NUTRIENT CHEMISTRY OF THE WATER COLUMN OF LAKE TANGANYIKA

Lake Tanganyika shows permanent thermal stratification with deep-water temperatures that have been stable over the period of observation (si nce 1939). The lake is anoxic below approximately 150-m depth. In gene ral the nutrients show Redfield behavior save in the deep waters of th e northern basin where large excesses of phosphate and ammonia are pre sent. Bacterial disproportionation of organic material probably plays an important role in producing these excesses. Inorganic desorption fr om fluvial detritus is also a possible source of excess phosphate in d eep waters. The oxic-anoxic boundary at approximately 150 m is a sink for all forms of fixed nitrogen. Thus the nutrient budget of the lake probably involves fixation of nitrogen in the surface layer in additio n to substantial inputs from rainfall and runoff, with the phosphate s upplied by vertical mixing. Because these processes are in approximate balance under present conditions, the productivity of the lake must b e very sensitive to changes in climatic forcing.