NITROGEN BIOGEOCHEMISTRY OF AQUACULTURE PONDS

Nitrogen (N) biogeochemistry of aquaculture ponds is dominated by biol ogical transformations of N added to ponds in the form of inorganic or organic fertilizers and formulated feeds. Nitrogen application in exc ess of pond assimilatory capacity can lead to the deterioration of wat er quality through the accumulation of nitrogenous compounds (e.g., am monia and nitrite) with toxicity to fish or shrimp. Principal sources of ammonia include fish excretion and sediment flux derived from the m ineralization of organic matter and molecular diffusion from reduced s ediment, although cyanobacterial nitrogen fixation and atmospheric dep osition are occasionally important. Principal sinks for ammonia includ e phytoplankton uptake and nitrification. The magnitude of losses by a mmonia volatilization and ammonium fixation to cation exchange sites i s minor, but unknown. Interactions between pond sediment and water are important regulators of N biogeochemistry. Sediment represents a sour ce of ammonia and a sink for nitrite and nitrate. The large volume of reduced sediment suggests that the potential for N removal by denitrif ication is high, although the magnitude of N removal by this mechanism is low because nitrification and denitrification are tightly coupled in aquatic sediments and sediment nitrification is limited by the dept h of sediment oxygen penetration. Nitrogen biogeochemistry of aquacult ure ponds is affected by feeds and feeding practices, water exchange a nd circulation, aeration, pond depth and other management procedures. Opportunities for management of N biogeochemistry are limited and goal s are based largely on the intensity of fish production. (C) 1998 Publ ished by Elsevier Science B.V. All rights reserved.