Impact of farming intensity and water management on nitrogen dynamics in intensive pond culture: a mathematical model applied to Thai commercial shrimp farms

A mathematical model is used to investigate the impact of farming intensity 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 model is calibrated for two commercial shrimp (Penaeus monodon Fabricius) farms in Thailand. Assimilation by phytoplankton with subsequent sedimentation or discharge is the principal process of ammonia removal. When inputs of ammonia exceed the algal assimilation capacity (carrying capacity), nitrification and volatilization of excess ammonia become significant. Carrying capacity is negatively affected by non-chlorophyll turbidity, and was estimated as 6 t ha^?1 cycle^?1 at a non-chlorophyll extinction of 2.6 m^?1. In ponds managed within their carrying capacity, ammonia concentrations are lowest at no water exchange, reach 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 exchange. Average exchange rates used in intensive shrimp farms (up to 0.2 day^?1) reduce phytoplankton abundance and sedimentation within ponds, but not ammonia concentrations. Discharges are high in particulate nitrogen at water exchange rates up to 0.3 day^?1, but contain mainly dissolved nitrogen at higher rates.