1. The penetration of light into Windermere is dependent chiefly on the numbers of phytoplankton organisms in the epilimnion.
2. It appears that the greatest depth at which rooted aquatic plants can grow must therefore be affected by the abundance of phytoplankton. The rooted plants are most active during July and August. Observations show that at this time the presence of blue-green algae reduces the light intensity at the limit of submerged vegetation (4·3 metres) by more than 50 per cent.