External Nutrient Sources for Lake Tanganyika

Abstract

This study assessed the external nutrient sources for Lake Tanganyika from August 1994 to August 1995. The physico-chemical characteristics of the three largest inflowing rivers (Rusizi, Malagarasi, and Lufubu) and the wet atmospheric deposition in Bujumbura (Burundi), Kigoma (Tanzania), and Mpulungu (Zambia) were analyzed. The magnitude of external loading of P and N were evaluated to determine their potential for supporting biological activity. Seasonal changes in the physico-chemical composition of riverine input were detected and were linked to the altitude and morphology of the river system. A flushing effect was noted. Higher discharge rates corresponded with increased concentrations of most constituents. Rusizi provided the most and Lufubu the least of total riverine nutrient input. Rusizi's load equalled or almost doubled, depending on the nutrient, the combined loads of the other rivers. Seasonal changes in the atmospheric deposition chemistry were detected and were linked to biomass burning and atmospheric transport processes. Highest atmospheric deposition rates were encountered in more populated and industrialized Bujumbura. Long- and short-term atmospheric washouts were observed. Concentrations of rainwater components were on most occasions higher at the beginning than at the end of the rainy season. A similar trend was observed during separate rain events. Wet atmospheric deposition provided approximately 83% of dissolved inorganic nitrogen (DIN), 37% of total phosphorus (TP), 63% of total dissolved phosphorus (TDP), 65% of soluble reactive phosphorus (SRP), but only 1% of soluble reactive silicate (SRSi) of external source loading. The remaining load was derived from riverine sources. DIN, TP, TDP, SRP, and SRSi yearly net fluxes through wet deposition and riverine input were calculated as 58, 5.4, 2.5, 2, and 99 mmol per m 2 lake surface, respectively. Annual external loading of N and P potentially induced a new production of 45 to 60 g C/m 2/yr (ca. 7 to 14 % of annual primary production). External loading forms the main pathway for nutrients to enter the productive layers of Lake Tanganyika during stratified and oligotrophic periods (February to May).