Not AvailableSeasonal changes in abundance of nitrifiers (ammonia-oxidizing and nitrite-oxidizing bacteria) in surface and
bottom water of freshwater ponds were examined with respect to temperature, DO, pH as well as concentration of ammonia and nitrite. The most probable number (MPN) of ammonia-oxidizers in different ponds varied from1297±3.6 to 1673.23±0.36 ml-l in bottom and 720.5±8.1 to 955.3±10.8 ml-l in surface water during the rainy season while the MPN ranged from 1074±1.07 to 1372.17±4.6 ml-l in bottom and 515±10.1 to 678±11.8 ml-l in surface water in winter. However, the MPN were greatly reduced in summer and ranged from 435.05±15.7 to 547.54±2.12 ml-l in bottom and 218.7±7.3 to 368.4±9.32 ml-l in surface water. Similar seasonal trends were also observed in MPN of nitrite-oxidizers. Among all the physico-chemical parameters, abundance of nitrifiers was more positively correlated with ammonia and nitrite concentration in all the seasons. The abundance of nitrifiers in surface and bottom water was highest in rainy season followed by winter and modest in summer. The potential nitrification activities and oxidation rates were shown to be linear and activity of ammonia-oxidizing and nitriteoxidizing bacteria was highest during rainy season.National Bureau of AgriculturallyImportant Micro-organisms (ICAR), Mau (India
