Abstract
The seasonal and vertical changes in photosynthetic products were determined in Lake Biwa, which is the largest lake in Japan, by combined 13 C and gas chromatography-mass spectrometry ( 13 C-GC-MS). The temperature dependence of the production rates differed among organic compounds. The seasonal change in the protein specific production rate (SPR) showed a strong temperature dependence, indicating that photoplankton growth rate was primarily governed by water temperature. In August and September, low protein SPR was, however, observed, probably due to low inorganic nitrogen concentration. Nitrogen deficiency in the late stagnation period was also suggested by high C:N ratios both in paniculate matter and photosynthetic products. The glucose SPR did not show any significant correlation with water temperature, irradiance and inorganic nitrogen concentration. Highly variable glucose SPR might be due to high turnover of glucose, and vertical mixing in the mixed surface layer may be one of the most important factors causing variability of glucose SPR.
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