Seasonal variations of carbon and nitrogen isotope ratios of plankton and sinking particles in Lake Kizaki.

Abstract

Seasonal variations in carbon and nitrogen isotope ratios of plankton and sinking particles in Lake Kizaki were observed from April to November 1985 and May to October 1986. Carbon isotope ratio ( δ13C) of plankton with the size of 58 to 100 μm, which was mostly composed of phytoplankton, showed the highest value in June (-19 to -15 ‰) and the lowest value in April (-35.3 ‰). The δ13C value of plankton larger than 100 μm (mainly zooplankton) was lower by 1 to 5 ‰ than those of 58100 μm plankton. The nitrogen isotope ratios ( δ15N) of phytoplankton samples ranged from 1 to 6 ‰, and those of zooplankton samples were from 5 to 10 ‰. These changes in carbon and nitrogen isotope ratios of plankton in Lake Kizaki seemed to reflect the growth conditions of phytoplankton, such as water temperature, nutrient concentration, and light intensity. It was also suggested that the relative contributions of NH4+ and NO3- uptakes to total nitrogen uptake might affect the nitrogen isotope ratio of phytoplankton. The seasonal change in carbon isotope ratios of sinking particles collected by sediment trap was not so large, compared with that of phytoplankton. This indicated that the decomposition of autochthonous organic carbon occurred actively in the water column and that the contribution of allochthonous organic carbon to the bottom sediment was relatively large in Lake Kizaki.