Sources and Fluxes of Organic Carbon in Lake Malawi/Nyasa

Abstract

Significant changes in land use of the watershed of Lake Malawi, one of the Great Lakes of East Africa, have occurred over the past 30 to 40 years and continue today. Deforestation and increasingly intensive agriculture have contributed to an increase in the mobilization of allochthonous organic material to the lake. Dissolved and particulate organic carbon transport by rivers as well as wet and dry atmospheric deposition were measured and used to construct an organic carbon budget for the lake. Although the concentrations of dissolved organic carbon (DOC) and particulate carbon (PC) in Lake Malawi are among the lowest measured in any large lake and approach oceanic concentrations, significant seasonal changes over the annual cycle were found. The lake had a mean mass of 62.6 × 10 10 moles of organic carbon and within the annual cycle of DOC the mass fluctuated by 35.5 × 10 10 moles of organic carbon from September 1997 (47.1 × 10 10 moles of organic carbon) to May 1998 (82.6 × 10 10 moles of organic carbon). The annual allochthonous organic carbon input to the whole lake from all sources was approximately 8 × 10 10 moles with 36% from atmospheric sources. Previous studies have estimated a mean annual phytoplankton production of 98.9 × 10 10 moles of carbon for the whole lake. Liberation of 20 to 40% of the algal production as DOC will account for the change in the mass of organic carbon. In this large tropical system, allochthonous particulate carbon may be an increasingly significant source of energy and nutrients to the food web. Due to the importance of primary production, land use changes that increase nutrient export the lake will also alter the organic carbon stores