Dynamic status of primary production in lake yunoko,a small eutrophic subalpine lake in central Japan

Investigation was made of the primary productivity of Lake Yunoko. The concentrations of PO₄-P and NH₃-N were highest in summer. The total amount of chlorophylla in the entire water column varied from 45 mg/m² to 405 mg/m² during the year with the maximum values occurring in late spring and in December. These values are very high compared with those reported for other lakes in the same district. The standing crop of phytoplankton as organic carbon was approximately 50∼70% of the total particulate organic carbon in the lake with values as high as 80∼90% being obtained in the spring and autumn. The rate of phytosynthesis under saturated light conditions was 3.7 mg C/mg chla/hr in the case of phytoplankton of the sun type and 1.2∼1.6 mg C/mg chla/hr in those of the shade type. The daily gross primary production in the lake varied from a low of 134 mg C/m² to a high of 1,003 mg C/m² during the year, the mean value being 372 mg C/m². The total amount of daily microbial respiration varied from 177 mg C/m² in winter to 1,476 mg C/m² in summer. Thus, the daily balance of production and consumption of organic matter in the lake itself was concluded to be negative. The main factor contributing to the rectification of this imbalance of material budget seems to be the leaf litter coming from the forest surrounding the lake. In conclusion, Lake Yunoko is a very productive and at the same time, fairly heterotrophic lake. Special Project Research supported by the Ministry of Education on Dynamic State of Biosphere. This study was carried out as part of JIBP-PF.