A numerical analysis of hypolimnetic nitrogen and phosphorus transformations in Lake Rotoiti, New Zealand: A geothermally influenced lake

Abstract

Measurements of chlorophyll, oxygen, and particulate and dissolved forms of nitrogen and phosphorus made over a year were used in a one‐dimensional diffusion model to calculate rates of generation or loss of these substances in the hypolimnion of Lake Rotoiti, New Zealand. During summer stratification the hypolimnion was always a sink for chlorophyll, particulate nitrogen (PN), particulate phosphorus (PP), and dissolved oxygen. The order of decomposition was chlorophyll >PP >PN. The hypolimnion was a source for NH 4 + and dissolved reactive phosphorus (DRP), whereas dissolved organic nitrogen (DON) and dissolved organic phosphorus (DOP) showed no distinct seasonal trends. N 2 O reached 8,800% of air saturation just before the hypolimnion became anaerobic. At this time, the hypolimnetic N 2 O pool comprised about 13% of the total dissolved inorganic nitrogen pool. Diffusion of dissolved inorganic nitrogen (DIN) and DRP from the hypolimnion to the epilimnion could increase mean epilimnetic concentrations during summer stratification by 0.10 mg DRP m −3 d −1 and 0.34 mg DIN m −3 d −1 . The ratio of DIN : DRP supplied to the epilimnion was 3.4 (by weight), which would exacerbate DIN deficiency in this lake.