A SIMULATION-MODEL OF AMMONIA DYNAMICS IN COMMERCIAL CATFISH PONDS INTHE SOUTHEASTERN UNITED-STATES

A mechanistic model was developed to simulate annual variation of ammo nia concentration in commercial (levee-type) catfish ponds in the sout heastern United States. A simple mass balance approach was used to des cribe ammonia concentration as the balance (residual) between nitrogen sources and sinks. Two primary source processes (fish excretion and s ediment diffusion) and two primary sink processes (phytoplankton uptak e and nitrification) were considered. Model output predicted that 25-3 3% of ammonia production was derived from sediment diffusion. The rate of phytoplankton uptake of N was expected to exceed nitrification dur ing the growing season (April-October), whereas nitrification rate wou ld exceed phytoplankton uptake during cooler months. Nitrification rat e was bimodally distributed, with peaks in spring and fall, and was re lated to the interaction between ammonia concentration and water tempe rature. Sensitivity analysis of model parameters indicated that averag e annual ammonia concentration was most sensitive to the partition of nitrogenous excretion between solid and dissolved fractions, average f eeding rate, and phytoplankton specific uptake rate. Average annual am monia concentration was relatively Insensitive to changes in feed conv ersion. Model output and results of the sensitivity analysis provide i nsight into the magnitude and relative importance of the processes aff ecting ammonia transformations in commercial catfish ponds and offer p ossibilities for the most effective management intervention. (C) 1997 Elsevier Science B.V. Science B.V.