Study on the unexpected sudden decrease and suppressed growth of the rotifer population in mass culture - I The effect of un-ionized ammonia on the population growth of the rotifer in mass culture

In mass culture tanks for the rotifer Brachionus plicatilis, the concentrations of inorganic. nitrogen compounds, pH, water temperature and specific gravity were measured daily at the Aqua-culture Research Laboratory of Nagasaki Prefectural Institute of Fisheries from April to July, 1984. The rotifer density decreased with an increased concentration of un-ionized ammonia nitrogen (NH3-N). The highest concentration of NH3-N observed was 3.6ppm. The rotifer densities above 150 rods./ml were observed, only when the pH ranged from 7.3-7.8. Acute toxicity test of NH3-N to rotifers showed that 24h LC50 value was 17.0ppm at 23°C. Chronic toxicity test showed that concentrations of NH3-N at which the population growth and the reproduction reduced by 50% of the controls were 13.2 and 7.8ppm, respectively. The concentrations at which the intrinsic rate of population growth and the net reproduction rate were reduced by l% of those of the control, were adopted as the “no-effect” concentration of NH3-N. They were estimated as 2.1 and 2.4ppm, respectively. The results from mass culture and toxicity tests showed that higher concentration of NH3-Nmight be one of restrictive factors affecting the increase of the rotifer.