Abstract
Infestations of rotifers ( Brachionus rubens) and cladocerans ( Diaphanosoma brachyurum) in a 0.1-ha high-rate algal pond were eliminated by temporarily raising the un-ionized (free) ammonia concentration to approximately 20 mg/l (as N) by addition of ammonium hydroxide solution. The 24-h LD 100 for Brachionus was found to be 17 mg/l free ammonia-N and that for Diaphanosoma just under 20 mg/l. Ammonium hydroxide treatments did not inhibit microalgal populations but rather promoted rapid recovery of algal densities to pre-infestation levels. Manipulation of free ammonia concentrations provides an economical and effective means of zooplankton control in mass algal cultures.
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