Comparative Assessment of Water Quality with the Trophic Level Index and the Delphi Method in Lakes Rotoiti and Rotorua, New Zealand

Abstract

Abstract We describe a case study involving the application of a water quality (WQ) assessment for Lakes Rotorua and Rotoiti, New Zealand. Our major objective was to compare an established water quality system based on a trophic level index (TLI) against an alternative Delphi method (DM) which uses an assembled expert panel including specialists in limnology and water resources management. The set of water quality indices (WQI) established with DM captures the major uses of water resources of the studied lake ecosystems: conservation and recreation. The set of permissible ranges for individual WQIs assessed with DM allows a “reference state” to be defined for WQ in the study lakes. The TLI and DM were closely correlated when eutrophication was the overarching issue, but much less so when resources and uses of the lake ecosystems were taken into consideration. Water resources management based around the TLI is commonly targeted at oligotrophication of aquatic ecosystems to improve WQ, often in association with reducing economic activities in watersheds. In contrast, the DM allows a more complete assessment that aligns with sustainable management of natural water resources but also includes the need for conservation.