The Complexity of Top-Down Control on Primary Productivity: Interactive Effects of Biomass Removal and Consumer Nutrient Recycling in a Low Nutrient Ecosystem

In Lake Tanganyika, high secondary production and stunning biodiversity exists in the face of extreme nutrient scarcity. The littoral cichlid community is dominated by algivores that consume epilithic periphtyon. We used laboratory experiments to separate the effects of biomass removal and nutrient excretion on periphyton across a gradient of grazer densities. Algal biomass and productivity both increased in response to grazer removal, but the biomass response (+57-201%) was much stronger than the productivity response (+9-22%). Area-specific biomass and productivity of periphyton released from grazing was positively related to fish density due to nutrient excretion. However, productivity and biomass of grazed periphyton was not correlated with fish density; rather, fish grew more slowly at higher densities. Parallel field experiments corroborated these results. We infer that in the absence of grazers, biofilms rapidly become light and nutrient limited, such that productivity is independent of algal biomass. Conversely, the small negative effects of fish on area-specific productivity are alleviated by nutrient recycling. These complex interactions generate highly stable consumer-resource dynamics across a range of consumer densities in this low nutrient, high productivity ecosystem.