Nutrient enrichment experiments in tropical Great Lakes Malawi/Nyasa and Victoria

Enrichment experiments with and without zooplankton (> 50 mum) removed were conducted in Lake Malawi during three seasons (stratified rainy, deep mixing, and stratified dry) and demonstrated that when light is adequate phytoplankton in containers quickly become nutrient deficient. The response to enrichment was assessed using chlorophyll a, photosynthesis, particulate stoichiometric ratios, PO4 turnover and N and P debt assays. The response to nitrogen (N), phosphorus (P), and iron (Fe) enrichments indicated that although N is the nutrient that becomes deficient most consistently, P deficiency is common as well. When Fe was added with N and P, the response by chlorophyll was four times the response to N and P without Fe. This suggests that, after N and P, Fe is the next most limiting nutrient in Lake Malawi. Light was a factor controlling phytoplankton growth in situ during the deep mixing season, and grazer removal experiments demonstrated that zooplankton > 50 mum are important in modifing the response of algae to light and nutrients. In Lake Victoria, experiments demonstrated that phytoplankton were primarily light-limited during the early-stratified season. Increased light levels resulted in N deficiency. Fe additions stimulated N uptake in both Lake Victoria and Lake Malawi and N-2 fixation in Lake Victoria.