Porewater profiles clearly show that nitrate and ammonia both diffuse out of sediments into bottom water (Gobeil et al., 1991) and therefore sediments act as a source of regenerated nitrogen. However, the budget suggests a net loss of DIN over the shelf, most likely due to denitrification in sediments (cf. Christensen, 1994). This loss (1.9 × 109 mol yr-1) implies a denitrification rate of 0.1 mmol m-2 day-1 which is much lower than rates measured in Bering and Chukchi sediments (1 mmol-2 day-1 - Devol et al., 1997). This discrepancy may be partly explained by organic nitrogen which is often an important component of pristine rivers, equaling or exceeding DIN (Caraco, pers. comm.).
3. CONCLUSIONS
The Mackenzie shelf is dominated by the Mackenzie River. Inputs from the river are estimated sufficiently well to provide one of the better constraints for shelf budgets (water, sediments). Nevertheless, dissolved organic nutrients could be a significant component of bioavailable nutrient supply and yet they are rarely quantified. Ice and brine entrain and export dissolved and solid-phase components and are vulnerable to change, yet we do not have good quantitative estimates for either of them or the components they carry. Particles provide an important medium of exchange for many components, including nutrients, but we do not have reliable estimates for imports or exports. Finally, shelf-edge exchanges remain the largest and most uncertain component in shelf budgets, and will have to be better quantified before change can be predicted confidently.
4. REFERENCES
Christensen J. P,, 1994: Carbon export from continental shelves, denitrification and atmospheric carbon dioxide. Cont. Shelf Res. 14, 547-576.
Devol A. H., Codispoti L. A., and Christensen J. P,, 1997: Summer and winter denitrification rates in western Arctic shelf sediments. Cont. Shelf Res. 17, 1029-1050.
Gobeil C., Paton D., McLaughlin F. A., Macdonald R. W.,Paquette G., Clermont Y, and Lebeuf M., 1991:Donnes geochimiques sur les eaux interstitielies et les sediments de la mer de Beaufort. Rapp. stat. can. hydrogr. sci. ocean. 101: iv +92pp.
Gordon, D.C, P.R. Boudreau, K.H. Mann, J.-E. Ong, W.L. Silvert, S.V. Smith, G, Wattayakorn, F. Wulff and T. Yanagi, 1995: LOICZ Biogeochemical Modelling Guidelines. LOICZ/R&S/95-5, vi+96pp, The Netherlands.
Hedges J, I., 1992: Global biogeochemical cycles: progress and problems. Mar. Chem. 39, 67-93.
Macdonald R, W., Paton D. W., Carmack E. C., and Omstedt A., 1995: The freshwater budget and under-ice spreading of Mackenzie River water in the Canadian Beaufort Sea based on salinity and 180/160 measurements in water and ice. J. Geophys. Res. 100(C1),895-919.
Macdonald R. W., Solomon S. M., Cranston R. E., Welch H, E., Yunker M. B., and Gobeil C., 1998: A sediment and organic carbon budget for the Canadian Beaufort shelf. Mar. Geol., in press.
Macdonald R. W., Wong C, S,, and Erickson P. E,, 1987: The distribution of nutrients in the southeastern Beaufor1 Sea: implications for water circulation and primary production. J. Geophys. Res. 92(C3), 2939-2952.
Melling H., 1993: The formation of a haline shelf front in wintertime in an ice-covered Arctic sea. Continental Shelf Research 13, 1123-1147.
Melling H. and Lewis E. L,, 1982: Shelf drainage flows in the Beaufort Sea and their effect on the Arctic Ocean pycnocline, Deep-Sea Res. 29, 967-985.
Melling H. and Riedel D. A., 1996: Development of seasonal pack ice in the Beaufort Sea during the winter of 1991-1992: A view from below. J. Geophys. Res.101(C5), 11975-11991.
Reimnitz E., McCormick M., McDougall K., and Brouwers E., 1993: Sediment export by ice rafting from a coastal polynya, Arctic Alaska, U.S,A. Arctic and Alpine Res. 25, 83-98.
Ruttenberg K. 1993: Reassessment of the oceanic residence time of phosphorus. Chemical Geology 107, 405-409.
Ruttenberg K. and Goni M. A., 1996: Phosphorus distribution, C:N:P ratios, and δ13C in arctic, temperate, and tropical coastal sediments: tools for characterizing bulk sedimentary matter. Mar. Geol. 139, 123-145.
