MEASUREMENTS OF THE NITROGEN BUDGET IN THE ROTIFER BRACHIONUS-PLICATILIS BY USING N-15

The nitrogen budget of the rotifer, Brachionus plicatilis, fed on alga e (Nannochloropsis oculate) was examined using a N-15 tracer technique . It was found that this technique could be used to measure the nitrog en budget of zooplankton using the difference in the turnover time bet ween egestion and excretion. When food was unlimited, the assimilation efficiency, the gross growth efficiency (K-1), and the net growth eff iciency (K-2) were 22.8, 17.4 and 77.5% respectively. Therefore, appro ximately 80% of the ingested nitrogen was egested into the culture med ium and about 80% of the assimilated nitrogen was utilized for reprodu ction. The turnover time of egestion and excretion were about 20 min a nd 2.5 h, respectively. The obtained short turnover time for excretion is consistent with the hypothesis that assimilated nitrogen is used i mmediately and primarily for reproduction and the excess is stored in body tissue when food is abundant.