The population size of SRB and TOB was enumerated at bimonthly intervals from water and sediment samples of six fish ponds (polyculture, monoculture and traditional systems) over a period of two and a half years. Density of SRB and TOB was maximal in polyculture and minimal in traditional systems of fish farming. Using sine and cosine waves of periodic functions or polynomial equations of high order, seasonal changes in the numbers of SRB and TOB were represented; the former occurred in largest numbers in summer while the latter in winter.Step wise multiple regression analysis revealed that organic carbon, organic matter, phosphate and different forms of N were important attributes to the seasonal changes of SRB and TOB in these fish ponds.
Treatment of single and double doses of fertilizers had a definite response on the bacterial population of water and bottom sediments in tanks after 7 days (F 2 , 2 1 > 3.61; P < 0.05), but did not cause any significant change after day 32 (F 2 ,1s8 1.67; P > 0.05).Although the generation time of water bacteria did not vary much in these treatments of trophic gradients (P > 0.05), there was always a distinct variation (P < o.ol) in the generation time of the bacterial population of bottom sediments. The population size of water bacteria was shown to maintain an inverse relationship with the diurnal variation of temperature (P < 0.05) and dissolved oxygen (P < 0.05) of the water. The changes in the size of the bacterial population of water were positively correlated with the variations of hardness (P < 0.05), chloride (P < 0.05) and ammonia nitrogen (P < 0.05) of water. Although the relationships between the heterotrophic bacterial population and the inorganic phosphate of water was not convincing (P > 0.05), an inverse relationship was established between the bacterial community of bottom sediments and the phosphate content of the water (P < 0.01).
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