1999 Volume 60 Issue 3 Pages 367-377
The first record of depth-time profiles for some physicochemical and biological parameters of Lake Bishamon-numa based on periodic observations during the ice-free season of 1996 are reported. Throughout the observed period, the thermocline was either absent or very weak. In accordance with this structure of water temperature, the dissolved oxygen was distributed uniformly in the water column at above a 90 % saturation level. The average and standard deviation of transparency was 5.1 ± 1.1 m. On the assumption that the trophogenic zone is from the surface to a depth of twice the transparency, the whole water body of the east basin in which the sampling station is located, constitutes the trophogenic zone. pH was fairly constant from July to December with an average and standard deviation of 6.2±0.1. Compared with past data, the pH of Lake Bishamon-numa has changed greatly within a relatively short time period in the past and has lately stabilized around pH 6. The concentrations of Na, K, Mg, and Ca were in the order of m mol 1-1, while those of Fe and Mn were of umol 1-1. Ranked in descending order of concentrations, they are Na, Ca, Mg, K, Mn and Fe. The annual maximum of Chl. a was 1.5 μg 1-1 in May. Though there were slight maxima and minima, the Chl. a concentration was below 1μg 1-1 in most seasons and averaged 0.51μg 1-1. This Chl. a level corresponds to those of other oligotrophic lakes. Nanophytoplankton ranging from 2 to 20 μm predominated, and macrophytoplankton greater than 20 μm were relatively scarce.