Urea metabolism and its significance in the nitrogen cycle in the euphotic layer of lake biwa japan ii. half saturation constant for nitrogen assimilation by fractionated phytoplankton in different trophic areas
Archiv fur Hydrobiologie 107(2): 167-182
To obtain some informations on the influence of natural phytoplankton cell size and the trophic state of the water region upon the response of urea, ammonia and nitrate nitrogen assimilation and urea decomposition, sample waters taken from surface layers at four stations showing a different trophic character in Lake Biwa were separated into three fractions. The assimilation of urea and ammonia nitrogen by phytoplankton populations in the smaller fraction (< 25 .mu.M) exceeded those in the middle (25-70 .mu.m) and the larger fraction (> 70 .mu.m). The results of fractionated experiments of the urea decomposition showed a similar trend to those of the urea nitrogen assimilation. The half-saturation constants (Ks) for the nitrogen assimilation of three compounds were lower or comparable to the ambient nitrogen concentrations. The Ks value approximately increased with the phytoplankton cell size and of the trophic state of the water region. This suggests that the phytoplankton population in the oligotrophic area is adapted to the lower nutrient concentrations and has the capacity for higher assimilation than in the eutrophic area. The smaller phytoplankton would have a competitive advantage over larger phytoplankton in the assimilation of nitrogen.