Both New Zealand and Australia are characterized by large proportions of human populations focused into coastal towns and cities, with most industrial and domestic wastewater discharges into the coastal environment. Eutrophication of lakes and reservoirs in both countries has resulted mostly from agricultural development. In New Zealand and south-east Australia, around 90% of wetlands have been cleared and drained, mostly for agricultural development. In New Zealand in particular, pastoral farmers traditionally used phosphatic fertilizers often supplemented with micronutrients (e.g., cobalt) to increase pastoral productivity, while relying on nitrogen fixation by clover to meet grassland nitrogen demands. With the advent of relatively cheap nitrogenous fertilizers, predominantly urea and ammonium, there has
been a rapid increase in their use in pastoral farming. The NZ Parliamentary Commissioner for the Environment provides statistics for nitrogenous fertilizers that show an increase in application rates of 160% in dairy farms and 680% in sheep and beef farms between 1996 and 2002. Nitrate loads to Lake Rotorua are projected to increase nearly 10-fold between the relatively unimpacted period of the 1950s and 2050, in response increased intensity of pastoral farming, notably nitrogen fertilizer applications, in the lake catchment. The duration of this response reflects the long lag time as water and nutrients are transported through the porous volcanic soils into the underlying aquifer and ultimately into the lake. Productivity in many New Zealand lakes, particularly the large volcanic lakes of the central North Island (e.g., Lake Taupo and Lake Rotorua), is limited by availability of nitrogen. Under these circumstances the rate of eutrophication is expected to increase rapidly and productivity may be increasingly controlled by availability of phosphorus rather than nitrogen. Specific case studies are given below of large New Zealand lakes that have undergone rapid deterioration in water quality.
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