Trophic level index: The lake trophic level index (TLI) indicates the health of a lake based on its degree of nutrient enrichment. The trophic states represent increasing nutrient enrichment, from a state of microtrophic (very good) to supertrophic (very poor).
Microtrophic (TLI < 2; very good) lakes are very clean and often have snow or glacial sources (eg Lake Pukaki in Canterbury)
Oligotrophic (TLI 2–3; good) lakes are clear and blue, with low concentrations of nutrients and algae (eg Lake Rotoma in Bay of Plenty)
Mesotrophic (TLI 3–4; average) lakes have moderate concentrations of nutrients and algae (eg Lake Rerewhakaaitu in Bay of Plenty)
Eutrophic (TLI 4–5; poor) lakes are murky, with high concentrations of nutrients and algae (eg Lake Rotoroa in Northland)
Supertrophic or hypertrophic (TLI > 5; very poor) lakes have extremely high concentrations of phosphorus and nitrogen, and are overly fertile; they are rarely suitable for recreation and lack habitats for desirable aquatic species (eg Lake Forsyth in Canterbury).