Role Of Water And Soil Parameters In Pond Productivity

Role of Water and Soil parameters in Pond productivity Dr. Subhendu Datta Sr. Scientist CIFE, Kolkata Centre, India

(A) Need of Water and Soil Analysis for Sustainable Aquaculture

Importance of water in aquaculture: If aquaculture is the rearing of aquatic organisms, it is important for an aquaculturist to understand the aquatic medium i.e. water, in which these organisms inhabits. If the water is “bad,” plants and animals won’t grow or reproduce. Animal stressed because of poor water quality are also prime targets for pathogens and parasites. Just as people who work in offices or factories that are stuffy and have smoke or chemical fumes in the air are more apt to be sick, so it is with aquatic organisms grown in poor quality of water. Water is the medium in which fish live, and from which they derive oxygen and nutrients. So the quantity and quality of the water very much affect the prospect of fish culture. As water is the basic element in fish culture, its specific properties as a cultural medium are naturally of great significance in the productivity of a pond. Pure water is unable to support living organisms, but its content of nitrogen, phosphorus, potassium and calcium salts, dissolved organic matter and gases like oxygen, nitrogen and carbon dioxide determine to a large extent the productivity.

Importance of soil in aquaculture: The properties of pond soil are of greater significance than is generally realised. When soil conditions are not favourable, the production will be limited. Productivity of fishponds depends on the occurrence of suitable environmental conditions and abundance of fish food organisms. The first step in the food chain (Fig. 1) of a fish pond is constituted by primary food organisms e.g. phytoplanktons, which derive their nutrients from the pond environment and with the help of solar radiation undergo photosynthetic activities. Occurrence of these nutrients in pond water and maintenance of its relevant chemical condition depends largely on the nature and properties of the bottom soil wherein a series of chemical and biochemical reactions continuously take place resulting in release of different nutrients in overlaying water and also their absorption in the soil mass. Considering this importance of bottom soil in maintaining the productivity

1

of fish ponds, Hickling (1971) described such soils as the “Chemical Laboratory of the fish pond.

Fish

Bottom Fauna -----------------------Zooplankton Bottom Flora -----------------------------------------Phytoplankton Nutrients in water Nutrients in Soil

Fig. 1: Food pyramid in a pond

The soil fertility is of special importance in the growth of benthic vegetation. While water fertility will contribute largely to the production of plankton; the pond bed releases nutrient material into the water and helps in fixation or chemical combination of such substances released in the pond itself or introduced from outside. Production in ponds with a bottom rich in fertilising elements is much greater than in those with poor soil. The colloidal content of the soil, especially of the muddy layer on the top, is of importance in its capacity to fix or chemically bind nutrients. The productive capacity of the pond bottom has to be preserved by alternate periods of mud formation and mineralisation – the practice of regularly draining fish pond. In view of this importance of overlaying water as well as bottom soil in determining the productivity of a fish pond, an intimate knowledge about the nature and properties of these two phases need to be understood thoroughly for developing a clear idea about the ecosystem and obtaining, thereform, good production of fishes.

Importance of water and soil analysis in sustainable aquaculture: As the fish catch from all sources of capture fisheries has nearly attained a saturation point, aquaculture has gained special attention to increase the fish production of the country. To achieve this goal, with time aquaculture has also shifted from conventional culture practice to semiintensive and intensive culture practices. In intensive cultural practices, there is very high 2

load of fish, feed, nutrients and chemicals used for controlling fish diseases (e.g. antibiotics) per unit area. Therefore, fish excreta, respiratory products, unfed materials; unutilised nutrients/chemicals and transformed/metabolites of nutrients or chemicals can severely deteriorate the fish pond/lake environment (i.e. water and soil quality) even in short run. Every system must be sustainable. Fishery resources are renewable source of resources i.e. fish stock in an aquatic system are able to reproduce or replace themselves or increase. The management of renewable resources involves, as a minimum, practices that will result in a sustained yield. This emphasise the management of human use of fishery resources so that it may yield the greatest sustainable benefit to present generation while maintaining its potential to meet the needs and aspirations of future generations. In layman’s language, we should practice the aquaculture in such a way so that present generation can get the earnings for their livelihood, at the same time, follow the good management practices and maintain the good fertile condition of the system (i.e. soil and water) so that it produces fish in the similar way to the future generations. Hence the importance of water and soil analysis lies for sustaining high yielding aquacultural practices. Regular monitoring of water and soil quality parameters can give an insight about the physical, chemical and biological environment of the aquatic ecosystems. This will assist to take decisions on management practices to be adopted both in terms of better fish production and maintaining the ecosystem for long run.

References 1. Hickling, C.F. (1971). Fish Culture. Faber and Faber. London. pp.225. 2. Conservation of Natural Resources (2nd ed) Gey-Harold Smith (ed)- John Wiley & Sons. Inc. Chapt. 19. Fisheries for the future. 3. Environmental Conservation. R. F. Dasman, John Wiley & Sons. Inc. Chapt. 9. Water and Fisheries.

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(B). Role of water parameters in pond productivity

Physical Parameters of Water The major important physical parameters of water on which the productivity of a pond depends upon are; 1. Depth. 2. Temperature. 3. Turbidity. 4. Light.

Depth : Depth of a pond has an important bearing on the physical and chemical qualities of water. On it, but varying with its turbidity, depends the limit of penetration of sunlight, which in turn, determines the temperature and the circulation patterns of the water and the extent of photosynthetic activity. Ideal depth for different kinds of fish ponds from the point of view of congenial biological productivity are as follows; Nursery Pond : Rearing Pond : Stocking Pond :

1 – 1.5 m 1.5 – 2.0 m 2.0 – 2.5 m

Ponds shallower than 1m get over heated in tropical summers inhibiting survival of fish and other organisms. Depths greater than 5 m are also not suitable for fish culture. In such ponds along with poor penetration of sunlight, there remains the formation risk of a permanently deoxygenated layer or the circulation of water is unable to carry oxygen down to the mud layer. Formation of H2S takes place in reduced layer of pond mud and in absence of oxidizing surface layer, this poisonous gas diffuse into the water and make the deepest parts of the pond uninhabitable by fish. In such ponds there must be the provisions of plenty of breeze flowing which can keep water circulating or arrangement of artificial water circulation (aerator). Turbidity (Transparency): 1 Transparency ∝ -------------Turbidity Transparency is inversely proportional to the turbidity of water, which in turn is directly proportional to the amount of suspended organic and inorganic matter. *Turbidity due to profusion of plankton is an indication of pond’s high fertility but that caused by silt or mud beyond a limit (up to 4% by volume) is harmful to fish and fish food organisms. * 4

Turbidity due to high concentration of silt, mud or algal growth causes death of fishes due to choking of gills. Suspended particles may be settled by application of lime and algal bloom can be restricted by application of Takazine – 50 (Cymazine) @ 2-4 kg / acre. If the pond water is covered by floating weeds, Wolfia. sp (microweeds) or Lemna minor, Lemna major, Spirodella for one week then also the algal growth is checked due to lack of penetration of sunlight. Euphotic zone is the visible zone of natural water body. *[Turbidity and transparency both are optical properties of light, turbidity causes light to be scattered thereby restricts its penetration and reduce photosynthetic activity. Suspended particles causing turbidity may also adsorb considerable amount of nutrient elements like phosphate, K, N2 in their ionic form and making them unavailable for plankton production, while transparency cause light to be transmitted in straight line through the sample.]* Secchi disk transparency : 20-60 cm is ideal for good productivity. It is a metallic plate of 20 cm diameter with four alternate black and white quadrants (to give a sharper end point but generally at a smaller depth) on the upper surface and a hook at the center to tie a graduated rope. The procedure in simply to observe the depth at which the disk let down from the surface just disappear from view. The observation must be made through a shaded area of water surface. It is usual to determine the point of disappearance as the disk is lowered (d1) allow it to drop a little further, and then determine the point of reappearance as the disk is raised (d2). The mean of the two readings is taken as the secchi disk transparency. The observation should not be made early in the morning or late in the afternoon, though both theory and observations show that the result is largely independent or illumination. Other instrument to measure transparency are photometer, lux-meter. Temperature: Variations in temperature in a water body has a great influence upon its productivity. Temperature influence all metabolic and physiological activities and life processes such as feeding, reproduction, movement and distribution of aquatic organisms. Temperature also affects the speed of chemical changes in soil and water. The oxygen content of water decreases with rise in temperature. Most of the tropical fish cann’t survive below 100C. Tilapia cann’t survive below 80C. Indian major carps are able to tolerate a wide range of temperature (20 to 370C), below 16oC and above 400C prove fatal to them. Many exotic species can’t survive at higher temperature. Fishes native to cold 5

water (e.g. Silver Carp) are unable to survive on the plains due to higher water temperature in summer months. Both silver carp and grass carp prefers temperature below 30o C. Hence, a knowledge of the range of temperature variation is necessary before introducing fishes for culture in a pond. The thermal stratification namely (i) epilimnion, (ii) thermocline and (iii) hypolimnion may not be prominent in shallow ponds. Observations conducted in Indonesia have shown that instead of an annual turnover, as found in temperate climates, a daily turnover takes place in tropical ponds. During the nights, circulation takes place, bringing about a mixing of the water. This turnover is of extreme importance in the circulation of oxygen and nutrients in pond water.

Light: Light is an important factor influencing productivity. Penetration of light depends upon the available intensity of the incident light, which varies with the geographical locations of the pond and turbidity of water. In shallow ponds, light reaches upto the bottom and causes heavy growth of vegetation. Light controls the flora and oxygen content of the water of the pond. Shade provided by the surrounding vegetation affects the incidence of light on the pond. Advantage of shading effect in often taken in pisciculture effect for the control of algal blooms and submerged weeds. Among other physical factors, shore conditions, pressure and movement of water plays some role on productivity of pond water.

Pressure and Movement of Water: All animals cannot survive in very deep water due to increase in pressure and variation of percentage of mineral salts. Movement of water causes erosion of soil and increase turbidity. Movements of water due to waves, currents and breeze favours productivity provided motion is not too strong to bring about unfavourable changes in the condition of water. Shore Conditions: Longer shoreline enhances productivity due to increase in the production of vegetation and phytoplankton. But shady shore trees, surface and submerged plants and turbidity due to silt lowers the productivity by cutting light.

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Chemical Parameters of Water

Water pH: Logarithm of the reciprocal of hydrogen ion concentration in moles per lit. pH = log

1 = − log [H + ] [H + ]

pH Range

Productivity of Water

<5.5

(Strongly acid)

Unproductive.

5.5 – 6.5

(Acid Water)

Low Productivity.

6.5 – 7.0

(Neutral

Average Productivity.

7.0 – 8.5

(Slightly Alkaline)

Most Productivity.

8.5 – 10.5

(Alkaline)

Low Productivity.

> 10.5

(Strong Alkaline)

Unproductive.

The pH of the water is indicative of its fertility or potential productivity. A slight alkaline reaction is of great help in the conversion of organic matter into assimilable substance, such as ammonia and nitrates (mineralization). The majority of the natural mater are alkaline and alkalinity is mainly due to the salts of calcium in the form of bicarbonate and carbonate. Dominated by

pH Range

Free CO2 dominates

5–7

HCO3

7–9

CO3

> 9.5

-

(OH ions arising from hydrolysis of HCO3- and CO3- - ions) Strong acid dominates

>4.0

The pH of pond water undergoes a diurnal change, it being most alkaline in mid-afternoon and most acidic just before daybreak. During night, due to respiration of plants and animals, CO2 is produced which on hydrolysis produce H2CO3 and make the water acidic. H2O + CO2 = H2CO3 Concentration of CO2 is highest just before daybreak. So, the water is most acidic. During daytime, this CO2 is used up for photosynthesis and makes the water alkaline (as in natural water, HCO3- and CO3-- responsible for basic reaction, are dominated when CO2 is used up). pH > 9.5 is not suitable as CO2 is absent and photosynthesis does not occur and fish die. Acidity reduces the appetite of the fish, their growth and tolerance to toxic substances; Increase toxicity of H2S, copper and other heavy metal to fish; 7

Impeding the circulation of nutrients by reducing the rate of decomposition; Inhibit the nitrogen fixation; Fish gets prone to attacks of parasites and diseases. Water more acidic than pH 5.5 are not fertilized until they are corrected by liming. In acid waters, it is desirable to use non-acid forming fertilizers. pH

MANAGEMENT

<4.5

lime with Ca(OH)2 to pH 6-6.5, use basic fertilizers.

4.5 – 7

lime with CaCO3, use alkaline fertilizers

7.0 – 8.5

Most suitable

8.5 – 11

use acid forming fertilizers.

Acidic fertilizers: NH4Cl > (NH4)2SO4 > Ammonium Sulphate Nitrate > Urea > S.S.P Basic fertilizers: Rock phosphate > CaCN > Sod. Nitrate > Ca-nitrate. Neutral

:Calcium ammonium nitrate (CAN).

Dissolved Oxygen This is the most important factor governing the carrying capacity of pond or lake. Sources of Oxygen (O2): (i)

Absorption from air at the water surface.

(ii)

Photosynthesis of chlorophyll bearing organism inhabiting pond.

Consumption of Oxygen (O2): (i)

Respiration of aquatic animals and plants in day and night.

(ii)

Decomposition of organic matter* [Do not stock fish in newly constructed pond].

The O2 available in pond at a given time is balance of these two processes. Value of dissolved O2 depends on temperature, partial pressure of O2 and water salinity. When temperature increases dissolved O2 decreases. Rate of respiration is more and rate of photosynthesis is low due to high temperature. When partial pressure of O2 in contact with water at the surface increases amount of O2 dissolved in water is also increases. When concentrated of dissolved salts (salinity) increases dissolved O2 concentration decreases. At 00C, fresh water contains slightly over 2.0 mg/l O2 than sea water (35% salinity) Concentration of O2

Pond Productivity

Below 3.0 ppm

Unproductive

3.0 – 5.0 ppm

Average productive

6.0 – 5.0 ppm

High productive. 8

When heavy infestation of aquatic weed and dense algal bloom (may be because of over-fertilization of pond) are present a marked diurnal fluctuations and dangerous oxygen (O2) deficiency may results. During day time, because of photosynthesis, water is supersaturated with O2, but during night, consumes more O2 than produced, which is severe during late hours. On cloudy days, the photosynthesis may be reduced due to lack of sunlight prolonging the night deficit in the O2 budget. But when there is a continuous cloudy days, most of the O2 fluctuation are below the critical level for fish survival (50% saturation of O2) and mass mortality of fish may occur. To Mitigate the oxygen deficiency: Direct ways or Physical Methods : i)

Beating by stick on all sides of ponds;

ii)

Use Aerator

iii)

Introduce fresh oxygenated water from other areas to pond.

iv)

Pumping of water by water pump

Chemical methods: i) Apply lime @ 60 – 70 Kg/ha. ii) Apply KMnO4 @ 4 Kg/ha iii). Use UltaSil-Aqua (Aqua Zeolite) of Neospark, Drugs and Chemical Private Ltd, Hyderabad @ 10 to 40 Kg per acre. iv). Use Aqua Clean of G M Chemicals, Ahmedabd @ 25 – 35 Kg per acre at every 20-25 days. Oxygen deficiency in lakes and river cause migration of fish, attack of parasites, fungus diseases and death due to suffocation.

Free CO2 Sources of CO2 in natural water i) From atmosphere: a) Through rain water contains 0.3 – 0.6 ppm. b) Air in contact with water surface. ii) Respiration of aquatic plants and animals . iii) Decomposition of organic matter in water body.

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Consumption of CO2 Photosynthesis by aquatic plants and phytoplankton for production of carbohydrates. Carbon dioxide is present in three forms bound CO3- -, half bound HCO3 and free state CO2. When CO2 comes in contact with water, it produce carbonic acid H2O + CO2 = H2CO3 which displays its weak acidic character through dissociation H2CO3 = H+ + HCO3- + HCO3- Just before day-break, concentration of CO2 is highest and water is therefore, most acidic. Pond which is on calcareous soil contains free CaCO3. This CaCO3 is helpful to prevent the water pH to fall below 5.0 according to the following reaction. CaCO3 + CO2 + H2O

Ca(HCO3)2

Ca(HCO3)2 is far less acidic than H2CO3, when the pond soil does not contain any free CaCO3, lime should be applied. Lime Corrects acidity - forward reaction. Ca of Lime acts as buffer Reserve of CO2 The solution of Ca(HCO3)2 remains stable only in the presence of certain surplus amount of CO2. Therefore, the CO2 which is necessary to retain the calcium in solution in the form of Ca (HCO3)2 is called equilibrium or free CO2. 2 to 10 ppm of free CO2 is ideal for good productivity of pond. 20 – 30 ppm of CO2 can be tolerated provided O2 is near to saturation. Above 30 ppm CO concentration cause depletion of O but air – breathing fish may survive at 100 ppm concentration.

Total Alkalinity: Since most of the organisms thrive and proliferate in alkaline waters, alkalinity is therefore, an important factor in pond productivity. The total alkalinity of water is mainly caused by cations of Ca, Mg, Na, K, NH4 and Fe combined either as carbonates and or bicarbonates or occasionally as hydroxides. Hydroxides alkalinity generally occurs in polluted water (pH > 11). In other waters it is occasionally encountered during mid afternoon is surface layers in waters showing intense photosynthesis. A mixture of bicarbonate and carbonate alkalinity is generally encountered is waters of pH ranging from 8.4 to 10.5. At pH values lies than 8.3 but more than 4.5, partially no carbonate is present, but free CO2 and bicarbonates may be present. 10

H2CO3 → HCO3- + H+ HCO3- + H2O → H2CO3 + OHCO3- - + 2H2O → H2CO3 + 2OHTotal Alkalinity

Pond Qualities

Below 60 mg/lit

→

unproductive

60 – 100 mg/lit

→

Average Productivity

100 – 250 mg/lit

→

Highly Productive

But a range of 4 to over 1000 ppm has been encountered in natural bodies of water. Water of hilly stream, sandy, rocky or very clayey areas, flooded rivers in rainy season, water in heavy rain fall areas and water infested with submerged weeds usually have low total alkalinity values. On the other hand stagnant waters is tropical plains in low rainfall areas during the summer season are likely to have high total alkalinity value

Total Hardness: Both total alkalinity and hardness of water is expressed in terms of ppm or mg/l of CaCO3 but both are not same. Hardness is the total soluble Ca and Mg Salts (in same cases Fe salts). It includes sulphates and chlorides along with CO3- , HCO3- and OH- salts. In most natural matters, the predominant ions are those of bicarbonates, associated mainly with calcium to a lesser degree with Mg, sulphates and chlorides of Ca and Mg predominate in waters contaminated with ocean salts or from dry land areas. Hardness may be temporary caused by soluble Ca and Mg bicarbonates, this is also called carbonates hardness or permanent caused by soluble Ca and Mg carbonates, sulphates and chlorides. Temporary hardness can be removed by boiling while permanent hardness cannot be removed by boiling. Name

Total hardness Level (Mg/ L CaCO3)

Soft Water

→

< 125 – Toxicity of pollutant is high.

Medium Hard H2O

→

125 – 250

Hard Water

→

250 - 375

Very hard Water

→

> 375 – cause osmoregulatory stress to fish.

Hardness of Water < 5 ppm

Good for aquaculture

Significance

Cause slow growth, distress and eventual death of fish. Such ponds required liming.

< 12 ppm

Require liming for higher production 11

≥ 15 ppm

Satisfactory for growth of fish and don’t require addition of lime.

Dissolved Solids: The total concentration of dissolved solids (both inorganic and organic) in a water body is a useful parameter in deserving chemical density as a fitness factor and as a general measure of edaphic relationship that contributes to the productivity of the water. Electrical conductivity, which gives the total amount of ionized materials is an important measure of total dissolved solids present in water and is usually expressed as micromhos. Electrical conductivity above 400 Mmhos does not limit productivity but productivity does not increase proportionately with conductivity. Dissolved solids may be organic or inorganic. Inorganic dissolved solids are: metallic ions (eg. Ca, Mg, Na, K, Fe) in combination with anions like Cl-, SO4-2, CO3-2, HCO3-, OH-, PO4-3, NO3-, NO2- etc. and there may be trace elements like Ni, Co, Mn, Zn, Cu, Cr, Al, Silica, etc. Whereas, organic dissolved solids are: organic state of nitrogen, phosphorus and sugars, acids, fats, vitamin etc. Since, nitrogen and phosphorus are important in the field of aquaculture, it is explained briefly.

Nitrogen: Nitrogen is available to plants in three forms Nitrate, Nitrite and Ammonium. i) Free ammonia NH3 (the fourth form of nitrogen) may be harmful to fishes if it is above 2.5 mg/lit of water. It denotes that pond bottom has become foul due to excessive decomposition of anaerobic nature. The unionised (NH3) form of ammonia exist in equilibrium with the ammonium ion in water as per the following reaction NH3 + H2O = NH3.nH2O =NH4OH +(n-1)H2O. NH3 is also excreted through gills epithelium by fishes and crustaceans. Nitrate is very much useful for growth of phytoplankton and vegetations in water. NH4 and NO3-N may be applied to pond from outside as fertilizers. Certain quantities of nitrogen maybe taken into soil and water from the atmospheric nitrogen by rain or lightning. There ware also soil bacteria (Rhizobium) that can fix atmospheric nitrogen in the roots of leguminous plants and then make it available to the phytobiota. However, ponds derive the major supply of nitrogen from the putrefaction of organic nitrogen. Organic nitrogen forms about 50% or more of the total soluble nitrogen in surface waters of lakes and ponds. Decomposition of this organic nitrogen by anaerobic bacteria produces NH4-N, 12

the process is called ammonification. Aerobic decomposition of NH4-N by two groups of nitrifying bacteria produces NO3-N via two steps. In the first step, nitrosomonas produces NO2-N, the NO3-N is produced by nitrobacter from NO2-N in second step. The immediate conversion of NO2-N to NO3-N is beneficial as NO2- is toxic to aquatic life. Nitrate (NO3-) is very much useful for growth of phytoplankton and vegetations in water. NH4 and NO3N may be applied to pond from outside as fertilizers. Both NH4- and NO3-N are taken up by aquatic flora and fauna, which is the major source of organic nitrogen in pond. The NO3-N is again transformed to elemental nitrogen via oxides of nitrogen (NO, NO2-, N2O etc.) by denitrifying bacteria (e.g. Pseudomonas).

Free Nitrogen in Air Denitrification by

Nitrogen fixation by BGA and Azotobacter

denitrifying bacteria Oxides of Nitrogen

Organic Nitrogen in Pond

NO2, NO, N2O

Fauna

Anaeorobic decomposition

Flora

by bacteria and fungi NO3

NH4

Nitrosomonas

Nitrobacter

NO2 Aerobic decomposition by nitrifying bacteria (Nitrification)

Fig. 2: Nitrogen Cycle in ponds and lakes.

The process of tying up nitrogen in organic form from simple elements or inorganic form is called immobilization, its slow release especially conversion from organic to inorganic or elemental form is called mineralization. Nitrogen is a very important element in pond fertility. Presence of available nitrogen @ 0.2 – 0.5 mg/lit of water is good for production NH4 – N :

0.2 – 0.5 mg/lit

NH3 – N :

0.02 – 0.2 mg/lit 13

NO2 – N :

<0.014 mg/lit

1 + − [O] 2H [O] [O] NH 4 → HONH 2 −→  HONNOH → NO 2 → H + + Energy 2 −

+ NO2  → NO3 + Nitrate

+

Energy

+

−2

[O]  → 2NO 3 → 2NO 2

−

2[O] [O] − → 2NO  → N 2O −  → N2

Nitric Oxides

Nitrous Oxides

Elemental Nitrogen

PHOSPHORUS:

Phosphorus is recognised to the most critical single factor in the maintenance of pond fertility. It occurs in three forms. 1. The soluble inorganic phosphate phosphorus (PO4) 2. Soluble organic phosphorus and 3.The particulate organic phosphorus occurring in plankton, detritus and sedimentation. Out of these three forms, form PO4 i.e. soluble inorganic phosphate phosphorus or dissolved phosphorus takes part in production. It is required for cell division, preparation of fat, protein, high energy compounds (ATP, ADP, AMP) etc in the body. If pond soil is acidic, phosphorus become unavailable and stays in compound form with Fe, Al, Mn, Zn, For this reason, phosphate fertilizers are applied with lime in acidic soil. But, if the condition is highly alkaline, the phosphate again remains in pond soil as a compound form with Ca and Mg. Availability of phosphorus is highest near neutral pH. Sources of phosphorus in natural water: (i) Weathering of phosphorus bearing rocks (apetite) (ii) Leaching of soils of the catchment area by rain (iii) (Cattle drop, night soil) organic manure and inorganic fertilizers (SSP, Nitrophosphate, DAP) added to the pond. Dissolved phosphate :<0.05 ppm – unproductive pond 0.05 – 0.20 ppm – medium to high productivity. Lack of phosphorus is often the chief cause of poor productivity of water. An amount of 0.2 to 0.4 mg/lit of phosphorus. Phosphorus pentaoxide is good for production in pond water. Excess of phosphate in open waters is a sign of heavy organic pollution.

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Phytoplankton

5

4

2 1

Soluble

Littoral Veg

3 7

6 Sediments

Fig 3. Phosphorus Cycle in Pond

The various phosphorus cycle involve following processes : Liberation of phosphorus into epilimnion due to decay of littoral vegetation by microbial degradation; Update of phosphorus by phytoplankton; Update of phosphorus from water by littoral vegetation, during the periods of their rapid growth. Loss of phosphorus as a soluble compound. Less assimilable then ionic phosphate, from phytoplankton, followed by show regeneration of ionic phosphate; Sedimentation of phytoplankton and other phosphorus – containing seston, perhaps largely faecal pellets, into the hypolimnion; Liberation of phosphorus from the sedimenting seston in the hypolimnion when it reaches the mud – water interface; Diffusion of phosphorus from the sediments into the water at those depths at which the superficial layer of mud lakes on oxidized microzone – like many other ions in presence of O2, phosphorus ions are absorbed on colloidal ferric hydroxide in the oxidized microzone of the bottom of mud, and in absence of O2, the ferric ion is reduced to ferrous from and phosphorus in a soluble form is released in the water, thus indicating that the presence or absence of oxygen in the critical factor for release of phosphorus. Even when it is released in water in soluble form, its availability will be determined by water pH and presence or absence of soluble Al, Fe, Mn and Ca and minerals containing these cations.

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PHYSIO – CHEMICAL PARAMETERS OF FISH POND PARAMETERS POND QUALITIES

WATER 1. p.H 2. 3.

D.O (Mg/lit) Free CO2 (Mg/lit)

4.

Total Alkalinity Temperature

5.

6. 7. 8.

9.

10. 11.

Colour Turbidity NH4 – N (Mg / Lit) NH3 – N (Mg/Lit) NO2 – N (Mg / Lit) Phosphorus (P2O5) (Mg/Lit) Hardness (ppm CaCO3) Transparency

SOIL 1. PH 2.

3.

4. 5. 6.

Available Nitrogen (Mg/100 gm soil) Available Phosphorus (Mg/100g) Free CaCO3 (%) Organic Carbon(%) Carbon Nitrogen Ratio

UNPRODUCTIVE

AVERAGE PRODUCTIVE

HIGHLY PRODUCTIVE

<6.5 >8.5 >3.0 (or nil)

6.5 – 7.5

7.5 – 8.5

3.0 – 6.0 Traces – 3.0

6-10 5.0 – 15.0

<60.0

60-100

100-250

OTHER REMARKS

20-30 ppm can be tolerated provided O2 is near saturation, >30 ppm cause depletion of O2 and mortality of fish, air

20-30 C for nursery pond & 20-35 C for stocking pond. Greenish 20 – 60 cm 0.2 – 0.5 0.02 – 0.2 <0.014 0.02-0.2 for nursery / rearing pond and stocking pond. 5

5 – 12

> 15 Optimum Range in production ponds is between 20 cm – 60 cm.

< 5.5 > 8.5 <25.0

5.5 – 6.5

6.5 – 7.5

25 – 50

50 – 75

<3.0

3.0 – 6.0

6.0 – 15.0

<1.0

1.0 – 2.0

2.0 – 5.0

< 1.0

1.0 – 2.0

2.0 – 5.0

< 0.5 <5

0.5 – 1.5 5 – 10

1.5 – 2.5 10 – 15

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ROLE OF SOIL PARAMETERS IN POND PRODUCTIVITY

A thin layer of soil covers most of the earth’s land surface. This layer, varying from a few centimeters to 2 or 3 meters thickness, might appear insignificant relative to the bulk of the earth. Yet it is in this thin layer of soil that the plant and animal kingdoms meet the mineral world and establish a dynamic relationship. Plants obtain water and essential nutrients from the soil. Animal depends on plants for their lives. Plant and animal residues find their way back to the soil and are decomposed by the teeming microbial population living there. Life is vital to soil and soil is vital to life. The soil is a very complex system. A given volume of soil is made up of solid liquid, and gaseous material. The solid phase may be mineral or organic. The mineral portion consists of particles of varying sizes, shapes, and chemical compositions. The organic fraction includes resides in different stages of decomposition as well as live active organisms. The liquid phase is the soil water, which fills part, or all of the open spaces between the solid particles and which vary in its chemical composition and the freedom with which it moves. Te gaseous or vapour phase occupies that part of the pore space between the soil particles that is not filled with water; its composition may change within short intervals of time. The chemical and physical relationships among the solid, liquid and gaseous phases are affected not only by the properties of each but also by temperature, pressure and light. The different types of soil found in Indian are alluvial soil, black (regur) soil, red soil, laterite soil, forest soil, desert soil, saline and alkaline soil, and peaty soil. Soil plays an important role in determining the fertility of fish ponds. The basic criterion for selection of a site for construction of ponds is that the soil should not be porous. The soil condition is an important environmental factor influencing water quality and controlling various production processes. Banerjea (1967) classified pond productivity into three categories – low, medium and high – based on status of available nitrogen and phosphorus and organic carbon as below: -

Pond Productivity

Available N

Available P2O5

Organic Carbon

(mg/100 g soil)

(mg / 100 g soil)

(%)

Low

Less than 25

Less than 3

Less than 0.5

Medium

25 – 30

3–6

0.5 – 1.5

High

Above 50

Above 6

Above 1.5

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Physical parameters of soil Pond mud

While water fertility contribute largely to the production of plankton, the pond bed releases nutrient material into the water and helps in fixation or chemical combination of such substances released in the pond itself or introduced from outside. Pond productivity is increased only when the pond mud is rich in nutrients (phosphorus, nitrogen, organic carbon etc.). The colloidal content of the soil especially of the muddy layer on the top, is of importance in its capacity to fix or chemically bind nutrient. The productive capacity of the pond bottom has to be preserved by alternative period of mud formation and mineralisation – the practice of regular draining of fish ponds. Clayey soil is most suitable for pond construction, as it has maximum water retentivity and can easily be compacted and made leak proof. However, well compacted loamy soil may also be used. Pond mud should be loose and well aerated. To achieve this, the pond mud is normally dried up after the harvesting of fish crop. Pond soil differs from field soil in many respects. Pond soil is water logged and gas phase is absent. Only the top 2 to 5 cm of soil is concerned with nutrient-ion exchange and below this, soil is unaerated and has negligible involvement in the production cycle. Pond having large catchment area, where from it receives dissolved nutrient and sedimentary particles by precipitated rain water. The sedimentation of organic matter on the pond modify its properties.Besides, production and decomposition of minute plant and annual organisations in pond also modify the properties of pond bottom to a great extent. A true pond mud (rich in nutrients) is made up of fine soil particles, which contains deposits of certain amount of organic matter derived from the bacterial breakdown of plant and animal material present in a water body. The broken – down organic matter may exist as humus (derived from acid and peaty soils) and it is a mixture of colloidal acids. Humus acid is made up of 32% of protein – like material in firm combination with about 68% of another complex containing no nitrogen and its acts as a weak base which results in high adsorption capacity.

Chemical parameters of soil

The major chemical factors of importance are: (a) pH (b) Nitrogen (c) Phosphorus (d) Organic carbon and C/N ratio 18

(e) Calcium and potassium A production pond soil should have a total and available quantity of raw material along with the requirements of the organisms as to the suitability of conditions of existence and supply of nutrients. (a) Hydrogen-ion-concentration( pH):

The pH of a soil is closely related to the relative amounts of acidic cations (H+ and Al+3) and bases on its cationic – exchange sites. The pH rise when the concentration of bases increases and drops when the concentrations of acidic cations increase. The Al+3 ion is much less common than H+ at pH values above 5 but becomes the dominant ion in extremely acid soils. The acidity of soil is formed by formation of H2S gas, methane and short chain fatty acids by the process of decomposition of organic matter due to absence of sufficient oxygen in the bottom of pond. So, the pond soil should be buffered naturally or else it may reduce the rate of bacterial action, which influences productivity. The pH of soil helps in transformation of soluble phosphates and controls the adsorption and release of ions of essential nutrients at soil-water interface. pH range

4.0-4.5 4.5-5.5 5.5-6.5 6.5-7.5 7.5-8.5 > 8.5

Soil Condition Highly acidic Moderately acidic Slightly acidic Near neutral Slightly alkaline Highly alkaline

Lime (CaCO3) dose/Kg/hac 1000 700 500 250 100-200 Nil

However, pH range of soil from 7.5-8.5 (slightly alkaline) has been considered favourable for fish ponds. (b) Nitrogen:-

About 99% of the combined nitrogen in the soil is contained in the organic matter (humus) in the form of amino acids, peptides and easily decomposed proteins. It may also be in the form of inorganic compounds such as NH4+ and NO3 which are utilized by green plants (phytoplanktons) Anaerobic organisms (bacteria) helps in the decomposition of organic matter into simple inorganic forms forming products such as CO2, water and ammonia which influences directly or indirectly in pond productivity. The range of available nitrogen 50 – 75 mg/10 gm of soil is relatively more favourable for pond productivity. Though nitrogen are mostly available from organic matter, it can also be made available by fixing atmospheric nitrogen into organic nitrogen

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with the help of nitrogen – fixing bacteria present n the soil and water, blue green algae and some micro-organisms. (c) Phosphorus:

Phosphorus has been called “the key to life” because it is directly involved in most life processes. It is second only to nitrogen in frequency of use as a fertilizer element. One or both of these elements are nearly always included when a fertilizer is applied. Phosphorus occurs in the soil in both inorganic and organic forms. The inorganic phosphorus are calcium phosphate, aluminium phosphate, iron phosphate and reductant soluble phosphate whereas organic phosphorus may occur as phytin or phytin derivatives, nucleic acids and phospholipids. The organic form constitutes about 35 – 40% of the total phosphorus content of the soil. The availability of phosphorus is important to aquatic productivity owing to the fact that PO4 ions in soil form insoluble compounds with iron and aluminium under acidic conditions and with calcium under alkaline conditions, rendering the phosphorus ion unavailable to water body. Experiments show that alkaline soil adsorbs more phosphorus than acidic soil. However, phytoplankton helps in uptake of available phosphorus, which is stored for use in their cells, and as a result it helps in production of their population, which may directly or indirectly affect pond productivity.

Soil Phosphorus (P2O5) < 3 mg / 100 gm (30 ppm) 3 – 6 mg/ gm (30 – 60ppm) 6 – 12 gm (60 – 120 ppm) > 12 gm (120 ppm)

Pond Productive Poor Productivity Average Productive High Productive Poor Productive

(d) Organic carbon and C/N ratio: Organic compounds present in the soil exert a profound influence on almost every

facet of the nature of soil. Organic compounds are usually broken up by bacteria into inorganic compounds, which are consumed by phytoplanktons and are passed on to fish crop. Microbiologist believe that the bacterial activity depends both on the carbon content and the ratio of C/N in the parent organic substance. The significance of organic fertilizers lies in their carbohydrate content. The nitrogen fixation stops when carbohydrates are absent in the organic compound. The bacterial activity is low when C/N ratio falls below 10 : 1 and high when the ratio is 20 : 1 or higher.

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Organic Carbon Content

20.5 % 0.5 – 1.5 % 1.5 – 2.5 %

Pond Productivity

Law Productive Average Productive Highly Productive

Raw cowdung to be applied (Kg / Hac) 20,000 1000 – 10000 No Need.

On the other hand, when the C/N ration is <5, the pond shows poor production. Better production is found in the ratio 5 to 10 and 10 to 15 (ideal conditions). However, the ratio above 15 appears to be less favourable for pond production.

(e) Calcium and Potassium

Calcium is generally present in the form of CaCO3 (Calcium Carbonate). It helps in translocation of carbohydrates, acts as an integral component of plant tissue, increase the availability of other ions and reduces the toxic effect of single salt solution of other elements. It was however, noted that no marked influence of exchangeable calcium upon productivity could be noted. On the other hand, potassium also helps directly or indirectly in pond production though its optimal concentrations in soil are not known. It is taken up readily by submerged weeds for growth. During rapid plant growth period, potassium from the water and soil is stored in the tissues. Ponds with sandy and non-absorptive soils have poor potassium content and respond most markedly to fertilization.

CHEMISTRY OF POND MUD

When pond is flooded with water, the first effect of flooding is to drive out the air from the soil.Then the aquatic bacteria in the soil become active, decomposing the organic matter in the newly water logged soil and using up the oxygen. This lead to anaerobic conditions and the pond mud is in a reduced state and the flooded soil comes to contain carbon dioxide (CO2) but no oxygen (O2). Under such conditions, sulphates are reduced to sulphides (SO4 to S) nitrogenous substances to ammonia (NH3), Iron occurs in the reduced form (Fe2+) and some of the organic matter to methane (CH4). Because of ammonia, the soil becomes alkaline and because of presence of ferrous ion complex, the colour of the soil becomes a more or less intense blue-black. The water overlaying the mud becomes oxygenated partly because the water dissolve oxygen from the air and partly due to the oxygen (O2) release during the photosynthesis by the aquatic plants presents in the pond phytoplankton, which soon develops there. 21

This oxygen will oxidize the surface skin of the pond mud (1 to few mm thick only) and develop on oxidized microzone. Where ferrous iron (Fe2+) becomes ferric (Fe3+), sulphides (S) becomes sulphates (SO4) and ammonia (NH3) becomes nitrate and nitrite. Because of disappearance of ammonia (NH3) and the appearance of acid, this layer becomes acidic and the surface of the pond mud turned from blue – black to yellow to brown in colour due to the presence of ferric compounds (Fe). This phenomenon can almost always be seen when the mud of a pond is exposed, for example when it is drained to take the crop of fish, the foot prints of men working in the mud are deep blue or black, the undisturbed mud is yellow. But almost as one looks, the exposed black reduced soil takes up oxygen from the air and turns yellow. MECHANISM OF RELEASE OF NUTRIENTS :

The yellow ferric iron compound chiefly the hydroxide at the oxidized surface layer are usually in a very finely divided or colloidal state and this colloidal ferric hydroxide together with colloidal humic substances make a mud which has highly absorptive properties for both acid and basic radicals. As long as the iron compound on the surface layer of the mud were in ferric state, the surface was strongly adsorptive of positive ions, such as ammonia, calcium, manganese and of negative ions such as phosphates and silicates. But nitrate and nitrite were not adsorbed. During temporary cutting of oxygen from the surface layer of the mud (which may be caused by excessive respiration at night, or by lack of circulation of water or it may be, in deep ponds, a longer term phenomenon due to layering of water), the adsorbed ions are released into the water often in considerable quantities. The reduced iron has no power to hold, then diffuse up into the pond water and are taken up by plants and then by fish.

DRY PERIOD

During the water – logged period where the pond mud is under anaerobic conditions and alkaline in reaction, oxidation processes of organic matter cannot be completed and oxygen (O2) debt is built up of these partially oxidized products of fermentation. Exposure to air completes this oxidation after resulting in the release of carbon dioxide (CO2) making the soils slightly acidic. The completion of oxidation releases the contained nutrient materials by mineralization and acidic condition cause these materials to remain adsorbed in the soil, ready for release when the pond is refilled and the oxidation – reduction system sets itself up again. 22

The chief advantage of dry period is the restoration of the fertility of the pond. REDOX POTENTIAL:

When the oxidation taking place in surface of the mud and reduction at lower levels, the electrical charges on the molecule of electrolytes and ions in these two soil layers are responsible for differences in potential i.e. Redox potential. Reducing conditions prevails when potential is below +350 millivolts and above it, oxidising conditions occur. Therefore, the values of redox potential in the pond mud. Binding or releasing the ions of nutrient materials from pond mud.

FATE OF ADDED FERTILIZERS:

The finding and releasing by adsorption on the pond mud applies not only to the nutrients naturally occuring in the soil, but also to fertilizer added to the pond. The phosphate in the fish pond remained in the soil, adsorbed on the oxidising layer of the mud on colloidal ferric hydroxide and in absence of the oxygen (O2). Ferric ion is reduced to ferrous ion and phosphorus is released in soluble form in the water. Phosphate is also incorporated in the bodies of micro–organisms. These two factors account for the residual effect of phosphatic fertilizers and subsequent release under suitable condition even after 4 years of application. H2S TOXICITY

The hydrogen sulphide gas which is frequently detected in the mud during the construction of the ponds could poison the fish. But so long, as the surface layer of the mud is oxidizing, this very poisonous gas, deadly to fish, could not possibly diffuse into the water, for the sulphide could soon be oxidized to harmless sulphate. If a free circulation of the pond water is impeded, as for example by dense vegetation, then the smell of the gas appears. Clearance of vegetation will minimize the chance of H2S toxicity in this case. Due to all these fact described above the pond mud has been described as the “Chemical laboratory’ of pond.

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