Abstract
The uptake rate of nitrogen by freshwater zooplankton was measured by use of a 15N tracer technique under experimental conditions. To detect the low quantities of excess 15N in less than several hundreds of small zooplankton individuals (as small as 5 μgN), an isotope dilution technique was adopted using a ratio mass spectrometer. The analytical differences between natural 15N abundance method and result obtained by this technique were -0.00210 to +0.00113 15N atom % (mean±S.D.=0.00048±0.0010 15N atom %) . We obtained uptake rates of 8.5×10-8mg atom-N⋅indiv.-1⋅day-1 by Kellicottia longispina in Lake Nakatsuna and 4.2×10-7 by Keratella quadrata in Lake Suwa. Net production rates calculated from the data of the uptake rates, carbon contents and nitrogen contents were 5.1×10-6 mgC⋅indiv.-1⋅day-1 for K. longispina and 2.5×10-5 mgC⋅indiv.-1⋅day-1 for K. quadrata. Using a combination of the ratio mass spectrometer and the 15N tracer experiment, the nitrogen uptake rates from nutrient salt to phytoplankton and further to zooplankton can be detected directly at the same time and on the same space scale.
