Summary
Limnological conditions of Lago Grande do Jutai in Central Amazonia, are mainly determined by the Solimões River. The fluctuations of the river directly affect the lake, as the lake is connected with the river almost all year round. The water level varies annually from 9-10 m. From January to June the lake is filled with rain water and water from the river, the water level decreasing from July to December. These fluctuations in the water level are mainly responsible for the large physical, chemical and biological variations which occur in the lake's enviroment. Seasonal fluctuations, such as temperature stratification, oxigen conditions and salt contents, including the most important constituents of the lake, are clearly shown in the analyses which have been carried out. Maximum transparency is obtained when the water lake is on its peak. Low oxygen concentration due to stagnation (and therefore production of H2S and decrease in oxygen concentration), turbulence (causing the gas to raise up to the Epiliminieo region with consequent decrease in oxygen concentration), during the months of May. June, July, August and sometimes September (as occured in 1974), make these months the most critical for fish. The highest concentrations of CO2, nitrites, nitrates, ammonia, total nitrogen, iron and phosphorus are in the anoxic region. According to Schmidt (1973) and to own observations, conditions in the great lakes of Central Amazonia are better than in the small lakes due to the high concentration of oxigen in water. The major portion of the salts in the lake is of calcium bicarbonate, the largest concentrations being found during low water level. Although annually incorporated into the lake with inflow of water from the river, phosphorus is the most deficient of the elements important for the ichthyoflora and fauna. During "drought", i.e.. low water level homothermy occurs as a result of the hydric circulation in the lake.