Permanently ice-covered Lake Bonney presents a special situation where turbulence and upper trophic levels are virtually nonexistent. During our past studies on the photobiology of both the east and west lobes of this lake (Priscu et al. 1990), we made preliminary vertical profiles of nutrients, nitrous oxide, and bacterial number and activity. These data reveal several unique features. First, greater then 250 micromolar nitrate and 30 micromolar nitrite exist in a region of the east lobe which is apparently devoid of oxygen (figure 1A, B). This is the first case, to our knowledge, of the existence of a large nitrate pool in apparently anoxic water. Second, nitrous oxide levels in the east lobe just below the chemocline reach almost 40 micromolar nitrogen (figure 1B). This value exceeds air saturation by over 500,000 percent and is the highest recorded in a natural aquatic system. Lack of oxidized nitrogen (for example, nitrate, nitrite, nitrous oxide) in the anoxic zone below the chemocline of the west lobe (figure 2A, B) implies intense denitrification, which conforms to conventional biogeochemical concepts for nitrogen within this lobe.

A tentative calculation of nitrous oxide diffusion across the 20.5-meter (m) plane of the east lobe (using a molecular diffusion coefficient of 10^-6 square centimeter per second) yields an upward flux of 1.2 micrograms nitrous oxide-nitrogen per square meter per day which, given the surface area of the lake and assuming a constant annual nitrous oxide gradient, equates to about 1.3 kilograms (kg) nitrous oxide-nitrogen per year. Our preliminary measurements of nitrous oxide within the air above the lake ice on the east lobe revealed that they were about 40 percent greater than the global average for nitrous oxide or that measured over nearby McMurdo Sound (Rasmussen and Khalil 1986; Priscu et al. 1990) indicating that nitrous oxide escapes from the lake despite a permanent ice cap. Although this is a small outward flux relative to the global annual production of nitrous oxide (about 2x10⁷ kg nitrous oxide-nitrogen per year; Thiemens and Trogler 1991), it may have regional importance.

These unusual results (from the east lobe), in concert with previous reports that nitrogen can limit phytoplankton growth during certain periods (Sharp and Priscu 1990), have led us to propose the following hypotheses:

• Regeneration of nitrate and nitrite by nitrifying bacteria and ammonium by heterotrophs supports primary production in the upper trophogenic zone whereas ammonium diffusing upward across the chemocline supports the majority of production in the lower trophogenic zone, particularly in the west lobe.
• Nitrous oxide is produced in the chemocline of both lobes by nitrification and nitrifier denitrification (of nitrite). This process is most pronounced in the east lobe.
• The nitrite maxima are the result of nitrification in excess of denitrification.
• Nitrous oxide and nitrite levels in the chemocline of the west lobe are considerably lower than in the east lobe because they act as terminal electron acceptors for dinitrifiers in the former, that is, denitrification is not a sink for nitrous oxide or nitrite in the east lobe.
• The production of nitrous oxide in the east lobe significantly lowers upward diffusion of available "new" nitrogen for phytoplankton primary production.
• Heterotrophic denitrification does not occur in the east lobe whereas high rates of denitrification exist in the deep waters (below 25 m) of the west lobe. The latter acts as a major loss of nitrogen from the west lobe of the lake.
• Lake Bonney is an atmospheric source of nitrous oxide.

These hypotheses have formed the basis of our current 3-year study on Lake Bonney. The available background data for the two lobes of Lake Bonney, each of which has distinctive characteristics with respect to nitrogen dynamics, indicate that Lake Bonney provides an ideal situation to test these hypotheses. The purpose of this report is to present some of our preliminary results (notably those for nitrous oxide) and discuss them briefly within the context of hypotheses derived from them. This article also serves as an introduction to the reports of Ward, Cockroft, and Priscu (Antarctic Journal, in this issue), Woolston and Priscu (Antarctic Journal, in this issue), and Smith and Priscu (,i>Antarctic Journal, in this issue) which address various aspects of the proposed hypotheses.

Bartlett et al. (Antarctic Journal, in this issue) describe a number of techniques that had to be developed (to overcome the high salt content common to antarctic lakes) to use nitrogen-15 successfully as an isotope for the measurement of nitrogen transformations within Lake Bonney. Finally, Wing and Priscu (Antarctic Journal, in this issue) present results from a microbial community we discovered within the permanent icecap of Lake Bonney. We plan to propose further research to determine the biogeochemical importance of this ice community to the lake ecosystem.

Our study incorporates direct-rate measurements of all major nitrogen transformations together with immunochemical and molecular probes for microorganisms responsible for specific transformations. These techniques, together with measurements of various nitrogen species within the lake over time, will provide a better understanding of biological nitrogen transformations in high-latitude aquatic systems and will complement recent studies on nitrogen transformations in arctic systems (Alexander, Whalen, and Klingensmith 1989).

This work was supported in part by National Science Foundation grant OPP 91-17907 to John C. Priscu.

References

Alexander, V. S.C. Whalen, and K.M. Klingensmith. 1989. Nitrogen cycling in arctic lakes and ponds. In W.F. Vincent and J.C. Ellis-Evans (Eds.), High latitude limnology. Dondrecht, The Netherlands: Kluwer Academic Publishers.

Bartlett, R., J.C. Priscu, and C. Woolston. 1993. Influence of high salinity levels on 15-nitrogen extraction efficiency in Lake Bonney, Antarctica. Antarctic Journal of the U.S., 28(5).

Priscu, J.C., M.T. Downes, L.R. Priscu, A.C. Palmisano, and C.W. Sullivan. 1990. Dynamics of ammonium oxidizer activity and nitrous oxide (N₂O) within and beneath antarctic sea ice. Marine Ecology Progress Series, 62(1), 37-46.

Priscu, J.C., T.R. Sharp, M.P. Lizotte, and P.J. Neale. 1990. Photoadaptation by phytoplankton in permanently ice-covered antarctic lakes: Response to a non-turbulent environment. Antarctic Journal of the U.S., 25(5), 221-222.

Rasmussen, R.A., and M.A.K. Khalil. 1986. Atmospheric trace gases: Trends and distributions over the last decade. Science, 232, 1623-1624.

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.

Smith, J.J., and J.C. Priscu. 1993. Microbial respiration potential in Lake Bonney using a novel tetrazolium-reduction method. Antarctic Journal of the U.S., 28(5).

Thiemens, M.H., and W.C. Trogler. 1991. Nylon production: An unknown source of atmospheric nitrous oxide. Science, 251, 932-934.

Ward, B.B., Cockroft, A.R., and J.C. Priscu. 1993. Nitrifying and denitrifying bacteria in Lake Bonney. Antarctic Journal of the U.S., 28(5).

Wing, K.T., and J.C. Priscu. 1993. Microalgal communities in the permanent icecap of Lake Bonney, Antarctica: Relationships between chlorophyll a, gravel, and nutrients. Antarctic Journal of the U.S., 28(5).

Woolston, C., and J.C. Priscu. 1993. Phytoplankton utilization of ammonium and nitrate in Lake Bonney: A preliminary assessment. Antarctic Journal of the U.S., 28(5).