Urea metabolism and its significance in the nitrogen cycle in the euphotic layer of Lake Biwa. 4. Regeneration of urea and ammonia
Mitamura, O.; Saijo, Y.
Archiv fur Hydrobiologie 107(4): 425-440
The regeneration rate of urea and ammonia from zooplankton excretion and microbial mineralization (microbial degradation in the case of urea) was measured in the in situ condition in the euphotic layer at two stations of Lake Biwa. The daily regeneration rate (sum of excretion and mineralization rate) for urea and ammonia was 0.88 and 5.60 .mu.g at. N .cntdot. l-1 .cntdot. day-1 in a eutrophic area, and 0.14 to 0.46 and 1.10 to 1.27 .mu.g-at. N .cntdot. l-1 .cntdot. day-1 in a mesotrophic area. The contribution of the excretion rate in the regeneration rate ranged from 38 to 64% for urea and 56 to 76% for ammonia. Urea excretion in the total excretion rate (sum of urea and ammonia excretion) was 3 to 30% and urea mineralization in the total mineralization rate (sum of urea and ammonia mineralization) was 10 to 45%. The average ratio of the daily regeneration rate to the daily assimilation rate by phytoplankton was 0.82 for urea and 1.18 for ammonia. The present study indicates that the nitrogen supply from the excretion and mineralization through these processes is in a state of dynamic balance with the nitrogen consumption by phytoplankton assimilation, and the urea and ammonia is rapidly recycling in the euphotic layer of Lake Biwa.