Heterotrophic bacterial populations and dehydrogenase activity in fish ponds under different fertilisation practices

Abstract

TThe ranges of aerobic heterotrophic bacterial populations in fish culture ponds (0.04 ha) under four manurial treatments, (1) cow manure at 10 t ha-1 yr-1; urea at 100 kg N ha-1 yr-1 and single super phosphate at 50 kg P ha-1 yr-1, (2) biogas slurry at 15 t ha-1 yr-1 and inorganic fertilisers, (3) biogas slurry at 30 t ha-1 yr-1, and (4) biogas slurry at 30 t ha-1 yr-1, with supplementary feeds assessed over a period of 15 months, were 0.28-4.44,0.30-3.86, 0.48-3.81 and 0.70-4.38 x103 ml-1 with means of 1.49,1.43,1.94 and 2.08 x103 ml-1. The sediment heterotrophic bacterial counts presented a similar pattern of variation with ranges of 1.20-6.71, 1.29-4.13, 2.04-9.92 and 2.66-11.07 x105 g-1 and mean values of 3.28, 2.62, 4.87 and 5.08 x105 g-1 in the four treatments. Water temperature and water depth were in the ranges of 20.8-30.2 °C and 0.8 to 1.8 m. The ranges of different water quality parameters were pH, D.O, NH4-N, NO2-N, NO3-N, PO4-P and dissolved organic matter were 7.4-8.5, 3.5-6.1 mg l-1, 3.25-22.66, 4.63-28.95, 10.04-48.98 mg-at N l-1, 0.03-0.54 and 2.1-9.5 mg l-1. The variations in the dehydrogenase activity of the surface sediment layers, indicative of the total bacterial activity were 0.06-0.46, 0.11-0.36, 0.19-0.87 and 0.02-0.94 mg triphenyl formazone per g sediment per day with the mean levels being 0.48, 0.41, 0.28 and 0.23 mg triphenyl formazone per g sediment per day. It is apparent that the activity was higher in treatments 3 and 4. This corroborates the observations on higher heterotrophic bacterial counts in the sediments of treatments 3 and 4, attributable to availability of processed substrate as compared to the ponds with application of raw cow manure. This also suggests better sediment-water interactions in slurry-applied ponds in comparison to traditional manuring practices.