Sources, transport and sinks of organic matter in Lake Malawi and Lake Victoria, East Africa

Lake Malawi, an ultra-oligotrophic lake, is the southernmost of the Rift Valley lakes of East Africa. Three questions were posed in an effort to determine the importance of allochthonous inputs to Lake Malawi: (1) How much allochthonous material is added to Lake Malawi? (2) Are the changes in the watershed due to anthropogenic disturbances apparent in the riverine chemistry and delta 13C and delta15N? and (3) What happens to the allochthonous material in the lake?; It was found that atmospheric deposition of organic carbon to the lake constitutes 36% of the organic carbon from outside sources. Within the annual cycle of the lake, the total lake DOC concentration can fluctuate by almost 50%.; Seasonal effects in Lake Malawi, and its surrounding catchment, are controlled by the monsoonal rainy season, from December through April, and the strong southeasterly winds in July and August.; In Lake Malawi, both particulate organic carbon and dissolved organic carbon are lower than observed in other Great Lakes of the world and approaching values observed in the ocean. Studies using chemistry and stable isotopes in Lake Malawi indicate that inputs of terrestrial dissolved and particulate organic carbon are not currently important to the overall concentration of carbon in the lake, although other nutrients associated with both the particulate and dissolved organic carbon may be important to autochthonous production.; Stable isotopes were also used to study the possible sources of carbon and nitrogen in Lake Victoria, East Africa. The changing source of carbon to the particulate organic matter was reflected in the carbon stable isotopes and concentrations of dissolved carbon dioxide. The inshore waters may become limited in carbon dioxide, which can cause a shift in the algal population to those capable of using other forms of dissolved inorganic carbon. The delta 15N of the particulate matter increased along the inshore to offshore continuum, as the algal population switched from using atmospheric nitrogen as the nitrogen source (nitrogen fixation) to having sufficient dissolved inorganic nitrogen in the upper mixed layer of the lake to sustain the algal growth. There was a strong negative correlation between the stable nitrogen isotope ratio of the particulate material and the rate of nitrogen fixation. (Abstract shortened by UMI.)...