In Lake Opinicon, Frontenac County, Ontario, the temperature gradient at midday from 10 cm below the water surface to soil, in clear weather in August, was found to be 10 Celsius degrees (C°)/m in shallow water where there was a large macrophyte biomass and only 0.2 C°/m in deeper water with fewer plants. There was a direct correlation between the log of the biomass per cubic metre and the steepness of the gradient. Reference is made to temperature gradients developed in calm weather during phytoplankton blooms in Scottish lakes.The biomass of aquatic macrophytes growing in a heated greenhouse in tubs with a capacity of 1 m3 increased from early January to the end of February. With increased biomass, the temperature gradient became larger. The size of the gradient correlated with the incoming radiation. Temperature gradients within large populations of the pondweed Potamogeton richardsonii were enhanced by low temperatures near the bottom.Diurnally produced temperature gradients with an algae bloom in the same tubs in the open in late June showed strong correlation with solar radiation, 8 C°/m with cumulative radiation of 22 MJ/m2 and 4.6 C°/m with 12 MJ/m2. The gradients produced by a population of P. richardsonii with tips 17 cm from surface were from 5.2 to 3.4 C°/m but showed less correlation with radiation when the wind velocity was above a threshold value. No gradient was produced in tubs of water without plants.