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19. Organic matter

Organic matter in water can be estimated by 3 methods:

  1. Determination of the low on ignition of dry residue after evaporation - a correction is made for CO2 loss form.

  2. Determination of the amount of an oxidizing agent such as potassium permagnate, required to oxidized the organic matter.

  3. Determination of organic carbon by combustion and organic nitrogen by kjeldahl method

All these methods though used are still imperfect. Organic matter derived from plankton has about 24% crude protein, with a C:N ratio of about 12:1. Organic matter derived from peaty materials or logs contain about 6% crude protein with a poorer C:N ratio of 45 – 50:1. Organic matter with higher N content, as indicated, is better for increasing productivity.

Nitrogenous organic matter - organic nitrogen accounts for 50 – 75% of total soluble nitrogen and half of this is in amino groups.

In fish ponds organic matter is present as living plankton, detri tus and dissolved organic matter. Besides the methods indicated organic matter in pond water can be estimated by measurement of biologica l oxygen demanded (BOD) and chemical oxygen demand (COD) (see Practical Handout for details). Normally BOD is increased by incubating a water sample with ample nutrients at 20°C for 5 hours - time and temperature can be changed eventhough these are the conventions. The amount of DO consumed during incubation period is considered the amount required to decompose the most reactive organic matter in the sample i.e. to stabilize the organic matter. Boyd (1982) is of opinion that standard BOD measures are meaningless for fish pond study because most of the BOD of pond water results from plankton respiration rather than from decomposition of organic waste. Since fish respond quickly to lowering of DO the hourly rate of O2 consumption is considered by Boyd as more meaningful than a standard BOD value. Boyd found that 6-hour BOD values of pond water correlated well with phytoplankton density.

COD represents oxygen requirements to oxidize all of the organic matter in a water sample to CO2 and water. Boyd (1982) observes that since there is good correlation between BOD and COD, BOD can be calculated from the less time consuming COD. For COD the sample is treated with sulfuric acid and potassium dichromate and digested for 2 hours. The O2 equivalent of dichromate consumed in oxidizing the organic matter is COD.

Boyd (1982) found a relationship between COD and hourly rate of oxygen consumption in pond waters:

O2 consumption in mg/l per hour =

1.006 - 0.00148 - 0.0000125 C2 + 0.0766 T - 0.00144 T2 + 0.0000253 CT

where C = COD in mg/l and T = °C.
(r = 0.92, COD range: 20 – 140 mg/l; T = 20 – 30°C)

Boyd also found that COD is highly correlated with chlorophyll a concentration (phytoplankton). Since plankton is the major causative factor for COD changes and in view plankton density varies with turbidity, Boyd (1982) also showed that the hourly rate O2 consumption in ponds is closely correlated with turbidity or Secchi disc visibility. More information on plankton will be given separately. This discussion however, shows how organic matter and plankton are important for studying the water characteristics and productivity in fish ponds.

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