The turnover in a lake in spring and autumn results in

Options

A. Free mixing of `O_(2)` and nutrients
B. Algal bloom
C. Temperature inversion
D. Both (1) and (2)

Answer

### Step-by-Step Solution:

1. **Understanding Lake Turnover**:
- Lake turnover refers to the process where the water layers in a lake mix. This occurs when the temperature of the water changes, causing the denser, cooler water to sink and the warmer, less dense water to rise.

2. **Seasons of Turnover**:
- Turnover typically happens in spring and autumn. In spring, as the ice melts and the water warms, the upper layer (epilimnion) becomes less dense and mixes with the cooler, denser water at the bottom (hypolimnion). In autumn, as the temperature drops, the surface water cools, becomes denser, and sinks, causing a similar mixing effect.

3. **Effects of Turnover**:
- During turnover, two significant events occur:
- **Mixing of Oxygen and Nutrients**: The turnover process facilitates the mixing of oxygen-rich water from the surface with nutrient-rich water from the bottom, enhancing the overall health of the lake ecosystem.
- **Algal Bloom**: The increased nutrient availability in the upper layer can lead to algal blooms, which are rapid increases in algae populations.

4. **Evaluating the Options**:
- **Option A**: Free mixing of oxygen and nutrients - This is correct as turnover promotes this mixing.
- **Option B**: Algal bloom - This is also correct due to the nutrient increase in the upper layer.
- **Option C**: Temperature inversion - This is incorrect as temperature inversion is a separate phenomenon that does not result from turnover.
- **Option D**: Both 1 and 2 - This is the correct choice as both mixing of oxygen/nutrients and algal bloom occur during turnover.

5. **Conclusion**:
- The correct answer is **Option D**: Both 1 and 2.