Lake Bonney is characterized by strong gradients in nutrients and algal concentrations (figure 1 and Priscu, Ward, and Downes, Antarctic Journal, in this issue). In theis permanently ice-covered quiescent environment, nutrient transformations and microplanktonic activity are hypothesized to be tightly coupled. This should be particularly true for transformations of inorganic nitrogen, because nitrogen has been shown to limit phytoplankton photosynthesis in a number of antarctic lakes (Priddle et al. 1986; Priscu, Vincent, and Howard-Williams 1989; Sharp and Priscu 1990). Our current research on Lake Bonney is based on the hypothesis that regeneration of ammonium (NH₄⁺) and nitrate (NO₃^-) through heterotrophic remineralization and nitrification, respectively, support the bulk of productivity in the upper trophogenic zone of Lake Bonney (Priscu et al., Antarctic Journal, in this issue).

This report focuses on a preliminary assessment of the physiological capacity of planktonic assemblages for inorganic nitrogen uptake in the trophogenic zone of the east lobe of Lake Bonney. Specifically, our study provides estimates for the effects of substrate concentration on uptake kinetics as well as ambient uptake rates of inorganic nitrogen and carbon. The results provide preliminary insight into the interactions of nutrients and microplankton; furthermore, they set the groundwork for future investigations of nutrient cycling and primary production in the lake. In this report, uptake refers to transport plus assimilation, two distinct pathways which could not be distinguished by our experimental designs.

The Michaelis-Menten parameters V_max (the maximum specific uptake rate) and K_m (the substrate concentration at which uptake is half of the maximal rate) were determined with substrate kinetics experiments. In these experiments, water samples from 4.5 meters (m) received varying levels of enrichment of nitrogen-15-labeled NH₄⁺ or NO₃^- and were incubated for approximately 24 hours. The suspended particulate matter in the water samples was collected on precombusted GF/C filters which were frozen until analysis. The accumulation of nitrogen-15 in the particulate matter was measured by atomic emission spectrometry following Dumas combustion of filters (Timperly and Priscu 1986).

Uptake rates were calculated fom the equations formulated by Dugdale and Goering (1967).

The uptake rates were fitted to the Michaelis-Menten model with a nonlinear curve fitting program utilizing Marquardt's algorithm (Dodds, Priscu, and Ellis 1991). From data collected on 29 November 1990, V_max (plus standard deveiation) and K_m (plus standard deviation) for NH₄⁺ at 4.5 m were 0.00083 per hour + 2.81 x 10^-14 and 1.000 micromoles per liter + 1.65 x 10^-10, respectively. V_max and K_m for NO₃^- at 4.5 m, calculated from data collected on 18 November 1990, were 0.00039 per hour + 6.87 x 10^-15 and 4.83 micromoles per liter + 3.36 x 10^-10, respectively.

The high V_max and low K_m for NH₄⁺, relative to NO₃^-, indicate a high affinity for this nutrient at both saturating and limiting levels (Goldman and Glibert 1983). Two potential explanations for this affinity are, first, that NH₄⁺ requires less energy for assimilation than does NO₃^-, and second, that increased capacity for NH₄⁺ uptake may represent an adaptive response for utilization of temporary micropatches of NH₄⁺ produced through regeneration. In Lake Bonney, both regenerated NO₃^- and regenerated NH₄⁺ are hypothesized to support primary production in the euphotic zone (Priscu et al., Antarctic Journal, this issue). Because of the virtual lack of zooplankton capable of producing NH₄⁺ patches, the differential affinity can perhaps best be explained by the energy requirement of assimilation.

Estimates of ambient uptake rates (figure 2) were computed using ambient nutrient concentrations in conjunction with Michaelis-Menten parameters for uptake. These ambient rates indicate that NH₄⁺ is the predominant source of nitrogen utilized throughout the trophogenic zone. Furthermore, the relative contribution of NH₄⁺ to nitrogenous nutrition increases with depth from 6 to 12 m. This supports the hypothesis that phytoplankton near the first chemical gradient (at approximately 11 m) are supported by the upward diffusion of NH₄⁺ (Priscu et al., Antarctic Journal, this issue).

There is reason, however, to interpret these data with caution. First, because nutrient distributions and physiological response may change with time, any estimate of uptake based on single measurements of nutrient concentration and Michaelis-Menten parameters will reflect instantaneous conditions only (Goldman and Glibert 1983). Second, nitrogen-15 in the substrate during incubation could be significantly diluted by heterotrophic regeneration of inorganic nitrogen. Because such an effect could lead to underestimation of inorganic nitrogen-14 uptake, rates of isotope dilution in Lake Bonney remain tentative until further investigations are made.

Addition of carbon-14-bicarbonate to water samples and subsequent incubation allowed for measurements of inorganic carbon uptake (primary production) in units of micromoles per liter per hour. To achieve similar units and to compute total inorganic nitrogen uptake, the specific uptake rates of NH₄⁺ and NO₃^- (V) were added and then multiplied by the concentration of particulate nitrogen at the depths where rates were to be obtained. Ratio of inorganic carbon uptake to inorganic nitrogen uptake (figure 3) indicate that nitrogen limitation, if present, is most pronounced above the layer of intense chemical stratification. That nitrogen uptake exceeds carbon uptake below 15 meters may represent a dominance of heterotrophic nitrogen utilization below this depth.

In conclusion, our preliminary study suggests that NH₄⁺ is the primary nitrogenous nutrient supporting microplankton production above 20 m in Lake Bonney. Substrate kinetics experiments suggest a high affinity for this nutrient, and nutrient profiles indicate that it is more abundant than NO₃^- and nitrite (NO₂^-) combined. Our future investigations of heterotrophic regeneration combined with direct measurement of inorganic nitrogen uptake in the water column will clarify the exact relationship between nutrient transformations and planktonic activity.

We thank T. Sharp for conducting many of the field experiments. This work was supported in part by National Science Foundation grant OPP 91-17907 to John Priscu.

References

Dodds, W.K., J.C. Priscu, and B.K. Ellis. 1991. Seasonal uptake and regeneration of inorganic nitrogen and phosphorus in a large oligotrophic lake: Size fractionation and antibiotic treatment. Journal of Plankton Research, 13(6), 1339-1358.

Dugdale, R.C., and J.J. Goering. 1967. Uptake of new and regenerated forms of nitrogen in primary productivity. Limnology and Oceanography, 12(2), 196-206.

Goldman, J.C., and P.M. Glibert. 1983. Inorganic nitrogen uptake in phytoplankton. In E.J. Carpenter and D.G. Capone (Eds.), Nitrogen and the marine environment. New York: Academic Press.

Priddle, J., I. Hawes, J.C. Ellis-Evans, and T.J. Smith. 1986. Antarctic aquatic ecosystems as habitats for phytoplankton. Biology Review, 61, 199-238.

Priscu, J.C., W.F. Vincent, and C. Howard-Williams. 1989. Inorganic nitrogen uptake and regeneration in Lakes Fryxell and Vanda, Antarctica. Journal of Plankton Research, 11(2), 335-351.

Priscu, J.C., B.B. Ward, and M.T. Downes. 1993. Water column transformations of nitrogen in Lake Bonney, a perennially ice-covered antarctic lake. Antarctic Journal of the U.S., 28(5).

Sharp, T.R., and J.C. Priscu. 1990. Ambient nutrient levels and the effects of nutrient enrichment on primary productivity in Lake Bonney. Antarctic Journal of the U.S., 25(5), 226-227.

Timperly, M.H., and J.C. Priscu. 1986. Determination of nitrogen-15 by emission spectrometry using an atomic absorption spectrometer. Analyst, 111, 23-28.