What characterizes a meromictic lake?

A meromictic lake is characterized by having distinct layers of water that do not mix. This stratification occurs due to differences in temperature, salinity, or density between the layers, causing them to remain isolated from one another. In a meromictic lake, the bottom layer, or monimolimnion, is typically more stable and does not mix with the upper layer, or mixolimnion, which can lead to the accumulation of nutrients and other substances in the deeper water over time. This phenomenon is significant as it influences the biological and chemical processes within the lake.

The other options do not accurately describe a meromictic lake. Complete mixing refers to a condition found in polymictic lakes, while no mixing at all does not fully capture the nature of meromictic lakes, as they do have layers that interact at the surface but remain distinct. Lastly, the depth of the lake does not define its meromictic status; meromictic lakes can be deep or shallow. The defining feature remains the distinct non-mixing layers.