General nutrient distribution in the water column of Northern Lake Tanganyika

A one-year's sampling programme of ammonium, nitrate and nitrite, and silica was performed at a station located in the pelagic area of northern Lake Tanganyika, about 4 km offshore in the vicinity of Bujumbura city. Sampling was generally conducted twice a month and water samples were collected at 10 m intervals between 0 and 100 m depth and analyzed spectrophotometrically for dissolved inorganic nutrients. Physico-chemical parameters were determined during field work with specific probes, and further work was done in the laboratory for dissolved oxygen, which was measured using the Winkler method. Meteorological data allowed for the distinction of four contrasting seasonal situations: 1) September to December 2004 with low rainfall and decreasing wind speed, 2) December 2004 to April 2005 with low wind speed and high rainfall, 3) April to June 2005 with sporadic rainfall and slightly increasing wind speed, and 4) June to August 2005, a typical dry season with no rainfall and high wind speed. Low to high temperature stratification periods were recorded and seemed to be linked to the hydrodynamics of the lake. Below the mixed layer the water is generally anoxic, but in July a deepening of the oxygenated mixed layer down to the bottom was recorded. Within the photic zone, phytoplankton activity implied most of the year undetectable levels of nitrate and nitrite. However, strongly fluctuating and surprisingly high levels of ammonium and the lowest concentrations for silicates were observed. While ammonium accumulates in the dark anoxic waters, nitrate and nitrite peak levels are mostly observed between 30 and 70 m depth due to nitrification processes. Below the photic depth and down to the bottom, silicates increased due to biogenic silica dissolution. Our data mostly corroborate earlier works and confirm the strong coupling of meteorological data and water column properties in northern Lake Tanganyika.