Studies in circulated fish ponds: organic matter recycling and nitrogen transformation

Abstract. Three types of circulated systems slocked with tilapia were studied: tanks: miniponds; and ponds. Water was continually circulated and aerated. Daily drainage of non-suspended material was applied in most systems. The continual circulation and resulting resuspension of the organic particular ma Her led to a very high rate of microbial activity. Organic carbon was efficiently metabolized and utilized in the food chain. Inorganic nitrogen was completely oxidized and accumulated in water as nitrates, unlike conventional ponds. The establish men! of an active nitrifying population took, however, a period of about 3 weeks during which high levels of ammonium and nitrite were built up. The water body was continually aerated, yet local anaerobic conditions developed, especially in non-drained tanks or in miniponds where the plastic bottom was covered by a soil layer. Such conditions led to a less efficient organic carbon metabolism, to reduced nitrification and to denitrifcation. Fish growth seemed to be retarded in systems where denitrifcation took place, indicating that the growth-retarding factor is associated with production of anaerobic metabolites in the pond. The role of daily drainage and a proper aeration system is to avoid formation of anaerobic pockets.