DISSOLVED OXYGEN MASS BALANCE IN AQUACULTURE PONDS
SAMIR AHMAD ALI
The prediction of dissolved oxygen in aquaculture ponds throughout the year is essential to the design and evaluates the potential aquaculture sites. A computer model has been developed to simulate dissolved oxygen in a fish pond. A short-term Dissolved Oxygen (DO) fluctuation of a fishpond was developed by using various simple equations and continuous measurement of DO, temperature and solar intensity. Numerical computation has been performed for a typical winter (17th of January) and summer (17th of July) days.
Results from model verification runs showed that the model performance was satisfactory with respect to aquaculture pond dissolved oxygen. The relative percentage of error (RPE) for the 24 hours of simulation was 0.2818% and the correlation coefficient between predicted and measured dissolved oxygen was 0.97. The predicted dissolved oxygen was fluctuated between -0.101 to 0.113gO2m-3lower and higher than the measured dissolved oxygen for most of the 24 hour simulation.
The predicted results indicate that DO is affected by weather variables, especially solar radiation. The dissolved oxygen (DO) values ranged from 4.4 to 8.7 g m-3, where it reached the highest value (8.7) at 17:00 h, while it reached the lowest value (4.4) at 6:00 h.
The fish growth model results indicated that the total cycle time between the stocking and the harvesting is about 180-190 days during the summer months; compared with the total cycle time in natural setting is about 210-240 days.
Dissolved Oxygen (DO) is one of the most important factors affecting most aquaculture species. For fish culture, maintaining dissolved oxygen at a level suitable for fish survival and growth does pond management. When DO levels in aquaculture ponds become low, the cultured organisms may become stressed or even die. A healthy balance pond provides a fluctuation in oxygen levels between day and night that leaves an adequate concentration of oxygen in the
*Agric. Eng. Dept., Fac. Agric., Moshtohor, Toukh, Qalubia, P.O. Box, 13736, Egypt.
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water that can support aquatic animal life during both day and night hours. Phytoplankton can exert a profound effect on water quality constituents, especially dissolved oxygen, by producing supersaturated concentrations during the day and reduced levels during the night due to biotic respiration and chemical oxidants result in a net loss of oxygen which can reach critically low concentrations (Muhammetoglu and Soyupak, 2000). The highest oxygen levels in a pond are usually measured on sunny afternoon when phytoplankton and other aquatic plants are producing oxygen through photosynthesis. The lowest level occurs just before daybreak after a night of oxygen consumption by aquatic plants and animals. Dissolved oxygen consumption and regeneration by phytoplankton is directly related to their rates of photosynthesis and respiration.
The intensity of solar radiation strongly regulates rates of photosynthesis and oxygen evolution in fishponds (Romaire and Boyd, 1979). The rate of oxygen production is a function of the concentration of algae and other forcing functions. Because the growth of algae is light and temperature dependent, hence the rate of photosynthetic oxygen production follows the same pattern. Temperature is a parameter that shows a marked seasonal and daily variation in fishponds. It influences photosynthesis, growth of algae and bio decomposition of organic matter in the pond. Other factors such as density of fish, turbidity, organic matter levels and wind velocity also greatly influence dissolved oxygen budgets for fishponds (Boyd et al., 1978). In ponds showing marked stratification, surface waters may be harmful to fish due to supersaturated DO conditions in combination with high temperatures, while in the same pond near anoxic conditions may exist close to the bottom (Chang and Ouyang, 1988; Losordo, 1988; Boyd, 1990).
There are several reports of DO models incorporating mechanistic characterization of the chemical, physical and biological processes in an open pond which governs the resulting DO levels (Losordo, 1988; Losordo and Piedrahita, 1991). The intent of the present study describes to develop a dissolved oxygen model using input variables in low cost earthen fishpond. In this study additional modification have been implemented to the predictions of DO performance in earthen fishpond from calculation of solar radiation falling on the pond water surface.
This model simulates the hourly variation of DO in a fishpond over a 24 h period as influenced by the consumption and production of oxygen by phytoplankton and fish. Measurable rates of photosynthesis
and respiration are needed for proper calibration of the model. This model is neither site nor species specific and input variables can be adjusted to accommodate most pond conditions. In freshwater fishponds effects of solar radiation on temperature and oxygen variations have been described in detail (Boyd, 1979). The model was developed with the following objectives under consideration:
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