Why are there no convection currents in a lake during the summer?

Question

Why are there no convection currents in a lake during the summer?

Answer 1

Convection currents are driven by a temperature gradient, warm material rises and is replaced by colder material coming from somewhere else (mantle convection, air convection...).  

In a lake the top layer is usually warmer in the summer, being heated by the Sun and the warm air, so is lighter (has a lower density) while the deeper regions of a lake are colder and denser, therefore heavier.