What is the main difference between oligotrophic and eutrophic lakes?

The amount of nutrients in the water

Oligotrophic lakes are characterized by low nutrient values, which limits the lake's ability to support animal life. Consequently, the water remains clear.

Eutrophic lakes are characterized by high nutrient values, which allows microorganisms and algae to grow in large numbers, which then allows animals that feed on those algae to also be supported.