In Lake Shinji, it is known that cyanobacterial water-bloom grows under the fleshwater condition in the pluvial. In the middle of November 1999, however, the water-bloom grew under the restrained condition of the saline water(4.6 psu) and the low temperature (lower than 15℃). The purpose of this study is to clarify geochemical features such as carbon isotope ratio of the water-bloom in November 1999, and to discuss the factors controlling the occurrence of water-bloom based on geochemical features and environmental information.
The carbon isotope ratios of ＜63μm suspended particulate organic matter(POM) from the Shinji Lake Surface Water(SSW) are -22.45±0.17‰, showing a intermediateδ^<13>C value between typical oceanic and freshwater phytoplankton. It is considered that highδ^<13>C values of POM are caused by ^13C concentration increased in dissolved inorganic carbon(DIC) as a result of high production of phytoplankton for the prior absorption of ^<12>C. The δ^<13>C value of POM in the pycnocline is .25.05‰, which is lower than that of POM in SSW. This lowδ13C value may have shown the carbon fixation by the photosynthesis using the ^<12>C-enriched DIC accumulated in pycnocline and Shinji Lake Bottom Water(SBW).
The δ^<13>C values of water-bloom are -23.82±0.03‰, and are 1.1 to 1.4‰ lower than that of POM in SSW. This low δ^<13>C value may be caused by the water-bloom produced on/in the water surface where inorganic carbon exchange with the atmosphere. The negative correlation is recognized between δ^<13>C values of the water-bloom and salinity. This correlation shows that high salinity water mass below pycnocline including the ^<12>C-enriched DIC is added to the SSW.
Judging from geochemical characteristics and the water-bloom condition, the cyanobacterial water-bloom in November, 1999, may be caused by the lower layer including the nutriment such as NH_4-N is thrust up by the inflow of the saline water, and partially diffused in SSW.