What are convection currents in a pond?

Convection currents in water (or any fluid) are the movement of the water caused by a difference in temperature between the upper and lower layers of the water. As water is heated it becomes lighter than cooler water so it floats up. Conversely, as water is cooled it get heavier so it sinks.

Assume the weather has been warm and the water in a pond is warm, then a cold spell comes through. The cold air cools the water at the surface causing it to sink. As it goes down into the warmer water it's heated and as more cooled heavier water sinks it pushes the newly warmed water up. This continuing process will eventually cause this original cooled and rewarmed water to reach the surface where it will be cooled and sink again and the process repeats until the entire pond is at the same temperature. This sinking and rising is called convection and the moving water is called convection currents.