1Tg P y-l while the amounts released from sediments annually are estimated t o be less than 1 Tg P (Pierrou, 1976). Emery et al. (1955) calculated that the amount of phosphorus deposited in ocean sediments is 13 Tg P y-l. The amount of phosphorus released from ocean sediments is unknown but is probably relatively small, as the reducing conditions (lack of oxygen) required occur relatively rarely in the ocean. The uptake of phosphorus by phytoplankton in the ocean has been variously calculated as 1300 Tg P y-l (Emery et al., 1955) and 990 Tg P y-l (Stumm, 1973). A similar estimate, about 1000 Tg P y-l, can be made for the amount of phosphorus deposited in oceanic detritus (Pierrou, 1976). The large storage of phosphorus in the ocean (see Table 3.2.1) and the large internal circulation within the ocean can absorb relatively large additions of phosphorus without causing any noticeable effects on the concentration of phosphorus in the water. However, even small increases in phosphorus concentration will increase the fraction of the ocean bottom covered by
209 anoxic waters (Stumm, 1973). According to Broecker (1974), the ocean may balance this oxygen decrease by changing the dynamics of loss and gain of phosphorus between water and sediments. The reaction time is in the order of 10-100 ky. Since the additions of phosphorus to the ocean may multiply over decades, there is a risk of an excessive enrichment of phosphorus in the ocean, especially coastal waters. Because of this, low oxygen conditions may be created which in turn will eliminate the sensitive higher forms of life in these waters. CONCLUDING REMARKS
The main fluxes of the global phosphorus cycle are summarized in Fig. 3.2.2 and the major phosphorus reservoirs in Table 3.2.1. Man’s transport of phosphate rock, fertilizers, meat and cereals may be of importance on a global scale since these transports tend to concentrate phosphorus in geographically limited areas where large problems on a regional and local scale may develop. Indeed, archaeologists have exploited the use of soil phosphorus anomalies for locating ancient settlements (Arrhenius, 1931). In the modern industrialized environment, man’s tendency to accumulate creates a problem of a different magnitude. Regionally man’s impact on the phos-
Atmosphere
26-35
36-9 3
.-. . . A
I
I._._
136
1
10
-
I+
10 Freshwater
w 231
Soil
I
2.-5-123
I,
I&
I
99011300
Marine w a t e r
I
M a r i n e sediment
I
Fig. 3.2.2. A summary of global flows of phosphorus (Tg permission).
(from Pierrou
210
phorus cycle is expressed primarily in aquatic systems. The freshwater ecosystem is very sensitive to additions of phosphorus and reacts to phosphorus additions with increased production of plants and algae. These increased amounts of plant material will use large amounts of oxygen during decay, thereby creating low oxygen conditions in the bottom waters. Foul-tasting and foul-smelling algal products, and toxic compounds from the decaying plants and algae, will diminish the uses of water, for example, for drinking and recreational purposes. Heavily polluted waters may be sources of epidemic infections because of the bacterial content of the phosphorus-containing sewage. Coastal waters react to small-to-moderate additions of phosphorus only if the water exchange with the ocean is impeded, e.g. in fjords and in bays with large inflows of freshwater. In short, the biogeochemical cycling of phosphorus is greatly affected by man, who thereby creates several environmental problems. REFERENCES Arrhenius, O., 1931. Bodenanalyse in der Archaologie. Z. Pflanzenernaehr. Dueng. Bodenkd., Abt. B, 10: 427-439. Bazilevich, N.I., 1974. Energy flow and biogeochemical regularities of the main world ecosystems. In,: A.J. Cave (Editor), Proceeding of the First International Congress of Ecology. Structure, functioning and Management of Ecosystems. Wageningen: Pudoc, pp. 182-186. Bertine, K.K. and Goldberg, E.D., 1971. Fossil fuel combustion and the major sedimentary cycle. Science, 173: 233-235. Broecker, W.S., 1974. Chemical Oceanography. Harcourt Brace Jovanovich, New York, NY, 214 pp. Emery, K.O., Orr, W.L. and Rittenberg, S.C., 1955. Nutrient budgets in the ocean. In: Essays in the Natural Sciences in Honor of Captain Allan Hancock, University of Southern California Press, Los AngeIes, CA, pp. 299-309. FAO, 1975. Production Yearbook, 1974. Vol. 28: 1. Rome: FAO, 328 pp. Gulbrandsen, R.A. and Robertson, C.E., 1973. Inorganic phosphorus in seawater. In: E.J. Griffith, A. Beeton, J.M. Spencer and D.T. Mitchell (Editors), Environmental Phosphorus Handbook, John Wiley, New York, NY, pp. 117-140. Hutchinson, G.E., 1952. The biochemistry of phosphorus. In: L.F. Wolterink (Editor), The Biology of Phosphorus, Michigan State College Press, pp. 1-35. Phillips, A.B. and Webb, J.R., 1971. Phosphorus fertilizers. In: R.A. Olson, T.J. Amy, J.J. Hanway and V.J. Kilmer (Editors), Fertilizer Technology and Use, 2nd edn. Soil. Science Society of America, Inc., Madison, WI, pp. 271-301. Pierrou, U., 1976. The Global Phosphorus Cycle. In: B.H. Svensson and R. Soderlund (Editors). Nitrogen, Phosphorus and Sulphur - Global Cycles. SCOPE Report 7. Ecol. Bull. (Stockholm), 22: 75-88. Stumm, W., 1973. The acceleration of the hydrogeochemical cycling of phosphorus. Water Res., 7: 131-144. VaIlentyne, J.R., 1974. The Algal Bowl Lakes and Man. Miscellanous Special Publication 22. Department of the Environment, Fisheries and Marine Service, Ottawa, Canada, 186 pp.