Phosphorus in Aquatic Ecosystems PDF

Phosphorus 12 Abstract Phosphorus usually is the most important nutrient limiting phytoplankton productivity in both aquatic and terrestrial ecosystems. Phosphorus occurs naturally in most geological format...

Phosphorus 12 Abstract Phosphorus usually is the most important nutrient limiting phytoplankton productivity in both aquatic and terrestrial ecosystems. Phosphorus occurs naturally in most geological formations and soils in varying amounts and forms; the main source of agricultural and industrial phosphate is deposits of the mineral apatite—known as rock phosphate. Municipal and agricultural pollution is a major source of phosphorus to many water bodies. Most dissolved inorganic phosphorus in aquatic ecosystems is an ionization product of orthophosphoric acid (H3PO4). At the pH of most water bodies, HPO42 and H2PO4 are the forms of dissolved phosphate. Despite its biological signifi- cance, the dynamics of phosphorus in ecosystems are dominated by chemical processes. Phosphate is removed from water by reactions with aluminum, and to a lesser extent, with iron in sediment. In alkaline environments, phosphate is precipitated as calcium phosphate. Aluminum, iron and calcium phosphates are only slightly soluble, and sediments act as sinks for phosphorus. Concentrations of inorganic phosphorus in water bodies seldom exceed 0.1 mg/L, and total phosphorus concentration rarely is greater than 0.5 mg/L. In anaerobic zones, the solubility of iron phosphates increases; sediment pore water and hypolimnetic water of eutrophic lakes may have phosphate concentrations above 1 mg/L. Phosphorus is not toxic at elevated concentration, but along with nitrogen, it can lead to eutrophication. Keywords Biological role of phosphorus Phosphorus chemistry Sediment phosphorus Phosphorus dynamics in water bodies Phosphorus and eutrophication © Springer International Publishing Switzerland 2015 243 C.E. Boyd, Water Quality, DOI 10.1007/978-3-319-17446-4_12 244 12 Phosphorus Introduction Phosphorus is an extremely important element for all living things. It is contained in deoxyribonucleic acid (DNA) that contains the genetic code (or genome) that instructs organisms how to grow, maintain themselves, and reproduce. Phosphorus also is a component of ribonucleic acid (RNA) that provides the information needed for protein synthesis in organisms, i.e., DNA is responsible for RNA production which in turn controls protein synthesis. Phosphorus is a component of adenine diphosphate (ADP) and adenine triphosphate (ATP) that are responsible for energy storage and use at the cellular level. Phosphorus is a component of many other biochemical compounds such as phospholipids important in all cellular membranes. Moreover, calcium phosphorus comprises bone and teeth of vertebrates. Phosphorus—like carbon and nitrogen—is incorporated into plants and supplied to animals and the microorganisms of decay via food webs. The storage form of phosphorus in plant tissues is phytic acid, a saturated cyclic acid with the formula C6H18O24P6—a six carbon ring with a phosphate radical (H 2PO4 ) attached to each carbon. This compound is particularly abundant in bran and cereal grains. Phytic acid is not readily digestible by non-ruminant animals, but the enzyme phytase produced in the rumen by microorganisms allows ruminant animals to digest phytic acid. Thus, most animals get their phosphorus from other phosphorus compounds in plants rather than from phytic acid. But, microorganisms of decay can degrade phytic acid with the release of phosphate. Phosphorus concentrations typically are low in natural waters, but it is required in relatively large amounts by plants. Phosphorus is generally the most important nutrient controlling plant growth in aquatic ecosystems, and phosphorus pollution of natural waters is considered a primary cause of eutrophication. Phosphorus is absorbed strongly by bottom sediment where it is bound in iron, aluminum, and calcium phosphate compounds and adsorbed onto iron and aluminum oxides and hydroxides. The solubilities of the mineral forms are regulated by pH, and there are few situations in nature in which phosphorus minerals are highly soluble. The phosphorus in organic matter is mineralized, but when it is, it usually will be adsorbed by sediment unless it is quickly absorbed by plants or bacteria. Because sediment tends to be a sink for phosphorus, high rates of plant growth in aquatic ecosystems require a continuous input of phosphorus. Phosphorus has several valence states ranging from 3 to +5, but most phos- phorus found in nature has a valence state of +5. In contrast with nitrogen and sulfur, oxidation and reduction reactions mediated by chemotropic bacteria are not an important feature of its cycle. Despite its tremendous biological importance, the phosphorus cycle is largely a chemical cycle instead of a biologically-driven one. Of course, microbial decomposition is an important factor in releasing phosphorus from organic matter. This chapter will discuss the sources and reactions of phosphorus in aquatic ecosystems and consider the importance of phosphorus in water quality. Phosphorus in Aquatic Ecosystems 245 Phosphorus in Aquatic Ecosystems Unlike carbon, oxygen, and nitrogen, there is not a well-defined global phosphorus cycle. This results primarily because the major sources of phosphorus are phosphorus-bearing minerals, e.g. iron, aluminum, and calcium phosphates that occur widely in soils at relatively low concentrations and in massive deposits of calcium phosphates of high phosphorus content at a few locations. These massive deposits of calcium phosphate consist of the mineral apatite that is commonly known as rock phosphate. Rock phosphate is mined and processed to make highly soluble calcium phosphate compounds for use as agricultural, industrial, and household phosphates. In relatively unpolluted natural waters, the primary source of phosphorus is runoff from watershed soils and dissolution of sediment phospho- rus, because the atmospheric is not a significant source of phosphorus. Phosphorus concentrations in such waters reflect concentrations and solubilities of phosphorus minerals in soils and sediment. Phosphorus has many uses: in agriculture, processing of food, manufacturing of beverages, other industries, and the home. The greatest agricultural use is for phosphate fertilizers and in some pesticides; fertilizers also are used on lawns, gardens, and golf courses. Phosphoric acid is included in soft drinks to give them a sharper flavor and to inhibit the growth of microorganisms on sugar present in the beverages. Phosphorus is used for acidification, for buffering, as an emulsifier, and for flavor intensification in food. It is important in an industry as a component of surfactants, lubricants, metal treating processes, component of matches, and as a fire retardant. Trisodium phosphate may be used as a cleaning agent and water softener in both industry and in the home. Thus, it is not surprising that agricultural runoff, effluent from industrial operations, and municipal sewage contains elevated concentrations of phosphorus and can lead to higher phosphorus concentrations in water bodies into which they are discharged. Increased phosphorus concentrations in natural waters resulting from human activities normally stimulate aquatic plant growth and especially phytoplankton growth. If phosphorus additions to natural water are too great, eutrophication occurs with excessive phytoplankton blooms or nuisance growths of aquatic macrophytes. Phosphorus additions to natural waters are considered a form of water pollution. However, phosphorus often is applied to aquaculture ponds in fertilizers to increase natural productivity that is the base of the food web for fish production. The dynamics of phosphorus in aquatic ecosystems are illustrated in Fig. 12.1. Dissolved and particulate phosphorus enters water bodies from the watershed. Dissolved inorganic phosphorus is absorbed by plants and incorporated into their biomass. Plant phosphorus is passed on to animals via the food web, and when plants and animals die, microbial activity mineralizes the phosphorus from their remains. If not absorbed by plants, dissolved inorganic phosphorus is strongly adsorbed by sediment. There is an equilibrium between inorganic phosphorus bound in sediment and phosphorus dissolved in water, but the equilibrium is shifted greatly towards sediment phosphorus. Rooted aquatic macrophytes can utilize sediment phosphorus that would otherwise not enter the water column, because 246 12 Phosphorus Fig. 12.1 A qualitative model of the phosphorus cycle in an aquatic ecosystem their roots extract phosphorus dissolved in sediment pore water. Phosphorus is lost from aquatic ecosystems in outflowing water, intentional withdrawal of water for human use, and harvest of aquatic products. The sediment of water bodies tends to be a phosphorus sink, and it may increase in phosphorus content over time. As with nitrogen, in unpolluted natural waters, there tends to be an equilibrium among phosphorus inputs, outputs, and storage. Additions of phosphorus through water pollution can disrupt this equilibrium and cause undesirable changes in aquatic ecosystems. Phosphorus Chemistry To lead the reader to an understanding of why phosphorus concentrations in water tend to be low and controlled by interactions between water and sediment, a discussion of phosphorus chemistry is provided. Dissociation of Orthophosphoric Acid Inorganic phosphorus in soils, sediment, and water normally can be considered an ionization product of orthophosphoric acid (H3PO4) that dissociates as follows: H3 PO4 ¼ Hþ þ H2 PO4 K 1 ¼ 10 2:13 ð12:1Þ H2 PO4 ¼ Hþ þ HPO24 K 2 ¼ 10 7:21 ð12:2Þ HPO24 ¼ Hþ þ PO34 K 3 ¼ 10 12:36 : ð12:3Þ Phosphorus Chemistry 247 Fig. 12.2 Effects of pH on relative proportions (mole fractions) of H3PO4, H2PO4 , HPO42 , and PO43 in an orthophosphate solution When the hydrogen ion concentration equals the equilibrium constant for one of the steps in the dissociation, then the two phosphate ions involved in that step will have exactly equal concentrations. For example, in (12.1), when pH ¼ 2.13 [(H+) ¼ 10 2.13], the ratio K1/(H+) is 10 2.13/10 2.13 or 1.0. This means that the ratio of (HPO 42 )/(H3PO4) also equals 1.0, and the concentrations of the two forms of phosphorus are equal. The calculation of the proportions of the two forms of phosphorus in any one of the three dissociations of phosphoric acid may be done is illustrated in Ex. 12.1. This procedure can be applied across the entire pH range to provide the data needed for graphically depicting the effect of pH on proportions of H3PO4, H2PO4 , HPO42 , and PO43 (Fig. 12.2). There is no un-ionized H3PO4 except in highly acidic solutions, and PO43 predominates only in highly basic solutions. Within the pH range of most natural waters, dissolved phosphate will exist as H2PO4 and HPO42. At pH values below 7.21, there will be more H2PO4 , and HPO42 will dominate at pH 7.22 and above. Ex. 12.1: The percentages of H2 PO4 and HPO24 at pH 6 will be estimated. Solution: The mass action expression for (12.2) allows an expression for the ratio of HPO24 to H 2 PO4 ; 248 12 Phosphorus ðHþ Þ HPO24 7:21 ¼ 10 H 2 PO4 HPO24 10 7:21 ¼ H 2 PO4 ðH þ Þ and HPO24 10 7:21 1:21 ¼ ¼ 10 ¼ 0:062: H2 PO4 10 6 There will be 1 part of H2PO4 and 0.062 part of HPO42 at pH 6. The percentage of HPO42 will be 0:062 100 ¼ 5:83% 1 þ 0:062 and 100% 5:83% ¼ 94:17% H 2 PO4 : Polyphosphates also enter waters naturally or in pollution. The parent form of polyphosphate can be considered a compound such as polyphosphoric acid with the formula H6P4O13. Polyphosphate has a greater proportion of phosphorus than does orthophosphate, e.g., orthophosphate (PO4) is 32.6 % phosphorus while the polyphosphate P4O13 is 37.3 % phosphorus. However, when introduced into water, polyphosphate soon hydrolyzes to orthophosphate. Phosphorus-Sediment Reactions Inorganic phosphorus reacts with iron and aluminum in acidic sediments or waters to form slightly soluble compounds. Representative aluminum and iron phosphate compounds are variscite, AlPO 4 2H2O, and strengite, FePO4 2H2O. The dissolution of these compounds is pH-dependent AlPO4 2H2 O þ 2Hþ ¼ Al3þ þ H2 PO4 þ 2H2 O K ¼ 10 2:5 ð12:4Þ FePO4 2H2 O þ 2Hþ ¼ Fe3þ þ H2 PO4 þ 2H2 O K ¼ 10 6:85 : ð12:5Þ Decreasing pH favors the solubility of iron and aluminum phosphates as illustrated in Ex. 12.2. Ex. 12.2: The solubility of phosphorus from variscite will be estimated for pH 5 and pH 6. Phosphorus Chemistry 249 Solution: (i) From (12.4), Al3þ H 2 PO4 2:5 2 ¼ 10 ðH þ Þ and 2 ðH þ Þ 10 2:5 H 2 PO4 ¼ Al3þ Letting x ¼ Al3þ ¼ H2 PO4 , for pH 5 5 2 2:5 10 10 x¼ x x2 ¼ 10 12:5 6:25 x ¼ 10 : At pH 5, 6:25 7 H 2 PO4 ¼ 10 M or 5:62 10 M: There are 31 g of phosphorus per mole of H2 PO4 ; so 7 5 5:62 10 M 31 g P=mol ¼ 1:74 10 g P=L or 0.017 mg/L of phosphorus. (ii) Repeating the above for pH ¼ 6 or (H+) ¼ 10 6 , we get 0.0017 mg P/L. Calculation of the solubility of strengite for the same pH values used in Ex. 12.2, gives phosphorus concentrations of 0.00012 mg/L at pH 5 and 0.000012 mg/L at pH 6. Strengite is less soluble than variscite at the same pH by a factor of nearly 150. Acidic and neutral sediments usually contain appreciable iron and aluminum minerals, and aluminum tends to control phosphate solubility in aerobic water and sediments. To illustrate, two representative iron and aluminum compounds found in sediment are gibbsite, Al(OH)3 and iron (III) hydroxide, Fe(OH)3. The dissolution of these two representative compounds, like other iron and aluminum oxides and hydroxides, is pH dependent 250 12 Phosphorus AlðOHÞ3 þ 3Hþ ¼ Al3þ þ 3H2 O K ¼ 109 ð12:6Þ FeðOHÞ3 þ 3Hþ ¼ Fe3þ þ 3H2 O K ¼ 103:54 : ð12:7Þ The solubilities of the two minerals increase with decreasing pH as illustrated in Ex. 12.3. Ex. 12.3: The solubilities of gibbsite and iron (III) hydroxide will be estimated for pH 5 and 6. Solution: From (12.6) and (12.7) Al3þ pH ¼ 5 þ 3 ¼ 109 ðH Þ 5 3 and Al3þ ¼ 10 109 ¼ 10 6 M ðFe3þ Þ þ 3 ¼ 103:54 ðH Þ 5 3 and Fe3þ ¼ 10 103:54 ¼ 10 11:46 M: 6 3 pH ¼ 6 Al3þ ¼ 10 109 ¼ 10 9 M 6 3 and Fe3þ ¼ 10 103:54 ¼ 10 14:46 M: The Al3+ concentration is 0.027 mg/L and.027 μg/L at pH 5 and pH 6, respectively. The iron concentration is more than four orders of magnitude lower than that of aluminum at each pH—iron is essentially imperceptible below pH 6. Ex. 12.3 clearly shows that the amounts of iron and aluminum in solution in sediment pore water and available to precipitate phosphorus as iron and aluminum phosphates will increase as pH decreases. But, at the same pH, aluminum compounds are more soluble than iron compounds. Despite iron and aluminum phosphate compounds being more soluble at lower pH (Ex. 12.2), iron and aluminum oxides and hydroxides tend to be much more abundant in sediment than are aluminum and iron phosphates. Thus, when phos- phorus is added to acidic sediment there is adequate Al3+ and Fe3+—especially Al3+ because of the greater solubility of its hydroxides and oxides compared to those of iron (Ex. 12.3)—present to precipitate it. Thus, in reality, the availability of phosphorus from sediment tends to decrease with decreasing pH. Phosphorus Chemistry 251 When a highly soluble source of phosphorus is added to sediment at pH of 7 or below, the phosphorus will react with aluminum and iron and precipitate. This phenomenon is illustrated in Ex. 12.4. Ex. 12.4: The solubility of phosphorus from a highly soluble monocalcium phosphate [Ca(H2PO4)2] in a system at equilibrium with gibbsite will be calculated. Solution: Gibbsite provides Al3+ to solution, and Al3+ can react with phosphorus to precipi- tate AlPO4 2H2O (variscite). The solubility of gibbsite in water of pH 5 is 10 6 M (see Ex. 12.3). Dissolution of variscite may be written as AlPO4 2H2 O þ 2H þ ¼ Al3þ þ H 2 PO4 þ 2H 2 O 2:5 f or which K ¼ 10 : Thus; Al3þ H 2 PO4 2:5 þ 2 ¼ 10 ðH Þ 2 ðHþ Þ 10 2:5 H 2 PO4 ¼ Al3þ 5 2 2:5 10 10 6:5 ¼ 6 ¼ 10 M: 10 Expressed in terms of phosphorus, 10 6:5 M is 0.00000032 M 30.98 g P/mol or 0.01 mg P/L. In this example, phosphate solubility is controlled by variscite, because variscite is less soluble than monocalcium phosphate. Phosphate can be absorbed by iron and aluminum oxides as follows: H2 PO4 þ AlðOHÞ3 ¼ AlðOHÞ2 H2 PO4 þ OH ð12:8Þ H2 PO4 þ FeOOH ¼ FeOH2 PO4 þ OH : ð12:9Þ In soil and sediment—especially in tropical areas—some of the clay fraction is in the form of iron and aluminum hydroxides. Clays are colloidal and have a large surface area; they can bind large amounts of phosphorus. Silicate clays also can fix phosphorus. Phosphorus is substituted for silicate in the clay structure. Clays also have some ability to adsorb anions, because they have a small number of positive charges on their surfaces. However, absorption by silicate minerals and anion exchange is less important than phosphorus removal by aluminum and iron in acidic soils and sediments. Primary phosphate compounds in neutral and basic sediment are calcium phosphates. The most soluble calcium phosphate compound is monocalcium 252 12 Phosphorus phosphate, Ca(H 2PO4)2. This is the form of phosphorus normally applied in fertil- izer. In neutral or basic soils, Ca(H2PO4)2 is transformed through dicalcium, octacalcium, and tricalcium phosphates to apatite. Apatite is not very soluble under neutral or alkaline conditions. A representative apatite, hydroxyapatite, dissolves as follows: Ca5 ðPO4 Þ3 OH þ 7Hþ ¼ 5Ca2þ þ 3H2 PO4 þ H2 O K ¼ 1014:46 : ð12:10Þ A high concentration of Ca2+ and a high pH favors formation of hydroxyapatite from dissolved phosphate in water or sediment pore water. Apatite is not apprecia- bly soluble at pH 7 and above even at low calcium concentration as shown in Ex. 12.5. Ex. 12.5: The solubility of phosphorus from hydroxyapatite in water of pH 7 and pH 8 and 15 mg/L calcium 10 3:43 M will be estimated. Solution: Assuming that the reaction controlling phosphorus concentration is (12.10), 5 3 ðCa2þ Þ H 2 PO4 þ 7 ¼ 1014:46 ðH Þ at pH 7 7 7 3 10 1014:46 17:39 H 2 PO4 ¼ ¼ 10 M 3:43 5 10 5:80 H2 PO4 ¼ 10 M or 0:05 mg P=L: Repeating the calculation for pH 8 gives 8:13 H2 PO4 ¼ 10 M or 0:0002 mg P=L: At a higher calcium concentration, the phosphorus concentration would be less, e.g. at pH 7 and 20 mg Ca2+/L (10 3.30 M), the phosphorus concentration would be only 0.03 mg/L—40 % less than at 15 mg/L Ca2+. The maximum availability of phosphorus in aerobic soil or sediment typically occurs between pH 6 and 7 (Fig. 12.3). In this pH range there is less Al3+ and Fe3+ to react with phosphorus and a smaller tendency of aluminum and iron oxides to adsorb phosphorus than at lower pH. Also, at a pH of 6–7 the activity of calcium is normally lower than at higher pH. Nevertheless, in the pH range of 6–7, most of the phosphorus added to aquatic ecosystems still is rendered insoluble through adsorp- tion by colloids or precipitation as insoluble compounds. Organic Phosphorus 253 Fig. 12.3 Schematic showing effects of pH on the relative concentrations of dissolved phosphate in aerobic soil or sediment Iron phosphates contained in sediment become more soluble when the redox potential falls low enough for ferric iron to be reduced to ferrous iron. Phosphorus concentrations in pore water of anaerobic sediment may be quite high (Masuda and Boyd 1994a). This pool of phosphorus is largely unavailable to the water column because iron phosphate reprecipitates when ferrous iron and phosphate diffuse into the aerobic layer normally existing at the sediment-water interface. The aerobic layer at the interface is lost during thermal stratification of eutrophic lakes and ponds. Diffusion of iron and phosphorus from anaerobic sediment can lead to high iron and phosphate concentrations in the hypolimnion. Concentrations of 10–20 mg/L iron and 1–2 mg/L soluble orthophosphate are not unusual. When thermal destrati- fication occurs, hypolimnetic waters mix with surface waters, and concentrations of phosphorus increase briefly in surface waters. However, in oxygenated water, phosphorus concentrations quickly decline. Phosphorus either precipitates directly as iron phosphate (12.5) or it is adsorbed onto the surface of the floc of iron (III) hydroxide precipitating from the oxygenated water. Organic Phosphorus The dry matter of plants commonly contains 0.05–0.5 % phosphorus, while that of vertebrate animals such as fish may contain 2–3 % phosphorus or more. Crustaceans typically contain about 1 % phosphorus in their dry matter. As mentioned earlier in this chapter, phosphorus is present in plants, animals, and bacteria as phospholipids, nucleic acids, and other biochemicals. However, in plants some phosphorus is stored 254 12 Phosphorus in an organic form known as phytic acid, and bone made mainly of calcium phosphate contains much of the phosphorus in vertebrate animals. Phosphorus contained in organic matter is mineralized by microbial activity in the same manner that nitrogen is mineralized. The same conditions favoring decomposition and nitrogen mineralization favor phosphorus mineralization. Just as with nitrogen mineralization, if there is too little phosphorus in organic matter to satisfy microbial requirements, phosphorus can be immobilized from the environ- ment. The nitrogen:phosphorus ratio in living organisms and in decaying organic residues varies considerably ranging from around 5:1 to 20:1. Analytical Considerations The phosphorus in water consists of various forms to include soluble inorganic phosphorus, soluble organic phosphorus, particulate organic phosphorus (in living plankton and in dead detritus), and particulate inorganic phosphorus (on suspended mineral particles). The soluble fraction can be separated from the particulate fraction by filtration through a membrane or glass fiber filter. However, common analytical methods do not distinguish perfectly between soluble inorganic and soluble organic phosphorus, and a portion of the soluble organic phosphorus will be included in measurements of soluble inorganic phosphorus. Therefore, when the phosphorus concentration is measured directly in filtrates of water, the resulting phosphorus fraction is called soluble reactive phosphorus. Digestion of a raw water sample in acidic persulfate releases all of the bound phosphorus, and analysis of the digestate gives total phosphorus. Most of the information on phosphorus concentrations in natural waters is for soluble reactive phosphorus and total phosphorus. Phosphorus in sediment may be extracted with various solutions to give different fractions. A common way of fractioning sediment phosphorus is a sequential extraction with 1 M ammonium chloride to remove loosely-bound phosphorus, 0.1 N sodium hydroxide to remove iron and aluminum-bound phosphorus, and 0.5 N hydrochloric acid to remove calcium-bound phosphorus (Hieltjes and Liklema 1982). Other extractants also are used to remove phosphorus from sedi- ment samples. A widely used method of soil phosphorus analysis in soil testing laboratories is to measure phosphorus extracted by dilute (0.05–0.1 N) hydrochloric acid, sulfuric acid, or a mixture of these two acids. Soil can be digested in perchloric acid to release bound phosphorus for total phosphorus analysis. Phosphorus Dynamics Concentrations in Water Phosphorus concentrations in surface waters generally are quite low. Total phos- phorus seldom exceeds 0.5 mg/L except in highly eutrophic waters or in wastewaters. There generally is much more particulate phosphorus than soluble Phosphorus Dynamics 255 Table 12.1 Distribution of forms of soil and water phosphorus for a fish pond at Auburn, Alabama Phosphorus pool Phosphorus fraction Amount (g/m2) (%) Pond watera Total phosphorus 0.252 0.19 Soluble reactive phosphorus 0.019 0.01 Soluble nonreactive phosphorus 0.026 0.02 Particulate phosphorus 0.207 0.16 Soilb,c Total phosphorus 132.35 99.81 Loosely-bound phosphorus 1.28 0.96 Calcium-bound phosphorus 0.26 0.20 Iron- and aluminum-bound phosphorus 17.30 13.05 Residual phosphorusd 113.51 85.60 Pond All 132.60 100.00 a Average pond depth ¼ 1.0 m b Soil depth ¼ 0.2 m c Soil bulk density ¼ 0.797 g/cm3 d Phosphorus removed by perchloric acid digestion reactive phosphorus. For example, Masuda and Boyd (1994b) found that water in eutrophic aquaculture ponds contained 37 % dissolved phosphorus and 63 % particulate phosphorus. However, most of the dissolved phosphorus was non-reactive organic phosphorus, and only 7.7 % of the total phosphorus was soluble reactive phosphorus. Typically, 10 % or less of the total phosphorus will be soluble reactive phosphorus and readily available to plants. Most surface waters contain less than 0.05 mg/L soluble reactive phosphorus, and most unpolluted water bodies only contain 0.001–0.005 mg/L of this fraction. Sediment contains much more phosphorus than water. Total phosphorus concentrations found in the literature ranged from less than 10 mg/kg to more than 3,000 mg/kg. However, most of the phosphorus is tightly bound and not readily soluble in water. Phosphorus concentrations in the sediment of a eutrophic fishpond (Masuda and Boyd 1994b) are illustrated in Table 12.1. Notice that 85.6 % of the phosphorus was not removable by normal extracting agents and had to be released by perchloric acid digestion. Plant Uptake Phytoplankton can absorb phosphorus from water very quickly. In a water with a dense bloom of phytoplankton, phosphorus additions of 0.2–0.3 mg/L were completely removed within a few hours (Boyd and Musig 1981). Macrophytes also can remove phosphorus from water very quickly, and rooted macrophytes can absorb phosphorus from anaerobic zones in the sediment (Bristow and Whitcombe 1971). Plant uptake is a major factor controlling concentrations of soluble reactive phosphorus in water, and much of the total phosphorus in water is contained in phytoplankton cells. Macrophyte communities can store large amounts of phospho- rus in their biomass. 256 12 Phosphorus Fig. 12.4 Luxury consumption of phosphorus by phytoplankton Some plants can absorb more phosphorus than they need immediately, and they store it for use later. The absorption of phosphorus and other nutrients in excess of the amount required for growth has been demonstrated in many plant species including species of phytoplankton. This phenomenon is termed luxury consump- tion and is illustrated in Fig. 12.4. The ability to absorb and store more nutrients than needed at the moment is of competitive advantage for plants. The phosphorus can be removed from the envi- ronment thereby depriving competing plants of it. In phytoplankton, phosphorus in cells can be passed on to succeeding generations when cells divide and multiply. In larger plants, phosphorus can be translocated from storage sites in older tissue to rapidly growing meristematic tissues. Exchange Between Water and Sediment If some sediment is placed in a flask of distilled water and agitated until equilibrium phosphorus concentration is attained, very little phosphorus usually will be present in the water. For example, in a series of soil samples containing from 100 to 3,400 mg/kg of total phosphorus, water extractable phosphorus concentrations ranged from undetectable to 0.16 mg/L (Boyd and Munsiri 1996). The correlation between total phosphorus and water extractable phosphorus was weak (r ¼ 0.581), but the correlation between dilute acid (0.075 N) extractable phosphorus and water soluble phosphorus was much stronger (r ¼ 0.920). It can be shown by successive extractions of a sediment with water that there is a continued release of phosphorus to the water for many extractions (Fig. 12.5). Phosphorus Dynamics 257 Fig. 12.5 Quantities of phosphorus removed from a mud by consecutive extractions with phosphorus free water The amount released, however, declines with the number of extractions. Because of the relationship shown in Fig. 12.5, the sediment is a reserve of phosphorus available when plant removal causes phosphorus concentrations in the water to fall below the equilibrium concentration. Nevertheless, the concentrations of phos- phorus at equilibrium normally are quite low, and phosphate additions are neces- sary to stimulate abundant phytoplankton growth. Macrophytes—especially rooted, submerged macrophytes—grow quite well in waters that are low in phosphorus because they can absorb phosphorus and other nutrients from sediment. Sediment is not a readily available source of phosphorus or other nutrients for phytoplankton because of the difficult logistics of nutrient movement from sediment pore water to the illuminated zone where phytoplankton grow (Fig. 12.6). When phosphorus enters water through intentional additions as in fishponds or through pollution, stimulation of phytoplankton growth creates turbidity and shades the deeper waters. Restriction in light caused by nutrient enrichment may eliminate many species of macrophytes from aquatic communities in eutrophic water bodies. The relative concentrations of phosphorus in sediment, sediment pore water, at the sediment-water interface, and in surface water are illustrated in Fig. 12.7 for a small, eutrophic fishpond. There was roughly an order of magnitude difference in sediment phosphorus and pore water phosphorus concentrations, and another order of magni- tude difference between phosphorus concentrations in pore water and at the water at the sediment-water interface. Pore water is anaerobic and phosphorus in pore water tends to precipitate at the aerobic interface and little enters the pond water. 258 12 Phosphorus Fig. 12.6 Illustration of rapid uptake of phosphate by phytoplankton cells and slower exchange of phosphate between sediment and water Fig. 12.7 Concentrations of phosphorus bound in soil, dissolved in pore water, and dissolved in overlaying pond water Even when anaerobic conditions exist at the sediment-water interface, phosphorus must diffuse from the pore water into the open water, and diffusion is a relatively slow process. Once phosphorus enters the open water, it can be mixed throughout the water body rather quickly by turbulence. In pond aquaculture, phosphorus often is added to increase dissolved inorganic phosphorus concentrations. A portion of the added phosphorus is quickly absorbed by phytoplankton. The part that is not removed by plants will tend to accumulate in the sediment, and most of the phosphorus removed by plants also will eventually reach the sediment. Turbulence will allow soluble phosphorus to reach the sediment more quickly when compared to the movement of sediment-bound phosphorus into Eutrophication 259 Fig. 12.8 Average concentrations of total and particulate phosphorus in two fertilized fish ponds. Vertical arrows indicate fertilizer application dates the water. The removal of fertilizer phosphorus from fish ponds (Fig. 12.8) illustrates the rapidity of phosphorus removal from water. The ability of sediment to hold phosphorus is usually quite large. Fish ponds on the Auburn University E. W. Shell Fisheries Center at Auburn, Alabama that had received an average phosphorus input of 4.1 g P/m2/year (41 kg/ha/year) for 22 years were only about half-saturated with phosphorus and still rapidly adsorbed phosphorus from the water (Masuda and Boyd 1994b). Nevertheless, sediment can become saturated with phosphorus or have a very low capacity to adsorb phosphorus, e.g., sandy sediment. In such bodies of water, additions of phosphorus are particularly effective in stimulating phytoplankton growth. Sediment is not always necessary for phosphorus removal from the water. In waters with significant concentrations of calcium and pH of 7–9, phosphate will precipitate directly from the water as calcium phosphate. Eutrophication Average, annual total phosphorus concentrations and their ranges in lakes classified to trophic status by different investigators were summarized by Wetzel (2001) as follows: Status n Mean (mg/L) Range (mg/L) Oligotrophic 21 0.008 0.003–0.018 Mesotrophic 19 0.267 0.011–0.096 Eutrophic 71 0.084 0.016–0.386 Hyper-eutrophic 2 0.975 0.750–1.200 260 12 Phosphorus The range in phosphorus concentration for each trophic status level and the overlapping of the ranges for the different trophic levels reveal that there is not an exact relationship between phosphorus concentration and trophic status. One factor resulting in the wide ranges in phosphorus concentration for each trophic status is differences in the availability of other essential nutrients. Another is different degrees of turbidity resulting from suspended clay particles or humic substances in water that interfere with light penetration and phytoplankton photosynthesis. The amount of phosphorus that must be applied to a lake to cause eutrophication also differs with several factors. However, the most important one is the hydraulic flushing rate. The percentage of the phosphorus input (load) that is retained increases as hydraulic retention time increases. Thus, a greater phosphorus input would be necessary to cause eutrophication in a lake with a hydraulic retention time of 2 months than in a lake with a hydraulic retention time of 8 months assuming all other factors are equal. A lake with high pH and high calcium concentration also would require a greater phosphorus load to cause eutrophication than would a lake low in total alkalinity and total hardness concentration. Interaction with Nitrogen Nitrogen and phosphorus are key nutrients regulating aquatic plant productivity. But, the amounts and ratios of these two nutrients vary among species. Redfield (1934) reported that marine phytoplankton contained on a weight basis about seven times more nitrogen than phosphorus. This value is often used as the average N:P ratio in plants, but the ratios for individual species vary from 5:1 to 20:1. However, in most ecosystems, an increase in phosphorus concentration will cause a greater response in plant growth than will an increase in nitrogen concentration. This results because phosphorus is quickly removed from the water and bound in the sediment. There is limited recycling of sediment bound phosphorus, and to main- tain sufficient phosphorus in the water to promote high rates of plant growth, there must be a continuous external source of phosphorus. Nitrogen also is removed from water bodies by various processes, but as much as 10–20 % of added nitrogen is present in organic matter deposited in sediment. Sediment organic matter is decomposed, and nitrogen is continually mineralized. The internal recycling of nitrogen in aquatic ecosystems is much greater than it is for phosphorus. As a result, in order to achieve a nitrogen to phosphorus ratio of 7:1 (the Redfield ratio) in the water, it likely would require an addition of these two elements in a lower N:P ratio. Significance The popular literature on lake eutrophication often refers to phosphorus as a toxic element. Phosphorus is not actually a toxic element. But, high phosphorus concentrations in aquatic ecosystems cause excessive aquatic plant productivity References 261 and eutrophication that can result in low dissolved oxygen concentration and fish kills. Phosphorus and nitrogen are considered to be the two key nutrients associated with eutrophication, and like nitrogen, it is difficult to establish the concentration of phosphorus that will cause excessive plant growth. Also, it is difficult to establish phosphorus loads necessary to cause eutrophication in natural waters. Nevertheless, there is ample evidence that concentrations of total phosphorus of 0.005–0.05 mg/L can cause phytoplankton blooms in many lakes. Thus, limitations on phosphorus concentrations in effluents have been one of the main tools in combating eutrophication. References Boyd CE, Munsiri P (1996) Phosphorus adsorption capacity and availability of added phosphorus in soils from aquaculture areas in Thailand. J World Aquacult Soc 27:160–167 Boyd CE, Musig Y (1981) Orthophosphate uptake by phytoplankton and sediment. Aquaculture 22:165–173 Bristow JM, Whitcombe M (1971) The role of roots in the nutrition of aquatic vascular plants. Am J Bot 58:8–13 Hieltjes AHM, Liklema L (1982) Fractionation of inorganic phosphate in calcareous sediments. J Environ Qual 9:405–407 Masuda K, Boyd CE (1994a) Chemistry of sediment pore water in aquaculture ponds built on clayey, Ultisols at Auburn, Alabama. J World Aquacult Soc 25:396–404 Masuda K, Boyd CE (1994b) Phosphorus fractions in soil and water of aquaculture ponds built on clayey, Ultisols at Auburn, Alabama. J World Aquacult Soc 25:379–395 Redfield AC (1934) On the proportions of organic deviations in sea water and their relation to the composition of plankton. In: Daniel RJ (ed) James Johnstone memorial volume. University Press of Liverpool, Liverpool Wetzel RG (2001) Limnology, 3rd edn. Academic, New York