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17. Phosphorus

Biological productivity is mostly limited by the amount of phosphorus in water and soil. Phosphorus is present in natural waters as orthophosphate and undifferentiated organic phosphates. The total phosphorus is differentiated into soluble phosphate phosphorus (filtrable or soluble orthophosphate), organic soluble phosphate phosphorus and sestonic phosphorus, which is again divisible into sestonic acid-soluble phosphorus (mainly ferric and calcium phosphate) and organic sestonic phosphorus.

In a typical natural water body the total -P was estimated at 23 mg/M3, constituted by soluble phosphate -P (3 mg/M3), organic soluble phosphate -P (14 mg/M3) and sestonic -P (6 mg/M3) (Hutchinson, 1957). The total phosphorus in natural waters can be much high (30 mg/M3), with ratio usually of soluble PO4-P to total -P of 1:10.

The ionization products of orthophospheric acid in water are:

H3PO4 H+ + H2PO-4

H2PO4 H+ + HPO=4

HPO4 H+ + PO=4

The relationship of pH and activities of the various forms of orthophosphates (% mole fractions) are indicated Fig. 14. From this it is clear that at the pH levels often encountered in natural waters H2PO4 and HPO= 4 would be dominant.

As already indicated the phosphorus levels though usually small in water and mud in fish ponds are very important, and therefore the richness of the water body could be ascertained by measurement of their levels. Phosphate levels in ponds are maintained by addition of phosphorus fertilizers. Phosphorus input also comes from metabolic wastes and from uneaten feeds.

Both nitrogen and phosphorus levels are important in fish ponds, and a healthy P : N ratio of 1 : 4, in water is proposed; Hepher and Pruginin (1981) observed that levels of P over 0.4 mg/l and over 1.5 mg/l in fish pond water were not useful in increasing the productivity of fish ponds in Israel.

Fig. 14.

Fig. 14. Percent mole fraction of various forms of orthophosphate at different pH levels
(After Boyd, 1982).

These will be taken up again in “Fertilization” under Pond Culture. In spite of these higher levels maintained in the fish pond, it is known that at least for certain species of phytoplankton the phosphate - P concentration permitting optimal growth is quite low - e.g. Pediastrum barganum - 0.089 mg/l; Nitschia palea - 0.018 mg/l; and Asterionella gracillima (0.045 mg/l) (Hutchinson, 1957). Boyd (1982) finds that pond fertilization programmes often apply inorganic phosphorus fertilizers sufficient to increase filtrable ortho-phosphate - (soluble orphophate) by 0.1 - 0.5 mg/l as P, provided the fertilizer dissolves fully and mixes well in the volume of water. Variations in various forms of phosphorus and nitrogen in fish ponds are shown in Fig. 15.

The trends in the concentration of orthophosphate in pond water after application of three different fertilizers (ammonium polyphosphate, diammonium phosphate and triple super phosphate cast at 9 kg/ha as P2O5 in three different ponds) are shown by Boyd (1982) (Fig. 16). The levels go as high as 0.3 mg/l, and subsequently drops down exponentially and reaches close to original level in 20 days, in all cases.

The results show that levels required in the fish pond water can be reached by fertilization, but that there is need for repeated fertilization in small dozes to maintain the level. Hepher (1962) also comes to the same conclusion in his experiments in Israel.

It is well established that orthophosphate present in water after fertilization is absorbed by bacteria, phytoplanktons and macrophytes, (Rigler, 1964). Up to 41% of 0.3 mg/l orthophosphate added to fish pond water is removed by phytoplankton (Boyd & Musig, 1981) (Fig. 17). Both phytoplankton and macrophytes absorb and store phosphorus (“luxury consumption”) (though macrophytes consume less) and the stored phosphorus is used up later (Hayes and Phillips, 1958; Boyd, 1982). The role of sediment in absorbing phosphorus and releasing it when reduced (see Redox potential) must also be recalled here. The rooted plants can tap phosphorus from sediment.

Release of phosphorus from sediment to water is also known. Better productivity in water bodies with richer orthophosphate in the sediment has been recorded. Phosphate becomes unavailable when applied to high pH water with high calcium content, since insoluble calcium phosphate (Ca3 (PO4)2) is formed. Boyd (1982) observes that at a Ca++ concentration of 20 mg/l, more than 10 mg/l (as P) ortho-phosphate can exist in solutions at pH 8, but at pH 10 the orthophosphate concentration in water would not exceed 0.25 mg/l. The nature of the precipitating compound is not exactly known in alkaline waters and muds. The nature of fertilizer applied is really important and it is better to use fertilizers such as ammonium phosphates, which would bring down pH than calcium phosphate.

Eventhough the pond mud is known to serve as a sink for phosphorus added as fertilizer and also that the pond water could gain phosphorus from the mud, it is imperative that fertilizer have to be added to the pond for enhancing plankton production. Phosphate system in fish ponds is very complex and needs further elaborate studies under specific pond conditions. A knowledge of ‘P’ in water and soil are important in choosing the site and for subsequent pond preparation.

Fig. 15.

Fig. 15. Seasonal variation in different forms of nitrogen (A) and total and soluble orthophosphate (B) in catfish ponds in Alabama. (After Boyd, 1979).

Fig. 16.

Fig. 16. Concentration of soluble (filtrable) phosphate in fish pond water following fertilization (9 kg/ha of P2O5). 1. Ammonium polyphosphate (10-34-0) 2. Diammonium phosphate (18–46-0); 3. Triple superphosphate (0–46-0). (After Boyd, 1982).

Fig. 17.

Fig. 17. Decline in orthophosphate in earthen pond and in plastic pool (Mud-water-outdoor) (After Boyd, 1982).

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