Analysis of nitrogen and phosphorus transfer through aquatic food webs.

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In order to study the processes of carbon(C), nitrogen(N) and phosphorus(P) transfer through food webs in aquatic environments, contents of these elements in various plankton organisms were determined. Two study sites, Lake Ouwa as an example of eutrophic waters and lake okutama as oligotrophic ones, were selected to obtain planktonic samples. In laboratory studies the effects of nitrogen and phosphorus on the growth and production of bacteria, phytoplankton and zooplankton were assessed and the role of food webs was considered. The following information was obtained from the chemical analysis of size fractionated plankton samples. 1. Generally, higher N/C and P/C rations were recorded in L. Suwa samples than those from L. Okutama. 2. These two ratios were higher in zooplankton than phytoplankton; two times in N/C and four times in P/C, suggesting the necessity of condensation of the elements in their grazing activity. 3. C and N contents in bacterial cells were estimated; 0.06 to 0.19 pgC/um cell volume and 0.203 in mean N/C ratio. An experiment using H thymidine uptake revealed that phosphorus was a limiting nutrient for bacterial production in an oligotrophic lake. It was also shown phytoplankton production was controlled by P and N. The growth and element composition of zooplankton were affected by N and P-content of food phytoplankton especially under low food concentration. These results show that bacteria and zooplankton do not necessarily play the role of nutrient regeneration in aquatic environments as previously believed.