A simulation model of ammonia dynamics in commerical catfish ponds in the southeastern United States

Abstract

A mechanistic model was developed to simulate annual variation of ammonia concentration in commercial (levee-type) catfish ponds in the southeastern United States. A simple mass balance approach was used to describe ammonia concentration as the balance (residual) between nitrogen sources and sinks. Two primary source processes (fish excretion and sediment diffusion) and two primary sink processes (phytoplankton uptake and nitrification) were considered. Model output predicted that 25–33% of ammonia production was derived from sediment diffusion. The rate of phytoplankton uptake of N was expected to exceed nitrification during the growing season (April-October), whereas nitrification rate would exceed phytoplankton uptake during cooler months. Nitrification rate was bimodally distributed, with peaks in spring and fall, and was related to the interaction between ammonia concentration and water temperature. Sensitivity analysis of model parameters indicated that average annual ammonia concentration was most sensitive to the partition of nitrogenous excretion between solid and dissolved fractions, average feeding rate, and phytoplankton specific uptake rate. Average annual ammonia concentration was relatively insensitive to changes in feed conversion. Model output and results of the sensitivity analysis provide insight into the magnitude and relative importance of the processes affecting ammonia transformations in commercial catfish ponds and offer possibilities for the most effective management intervention.