the turnover in lake in spring and autumn results in

1. free mixing of O₂
2. algal bloom
3. temp. inversion
4. both (1) & (2)

The correct option is 4.
Lake turnover is the process of a lake's water turning over from top (epilimnion) to bottom (hypolimnion). During the summer, the epilimnion, or surface layer, is the warmest. It is heated by the sun. The deepest layer, the hypolimnion, is the coldest. The sun's radiation does not reach this cold, dark layer.

During the fall, the warm surface water begins to cool. As water cools, it becomes more dense, causing it to sink. This dense water forces the water of the hypolimnion to rise, "turning over" the layers.