The trophic level index is a value derived from four factors: water clarity, and the amount of chlorophyll, phosphorus and nitrogen in the water. The lower the value, out of zero to five, the higher quality the water. Trif said the data showed the overall quality of the Rotorua lakes had increased from an average TLI of 3.8 between 2001 and 2010 to an average of 3.6 in the decade between 2011 and 2020. “Lake Rotorua registered the biggest improvement, with a 0.5 reduction of its TLI, from 4.8 to 4.3. In terms of hitting the TLI targets, last year was the best since 2015, with four of the 12 lakes hitting their TLI targets.”