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The McMurdo Ice Shelf is in the northwestern corner of the Ross Ice Shelf, between Ross Island and Brown Peninsula. An area of about 1,500 km2 is known as Dirty Ice, an ablation zone covered by gravel. A large portion of this gravel originates from marine sediment, and much of the shelf ice is frozen seawater (7). During the summer melt, the area is covered by ponds of a wide size range between hummocks with a vertical profile as high as 20 m (12). Ponds form and disappear again over decades, leading to a wave-like cycling of the shelf surface through ponds and hummocks (2). Freezing, thawing, and evaporation often lead to pronounced solute gradients and water column stratification (9a). The bottoms of these ponds are covered with thick mats consisting of cyanobacteria, diatoms, and green algae (11, 12), below which is a layer of anaerobic sediment. Variations in chemical and physical conditions between the ponds lead to community differentiation within and between the mats and to differences in mat morphology, thus creating in a small area a variety of modern unlithified stromatolites unique on Earth (31). The mats harbor a small population of grazers, mainly the rotifer Philodinia gregaria, but their activity does not appear to have a key function in the ecosystem. Apart from the discrete ponds on the ice shelf, there are also ponds in estuaries. These ponds are connected by seawater at high tide and also contain diverse assemblages of algae and cyanobacteria with primary production characteristics similar to those for discrete ponds (8). Previous studies have shown that ponds may vary in conductivity, from highly saline to almost freshwater (12). While considerable knowledge has been gained about the photosynthetic activity and carbon flux attributed to the algal mats of these ponds, together with physical and chemical characteristics (2, 11), no studies on the processes occurring in the underlying pond sediments have been published. In this paper we describe some of the major heterotrophic processes occurring in the sediments of a low-salinity meltwater pond, Orange Pond, near Bratina Island. The rates of terminal anaerobic processes are given together with the contributions of these processes to terminal carbon and electron flow, and we discuss their significance for carbon flux in the mat-sediment ecocouple.

The acetylene block technique was used to assay denitrification in undisturbed sediment cores of an intertidal mud flat. Nitrogen loss due to this process was estimated at 1 to 41 kg of N/hectare (ha) per year. Anaerobic nitrate-saturated slurry of the same sediment had a denitrification capacity of 1,026 kg of N/ha per year. The acetylene block technique failed at low nitrate concentrations, so that denitrification at average in situ nitrate concentrations could not be determined. Denitrification followed zero-order kinetics at nitrate concentrations high enough to allow successful blockage of N 2 O reduction. Thus, an estimate of in situ rates based on kinetic parameters and in situ nitrate concentrations was impossible. No denitrification was observed in a slurry of the top 1.5 cm under aerated conditions and nitrate saturation. In undisturbed sediment, significant denitrification occurred in few discrete sites within a matrix of undetectable or low activity. Despite numerous errors contributing to the uncertainty of the estimate of in situ rates, the result obtained by this method was considered more valuable than the determination of denitrification capacities. Methods which include severe changes of physical and chemical parameters may frequently lead to overestimates of denitrification rates.