Nitrogen and phosphorus regeneration by cichlids in the littoral zone of Lake Malawi, Africa

Interaction between cichlid fishes of the nearshore rocky littoral zone of Lake Malawi with the epilithic periphyton community was addressed by measuring the importance of the fish community in recycling nitrogen and phosphorus through excretion, defecation, and mortality. A series of in situ fish incubations in 13-L Plexiglas chambers at two islands in Lake Malawi provided evidence that excretion and defecation by rock dwelling cichlids supplies 46% to 48% of both nitrogen and phosphorus demands, calculated using Redfield ratios, to support previously, measured rates of periphyton photosynthesis. Fish mortality was estimated to more than adequately supply the balance. Epilithic cyanobacteria are an additional source of fixed N. The role of benthic macroinvertebrates in recycling P and N has not been investigated but may be relatively unimportant based on the nutrient balance reported here. The results support the hypothesis that fish may be important in structuring the epilithic periphyton community composition by imposing a low N to P ratio on the benthic algae, favoring dominance by nitrogen-fixing cyanobacteria.