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4. Light turbidity and Colour

The spectral qualities of light (electromagnetic rediation spectrum) are shown in Fig. 8.5, in which the “visible” range is indicated. Originally the term light perhaps was restricted to this visible portion of the radiation spectrum only, but now the use of the term is broadned. The visible light extends from about 4000 to 7000 angstrom (1 angstrom (Ao) = 10-8cm).

Illumination of a surface such as that of a fish pond or tank is the luminous flux which it receives per unit area. Lux (1 lumen per m2) is the common unit used. EIFAC (1986) while reviewing the terminology and measurements used in flow/through and recirculation systems in aquaculture recommends that Lux be indicated in terms of radiant power rather than as lumen, the latter is evaluated in terms of its action on a selective receptor. The irradiance of a surface can be given as the radiant power:

  1. per unit area - Watts per m2 - (Wm-2)
  2. per unit area per second - Einstein per m2 per second (E.m-2.S-1)

Conversion of lux:
1 lux = 0.0027 Wm -2
        = 0.0125 MEM -2 -1

>Fig. 8.5

Fig. 8.5. Spectral radiation of radiant energy (after Hutchinson, 1957)

EIFAC (1986) recommends that both these specifications should be given when light intensities are indicated, especially in cases where controlled systems are concerned, along with the manufacturer's specifications for Watt measuring instrument and light source (i.e. wave length).

As already referred to, the light energy entering water depends on the geographical location of the water body. Both the intensity of light, which again depends on the angle of contact of light with surface and also the effective cover (cloud) or shade over the water body, and the duration light (photoperiod), are important to biological communities in water. These effects will not be referred to here except to mention that photosynthesis, the primary production mechanism, depends on light entering fish ponds and control of light falling on that is sometimes resorted to by provision of shades etc. for reducing algal bloom and weed growth. Light penetration is dependent on the turbidity of water, which we shall be referring to again.

4.1 Light Penetration in Water/Fish Ponds:

Of the total light falling on water a portion is reflected, which depends on the roughness of water surface and the angle of the radiation. The smoother the surface and closer the angle of radiation to the vertical the greater is the radiation penetrating the surface. The spectral quality and intensity of light changes as it enters deeper waters. In pure water 53% of incident light is lost as heat (quenching - light extinction) within the first metre of water. Light penetration is further decreased by impurities in water. Light penetration is further decreased by impurities in water (see also turbidity).

The upper water mass receiving a minimum of 1% of incident light and over is referred to as euphotic zone - in zones where light intensity is less than 1% of incident light photosynthesis cannot proceed at rates greater than respiration. In fish ponds usually the euphotic zone will be less than 1 metre due to the density of plankton. Boyd (1979) gives details of light penetration in Auburn fish ponds, where at 0.5m depth the incident light had “quenched” close to 1% (Fig. 8.6 Almazan (1977) points out that the depth of Secchi disc visibility multiplied by 2 gives an estimate of the depth of euphotic zone in Alabama fish ponds. Secchi disc visibility (see below) can also be used to estimate the extinction coefficient (K) of light penetration (Lambert's law equation);

K = 1.7/ZsD, where

ZsD is Secchi disc visibility in metres

Fig. 8.6

Fig. 8.6. Penetration of light in two separate fertilized fish ponds (after Boyd, 1979)

4.2 Turbidity:

We have referred to this briefly earlier. Turbidity is a condition of water resulting from the presence of suspended matters. Since there is suspended matter in all natural waters all are ‘turbid’; only the extent of turbidity is different. Turbidity producing substances may be devided into (1) Settling suspended matters and (2) Non-settling suspended matters. Settling suspended matters are those substances which in motionless water would sooner or later settle. This settlement time is related inversely to diameter of the particle. Coarse sand particles 1mm in diameter settle by 30cm in 3 seconds, while silt of 0.01mm and clay of 0.0001mm take 33 minutes and 230 days respectively.

The non-settling suspended matters consist of planktonic organisms and coarsely devided non-living substances whose specific gravity is less than that of water and exceedingly finely devided non-living materials and minute organisms such as nannoplankton.

Turbidity due to profusion of plankton is an indication of high fertility of a fish pond, but that caused by silt or mud is harmful to fish and fishfood organisms for they (silt) effectively reduce photosynthetic activity and nutrients are lost by adsorption to suspended particles.

A metal disc (20 cm in diameter) painted in black and white (Fig. 8.7), originally used by A. Secchi of Rome and hence known as Secchi disc, is lowered in the water to study the extent of light penetration. The level of induced bloom of dense plankton growth can be measured from the extent of light penetration.

Fig. 8.7

Fig. 8.7. Secchi disc, a metal painted black and white as shown, attached to a graduated chain for lowering in the water

There is a direct relation between the turbidity values obtained by using a Secchi disc and the concentration of particulate organic matter in fish pond water (“see organic matter” under “chemical features of water” in this manual), as indicated in Fig. 8.8. This in turn is also correlated with plankton counts and also chlorophll ‘a’ concentration in fish pond water. This will be referred to again.

Turbidity caused by settleable organic matter can also be obtained by using the Imhoff cone (Fig. 8.5.), where the sampled water is left to settle (Boyd, 1979). Settleable solids have been defined as the volume or weight (preferably dry) of solid material which will settle in one hour to the bottom of an Imhoff cone containing a measured volume of water (EIFAC, 1986).

Turbidity or light penetration can also be measured using a nephelometer. A detachable nephelometer component which can be used along with a field model spectrophotometer (e.g. Bausch and Lomb - Spectronic Mini-20) is ideal for field use (see also Water Quality - Practicals handouts). Monitoring of turbidity for evaluating the water quality is important for testing a water body and also for routine management in aquaculture. In intense culture systems (e.g. recirculation) it is recommended that suspended solids level should not go above 15 mg/l (dry weight) (Wickins, 1981).

During heavy floods in Rivers the turbidity could rise to 5,000 ppm (Jhingran, 1975). Some warm water fishes tested showed behavioural reaction to turbidity only when turbidity was increased to 20,000 ppm (as S.O2). Most tolerated turbidities of 100,000 ppm for about a week, but succumbed to 175,000 – 225,000 ppm turbidity due to gill clogging (Wallen, 1951). Nikolsky (1963) observed pronounced mechanical effects of turbidity in fish when exposed to 4% by volume of solid particles (i.e. roughly 400,000 ppm). Fishes living in turbed waters have adaptations such as reduced eyes and mucus secreted by their skin can rapidly sediment suspended particles. One drop of mucus of the fish Pisodonophis boro to ½ litre of turbid water cleans the water in 20 seconds. The coagulating property of the mucus protects their gills from clogging and the fish is always surrounded by an envelope of clear water.

Fig. 8.8Fig. 8.9
Fig. 8.8. Relationship of Secchi disc visibility and particulate organic matter in fish pond water (after Almazan & Boyd, 1978)Fig. 8.9. Imhoff cone used for measuring settleable solids in water

4.3 Colour

Colour of water can be due to hues inherent within the water itself resulting from colloidal substances and substances in solution. The term ‘true colour’ is used to designate colours of such causes. The colour of water caused by living or non-living substances in suspension and by extrinsic conditions, such as sky or colour of shallow substratum, is referred to as apparent colour. We have special interest in knowing the apparent colour of water caused by living and non-living substances in suspension.

As fish culturists we have to be familiar with ‘greening’ of water due to algal bloom and the reddish-brown colour of blooms of dinoflagellates often causing mortality to fishes.

Other apparent colours are:

Yellow - due to clayey turbidity

- also water overlying a clean sandy area

Bright-green - water over algae-covered depression

Brown - abundance of diatoms

Much of the colour due to suspended matter will be eliminated by filtration. Iron, as ferrous sulphate or as ferric oxide produces shades of yellow; humic matters produce varying colours iron-blue, green, yellow and brown. Colour can be measured by using colour standards (Platinum-cobalt standard - solutions)

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