Phosphate, nitrate, and chloride in a eutrophic coastal lake in New Zealand

Abstract

Tomahawk Lagoon. No. 2 is a small (9.6 ha) shallow (1 m) coastal lake in Otago (45° 53’ S, 170° 31’ E) with a drainage basin that is largely pasture. Chloride and phosphate concentrations in 1968–71 are compared with earlier figures. Nitrate was measured for 2 y, and nitrate and phosphate income were determined for 5 months in 1970. Chloride concentrations increased from 300 g.m‐3 to 1600 g.m‐3 in April‐May 1969 following a violent storm, and then declined during the next 3 y, though there was a slight increase each summer. Other large additions of chloride occurred before November 1964 and in 1972 or early 1973. In 1 month after superphosphate was applied in the drainage basin in July 1970, 35 kg of PO4‐P entered the lagoon. The average at other times was 0.6 kg.month‐1, and it is suggested that agricultural phosphate may be largely responsible for the highly eutrophic state of the lake. Phytoplankton productivity immediately increased in response to the addition of phosphate in July 1970. Reactive phosphate concentrations did not increase immediately, but they later reached the theoretically expected levels as the phytoplankton declined. The nitrate income was equivalent to 4.7 g.m‐2.y‐1 of nitrogen, which is consistent with the highly eutrophic state of the lake. Urban storm‐water drainage may at times contribute significant amounts of PO4‐P to the lake, but it was a minor source of inorganic nitrogen. Rainwater contained little inorganic nitrogen or PO4‐P. Phosphate concentrations were always relatively high in summer‐autumn and lowest in winter. In summer 1968–69 PO4‐P increased from 12 mg.m‐3 to 480 mg.m‐3 in 5 weeks as a large crop of epiphytic algae declined. The annual nitrate cycle contained irregular peaks of up to 3700 mg.m‐3 of NO3‐N which coincided with periods of high nitrate income. © 1975 Taylor & Francis Group, LLC.