When a water body becomes extremely low in nutrients, then it is called:

A water body extremely low in nutrients is described as oligotrophic.

Such water stays clear and well oxygenated with only sparse biological growth.

Dystrophic water is stained and acidic with high organic matter, not simply nutrient-poor.

Mesotrophic water carries a moderate, in-between level of nutrients.

Eutrophic water is the nutrient-rich opposite, prone to heavy algal blooms.

Deep, cold mountain lakes are typical examples of oligotrophic water bodies.