Preliminary assessment of nutrient loads on lake Taupo, New Zealand

Lake Taupo is an oligotrophic lake of 612.4 km 2 and 97 m average depth, situated in a catchment of 2849 km 2 on the Central Volcanic Plateau of the North Island of New Zealand. Annual loads of reactive phosphorus and inorganic nitrogen were estimated from aerial, surface stream, and groundwater seepage sources. The specific surface load for phosphorus of 0.189 g·m −2 ·y −1 suggests a mesotrophic status, but the nitrogen load of 1.073 g·m −2 ·y −1 fits the observed oligotrophic condition of the lake. The overall ratio of nitrogen to phosphorus in the inputs was only 5.68 : 1. Most of the phosphorus is derived from natural sources, and in general greater quantities leave wooded land (covered with native forest, pine plantations, and scrub) than leave mostly pastoral areas. The major nitrogen loads originate from one part of the catchment. Substantial variations in nutrient loadings and ratios in surface flows occur seasonally, with variations in flow rates, and between sub‐catchments. Obvious associations between sub‐catchment nutrient loads and vegetative cover were not found. The low ratio of nitrogen to phosphorus in inputs to the lake suggests that nitrogen may prove to be an important growth‐limiting nutrient. Variable climate is likely to affect nutrient ratios and loading estimates for lakes without major sources of man‐made pollution.