Isotopes of authigenic carbonate from lake sediments have great potential to decipher climatic changes over geologic periods. However, the climatic significance of proxies needs further verification. Genggahai Lake is in the central Gonghe Basin on the northeastern Qinghai-Tibetan Plateau. The lake is currently dominated by various submerged plants and is favorable for the formation of biogenic carbonates. Over the past two years, we have monitored changes in chemical and physical properties of lake water and analyzed isotopic compositions of carbonates, including stem incrustations and mollusk shells. In combination with oxygen isotope changes of the lake water (delta~(18)O_(Lake)) and water bodies in the study area, seasonal changes in isotopes of carbonates were investigated. The growth of Chara spp. was found to be seasonal, with maximum growth rate in summer. From May through August, the growth rate of Chara spp. was 5-6 cm per month. During this period, pH values of lake water increased while dissolved inorganic carbon (DIC) and calcium ions (Ca~(2+)) decreased, as a result of intense photosynthesis and calcification during the growth season of submerged plants. Seasonal fluctuations of lake level were mainly controlled by regional rainfall and lake evaporation. Changes in delta~(18)O_(Lake) are attributed to variations of groundwater delta~(18)O, evaporation processes, and/or regional precipitation. Because of the long growing season and slow calcification process of mollusk shells, oxygen stable isotope ratios in the shells (delta~(18)O_(Shell)) were mainly influenced by delta~(18)O_(Lake). Carbon stable isotope ratios in the shells (delta~(13)C_(Shell)) were primarily affected by the carbon isotope of DIC (delta~(13)C_(DIC)). There were no significant seasonal fluctuations in delta~(18)O_(Shell) and delta~(13)C_(Shell). Therefore, delta~(18)O_(Shell) and delta~(13)C_(Shell) indicated annual variations of delta~(18)O_(Lake) and delta~(13)C_(DIC), respectively. However, we observed extreme oxygen isotopic disequilibrium between the incrustations and lake water. Kinetic effects probably caused the extreme oxygen isotopic disequilibrium during intense photosynthesis. Because our data were limited, the disequilibrium stable isotope effects require further observation and discussion.