Introduction of Chemostat System into Ecological Studies on Zooplankters

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1. Last-year experiments made clear that those ecological numerals formerly obtained through batch-culture experiments had less reliability because of the delicacy of Brachionus plicatilis in the response on both feedind and growth rates afterencountered with food deficiency. Accordingly, it is nonsence to substantiate ourresults from a chemostat system with those "classic" numerals.The stable isotope (^<15>N) which enables very fine evaluation of metabolism was introduced for reconfirmation of last-year experiments. Digestivity, gross growth rate and net growth rate were 22.8,17.4 and 77.5%, respectively, when food was provided sufficient-ly. Those values mean that the rotifer extruded nearly 80% of grazed nitrogen as feaces, and 20% was assimilated. And more, as to the assimilated nitrogen, 20% was utilized for both growth and reproduction immediately, and 80% was excreted. These coincide with results from a chemostat system.2. Two chemostat systems were serially connected in order that the medium drawn from the front chemostat was introduced into the rear one after particles over 40mum was removed. Any suppressed growth was observed in the rear chemostat, in spite of two fold load of DON and NH_4. This suggests that the suppressed growth in a mass-rearing pond iscaused by high PON deriving from the more feeding relative to growing density.3. Concerning mass culture of the rotifer, the most effective way to prevent sudden mass mortality or suppressed growth is to remove PON which consists large part of organic load through adsorption or to renew the culture itself before the organic load reaches maximum.