IMPACT OF FARMING INTENSITY AND WATER MANAGEMENT ON NITROGEN DYNAMICSIN INTENSIVE POND CULTURE - A MATHEMATICAL-MODEL APPLIED TO THAI COMMERCIAL SHRIMP FARMS

A mathematical model is used to investigate the impact of farming inte nsity and water management on nitrogen dynamics in the water column of intensive aquaculture ponds. The model describes the input of ammonia , its assimilation by phytoplankton or nitrification, and the loss of nitrogen through sedimentation, volatilization, and discharge. The mod el is calibrated for two commercial shrimp (Penaeus monodon Fabricius) farms in Thailand. Assimilation by phytoplankton with subsequent sedi mentation or discharge is the principal process of ammonia removal. Wh en inputs of ammonia exceed the algal assimilation capacity (carrying capacity), nitrification and volatilization of excess ammonia become s ignificant, Carrying capacity is negatively affected by non-chlorophyl l turbidity, and was estimated as 6 t ha(-1) cycle(-1) at a non-chloro phyll extinction of 2.6 m(-1). In ponds managed within their carrying capacity, ammonia concentrations are lowest at no water exchange, reac h a maximum at exchange rates between 0.2 and 0.4 day(-1), and decline again at higher rates. When the carrying capacity is exceeded, excess ammonia concentrations decline continuously With increasing water exc hange. Average exchange rates used in intensive shrimp farms (up to 0. 2 day(-1)) reduce phytoplankton abundance and sedimentation within pon ds, but not ammonia concentrations, Discharges are high in particulate nitrogen at water exchange rates up to 0.3 day(-1), but contain mainl y dissolved nitrogen at higher rates.