Oligotrophic Lakes in Montana

Oligotrophic lakes are the clear, cold, nutrient-poor end of the trophic scale. The Carlson Trophic State Index measures this as TSI below 40 — water transparent enough to see 15+ feet down, total phosphorus under 12 µg/L, chlorophyll-a barely registering. Across Montana, 16 lakes register as oligotrophic — 0% of them break 50 feet of depth, which is the structural condition that underwrites the trophic class. Without depth and a low-nutrient watershed, oligotrophy is hard to sustain.

Oligotrophic lakes almost always carry the lowest phosphorus and chlorophyll-a readings on record, since the same low-nutrient conditions produce both the trophic class and the clear water.