Lake Tutira: The use of phosphorus loadings in a management study

Abstract

Decisions on lake managment are often made when data are scarce. A nutrient budget based on limited data of varying reliability and on information from the literature may help considerably. Lake Tutira''s [New Zealand] recreational uses are threatened by eutrophication. Artificial destratification is being used temporarily to protect trout fishery. The annual P input to the lake was estimated from nutrient loading graphs, from scanty stream input data and from land-use information to be about 3100 kg (1.8 g/m2 per yr). The 5-10 .times. reduction in P loading apparently required to achieve mesotrophic to oligotrophic conditions establishes the need for major changes in the catchment. The P budget helped in assessing the relative importances of the input streams and evaluating stream diversion proposals. P inputs came mostly from animal excreta, soil erosion and fertilizer, which provides support for farm management proposals. When the results of stream division and altered land management are known the P budget may help in deciding the optimum land area needed to be converted from pasture into forest.