Turbidity

• Refers to decreased ability of water to transmit light caused by suspended particulate matter in the water column of the pond ranging from colloidal to course.
• Transparency α1/Turbidity.

Causes of turbidity

1. Planktons: is generally desirable in fishponds.
2. Suspended particles of silts and clay: Is generally undesirable in fish ponds.
3. High concentrations of humus substances: May be desirable or undesirable in fish ponds.

Sources of turbidity

1. External sources:

• Surface runoffs, constructions, livestock, rainfall, dike erosion, etc.

2. Internal sources:

• Resuspension of particles
• Fecal material
• Feed, fertilizer, lime, etc.

Effects of turbidity

1. Reduced light penetration
2. Excessive turbidity may cause interfere gill function that cause respiration problem.
3. Cause siltation of bottom substrate- reduced pond depth.
4. Adsorbs and desorbs the nutrients of pond water.
5. Affects vision of fish.
6. Acidifies and sterilizes the water due to humus contents.

Measurement of turbidity

a. Secchi disk visibility method

• It is a weighted disk, 20 cm in diameter, and painted with alternate black and white quadrants.
• The average depth at which the disk disappears and reappears is the Secchi disk visibility.
• Desirable range for fish culture is 25-40 cm.

b. Suspended solids measurement method

• 1 liter of pond water is taken and filtered using glass fiber filter.
• The solid particles are dried, weighed and calculated in mg/L.
• Desirable range for fish culture is 25-80 mg/L.

c. Nephelometric method

• This method is ideal for more accurate measure.
• A nephelometer is an instrument that measures scattered light.
• A cuvette containing a sample of turbid water is placed in the nephelometer and a light source focused on the cuvette.
• The greater the turbidity of water sample, the greater the amount of scattered light, and larger the scale reading of the nephelometer.
• The unit is nephelometer turbidity unit ( NTU).