1. Vertical movements of cyanobacteria during summer stratification were studied in Cauldshiels Loch, a small dimictic temperate lake. 2. Filaments of the three species of cyanobacteria dominant in the summer phytoplankton were recorded moving vertically in both directions, by simple traps deployed just below the thermocline. 3. These phytoplankters could potentially exploit opposing ends of the vertical light/nutrient gradient that existed in the water column during stratification. 4. The hypolimnion and water overlying the sediments were enriched by phosphorus released from the sediments. This internal loading to the lake provides a source of phosphorus available to migrating cyanobacteria to sustain production and growth in the epilimnion. 5. In this study, cyanobacteria filaments moving from the hypolimnion were found to be translocating phosphorus to the epilimnion. 6. Following reduction of external phosphorus loading, utilization of such internal phosphorus sources may delay expected reductions of bloom-forming cyanobacterial communities, and consequent improvements in other aspects of water quality.