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

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 NH4+ and dissolved reactive phosphorus (DRP), whereas dissolved organic nitrogen (DON) and dissolved organic phosphorus (DOP) showed no distinct seasonal trends. N,O reached 8,800% of air saturation just before the hypolimnion became anaerobic. At this time, the hypolimnetic N,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-l and 0.34 mg DIN m3 d-l. The ratio of DIN : DRP supplied to the epilimnion was 3.4 (by weight), which would exacerbate DIN deficiency in this lake. Quantitative studies of nutrient cycles, particularly those of N and P, are important for understanding the basic processes governing biological production in lakes. These cycles are influenced by both external and internal events. In general, the transformations of biologically essential nutrients depend on internal rates of organic production and subsequent decomposition (Bloesch et al. 1977), much of which occurs below the euphotic zone (Priscu et al. 198 1; Takahashi and Saijo 198 1; Stewart et al. 1982). Although the ecological importance of N and P decomposition and regeneration to the overall production of lakes is often considered, there are few quantitative measurements, primarily because direct measurement of these processes is difficult (Saunders 1976). Most field estimates of organic matter decomposition and nutrient regeneration in aquatic systems are based on correlations of observed changes in organic matter with its decomposition products (e.g. Barica 1974; Priscu et al. 1982), on the dilution rate of an isotope (e.g. 15N) added to the system (Stewart et al. 1982), or on numerical models based on field data which distinguish the relative effects of physical and biological processes on the concentrations of the elements under concern (e.g. Takahashi and Saijo 198 1; Jahnke et al. 1982). Laboratory experiments used to determine rates of decomposition and transformations of a particular nutrient under various environmental conditions show that the rates of transformation may depend strongly on the age and type of organic material (usually phytoplankton) used, as well as on temperature and pH (Von Brand et al. 1937, 1942; Gunnison and Alexander 1975; Romancnko 1977). However, in many of these studies concentrations of organic matter were unrealistically high or incubation times in closed vessels were exceedingly long, so that the results are hard to apply to field situations. We present here results from a one-dimensional diffusion model used to quantify