| Summary: | This study uses Lake Naivasha, Kenya as an example of a shallow tropical freshwater lake. In common with many tropical lakes it experiences fluctuating water-levels which influence its area and productivity, and is currently considered moderately eutrophic. The light regime experienced by phytoplankton in Lake Naivasha dominates other controls as it determines the potential level of primary production. Photoinhibition reduces productivity by 25% at the surface with maximum productivity at a depth of approximately 0.5m. Light attenuation reduces productivity by 50% at 1m depth with zero productivity at 5m depth. Self-shading causes a 17% loss of productivity under conditions of below average productivity, but a 46% loss when productivity is above average. Hydrological factors form a primary control as the mixing regime determines the light regime. Lake Naivasha is generally well mixed, but where temporary stratification occurs there is nutrient resupply due to sediment anoxia. Without mixing, there is a 75% loss of productivity by cell sedimentation. Low sinking rates, tropical conditions and high nutrient availability favour low Surface Area:Volume species such as Aulacoseira which is the dominant genus. Changing conditions such as increased water-column stability could favour cyanobacteria. Bottom-up controls were the most important in Lake Naivasha, reducing potential productivity by 50%. Nitrogen was found to be more limiting than phosphorus with an algal preference for ammonium over nitrate. Minor nutrients were not limiting. The main source of allochthonous nutrients was from river inflow with underflow and circulations around the lake. Top-down control by grazing imposes a 15% reduction in productivity with zooplankton preferring large 'production' cells over small 'standing-stock' cells.
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