CARBON ISOTOPE DISCRIMINATION BY PHYTOPLANKTON AND PHOTOSYNTHETIC BACTERIA IN MONOMICTIC LAKE FUKAMI-IKE

Carbon isotope ratios of particulate, settling and sedimentary organic carbon, and dissolved inorganic carbon (DIC) were analyzed to study the eco-physiology of phytoplankton and photosynthetic bacteria (green sulfur bacteria) and the fate of organic matter in Lake Fukami-ike, Japan, together with C-13 and N-15 tracer experiments. Carbon isotope ratios of particulate organic carbon (POC) decreased from -24.8 approximately -17.8 parts-per-thousand at water surface, to -29.2 approximately -23.6 parts-per-thousand at 8 m depth, while those of dissolved inorganic carbon increased with depth and ranged from -17.7 to -4.4 parts-per-thousand. Calculated carbon isotope discrimination between POC and HCO3- in the euphotic zone, which can provide long-term integrated information on the kinetic mode of photosynthesis, ranged from 0.3 to 21.3 and increased with depth, indicating a relatively high photosynthetic rate at the surface. Discriminations by the photosynthetic bacteria at 5 m depth were consistently larger than those by surface phytoplankton. This finding suggested that photosynthetic bacteria populations had grown at a slow rate in L. Fukami-ike from June to July, 1988, consistent with low carbon and nitrogen uptake rates in the tracer experiments. By using a clear-cut difference in delta-C-13 between phytoplankton and photosynthetic bacteria, the dynamic mode of settling particles was also discussed. The high delta-C-13 values of DIC in deeper layers seem to result partly from isotope fractionation in the process of methane production in this lake.