The measurement of the nitrogen and phosphorous nutrients in the sediment is normally done after drying the mud as in the past. These methods bear no relevance to the real quantification of the basic production processes in the fish pond and they do not give a true representation of mineral cycling. Therefore, measurement of pH and redox gradient profiles at the sediment water interface has been done in situ. Direct measurement of the nitrogen and phosphorous nutrients has been done from the interstitial water freshly collected by the squeezing apparatus fabricated at FARTC. Further, the indofenol blue spectrophotometric method for measurement of ammonia, the cadmium reduction method for nitrite and the ascorbic acid method for phosphate have been followed instead of the old methods of determination which were slow, expensive and insensitive.
There are several methods of measuring or estimating the daily food production in fish ponds. The less tedious, most reliable and informative approach is the measurement of the primary production, which also has many disadvantages. Therefore, this parameter was quantified by use of the diel oxygen curve method. This approach enables analysis of the whole community metabolism or the whole oxygen budget of fish ponds. Besides primary production estimation at the diel curve, this analysis offers an excellent possibility to predict the oxygen deficient periods so often occurring in subtropical fish ponds and resulting in complete fish kill. The classical Winberg-Odum diel curve analysis and all the models formulated so far, especially for hypertrophic systems like fish pond ecosystems, could not provide satisfactory results for the total oxygen metabolism. Thus a new diel model was developed with the following principles:
Oxygen production is linear with the global radiation;
Oxygen consumption (dark) is linear with oxygen concentration;
Photorespiration is linear with a factor of global radiation and the actual oxygen concentration dependent proportion factor;
Diffusion at the air interface is linear with the saturation deficit of the surface water.
The community metabolism estimation model is suitable for computer analysis:
| where | t | = time |
| O2 | = average oxygen concentration of water column g m-3 | |
| q(t) | = global radiation on a unit surface KJ cm-2 | |
| h(t) | = saturation deficit of water surface in situ | |
| OTS | = saturation concentration of water as related to oxygen atomosphere, at a given temperature, g m-3 | |
| F | = production constant | |
| K | = respiration constant | |
| L | = photorespiration constant | |
| D | = diffusion constant |
This computer-adapted estimation model gives a detailed description of the oxygen producing and consuming processes in the fish pond and offers a wide range of further application to studies on the production processes in the drainable and undrainable fish ponds with varying degrees of management. In addition to this model, the simpler MacConnel model was also introduced at FARTC for a routine evaluation of the diel oxygen curve. For this simple model, three oxygen measurements were made at dusk, dawn and dusk. Normally, 1 mg oxygen increase in the water indicates about 50 kg ha-1 daily production of the primary organic matter.
