| Thermal and dissolved oxygen stratification in aquaculture ponds have been characterized and a computer model describing these processes has been developed. The characterizations were accomplished using data collected with an automated water quality data acquisition system that was developed for use in shallow aquaculture ponds. The self-contained microprocessor based system consisted of a micrologger, a weather station and a unique water sampling raft. The system was used to monitor and record the following information: (a) weather data including solar irradiance, photosynthetically active radiation (PAR), wind speed, wind direction, air temperature and relative humidity; (b) pond environmental data, taken at 6 depths to a maximum of 100 cm including dissolved oxygen concentration, temperature, pH, and underwater PAR.;A mathematical model to simulated thermal and dissolved oxygen stratification in shallow aquaculture ponds was developed. The dynamic, mechanistic model simulated the water column of ponds in discrete, completely mixed, horizontal volume elements. Energy exchanges between the pond's surface and atmosphere were calculated with theoretical and empirical relationships commonly applied to heat balance calculations in lakes, reservoirs and waste treatment ponds. Energy and mass transfer between the volume elements caused by turbulent mixing were simulated as functions of the temperature and concentration gradients in the water column and a diffusion coefficient. The value of the diffusion coefficient was calculated in each time step as a function of wind speed, depth, and water column density gradient. The model was implemented in a dynamic simulation language (STELLA;The system was deployed in five freshwater aquaculture production ponds at 3 sites in California and one pond in Hawaii. Thermal and water quality stratification and associated weather conditions were monitored continuously for a minimum of one week in each pond. Pond thermal and dissolved oxygen stratification were found to be diurnal phenomena. During hot and quiescent weather conditions, vertical thermal and dissolved oxygen gradients of up to 14... |
