Modeling nutrient upwelling in Lake Malawi/Nyasa

In response to a need for improved understanding of the nutrient budget for an African Great Lake, a field investigation of the physical and chemical limnology in Lake Malawi/Nyasa was undertaken. Continuous water temperature profiles at two stations along the longitudinal axis in the southern portion of the lake, shore-based winds, and frequent nutrient profiles at one of the temperature measurement sites provide the basis for a simple time dependent mathematical model of the vertical fluxes of six key nutrient species. Two components of nutrient flux are simulated, namely, the turbulent and the upwelling flux. Due to persistent southerly winds, vertically directed currents known as upwelling flows dominate the transport except near the surface where turbulent transport plays a role. On four occasions from May to August, 1997, AVHRR thermal imagery, nearshore winds, and longitudinal temperature transects demonstrated upwelling in the southern extremity of the lake. Thus, upwelling is responsible for the bulk of nutrient transport into the photic zone in the area studied. These results represent the first quantification of nutrient upwelling for a large African lake.