The deep-sea benthic community at the Porcupine Abyssal Plain (NE Atlantic) is a highly food limited system. The annual input of sedimenting phytodetritus, which reaches the sea floor around May/June, is the major input of energy. The relative trophic position of the most abundant components of the benthos (90 species or higher taxonomic groups), including meiofaunal, macrofaunal, and megafaunal organisms, was evaluated by stable isotope analysis. The majority of the macro-and megafaunal organisms investigated were deposit feeders (N=35), less numerous were suspension feeders (N=17) and predators/scavengers (N=29). Stable nitrogen values overlap and cover a large range within feeding types, indicating a strong overlap in food sources and a high degree of competition for food. Suspension feeders, mainly cnidarians, have a broad trophic spectrum through feeding on resuspended material as well as capturing pelagic prey; thus during the greater part of the year they can compensate for any shortage in sedimenting fresh POM. Benthic deposit feeders use a variety of feeding strategies to exploit their common food resource. The holothurians, the dominant megabenthic group at PAP, included some highly mobile species, which seem to be quite efficient in tracing and exploiting localised patches of nutritious phytodetritus. Other holothurian species, however, forage successfully on more refractory material, possibly assisted by enteric bacteria. Predators/scavengers fall into two groups, representing two major trophic pathways. Firstly, several of the invertebrate predators prey on deposit-feeding organisms and so are the end consumers of an exclusively benthic food web. Secondly, there are highly mobile benthopelagic predators/scavengers, which are a major link with the benthopelagic food web through their feeding on pelagic prey.Generally, within the benthic community at PAP competition for food is reduced by two alternative evolutionary adaptations: (1) specialization on slightly different food sources and (2) vertical expansion of the trophic spectrum. This leads to a rather complex food web, covering a total δ 15 N range of at least 10‰. 

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Equilibration technique suitable for a large amount of samples is described for hydrogen and oxygen isotope analyses of ground ice, especially ice wedges, including the sampling strategy and the analytical procedure as well as the calibration of the Finnigan MAT Delta-S mass spectrometer in June, 1999. Since for future analyses of ice wedges, a higher sampling resolution with limited sample volume is required, the limit of the equilibration technique for small water sample sizes of between 0.05 and 5 ml was checked. For water samples smaller than 1 ml, corresponding to a molar ratio [H2O]/[H2] of smaller than 0.994, a balance correction has to be applied. The experimental errors due to partial evaporation during evacuation, the balance calculation of the isotope equilibration process, the linearity as well as memory effects of the mass spectrometer for samples with large differences in delta18O and deltaD are tackled in this paper. In the polar regions of Northern Siberia without Late Pleistocene and Holocene glaciation, ground ice is used as an archive for paleoclimate studies. First results of stable isotope measurements on ice wedges clearly show a shift towards heavier isotopes and thus warmer winter temperatures as well as a change in the source of the precipitation between Late Pleistocene and Holocene. These results indicate the high potential of ground ice for paleoclimate studies.

We found unprecedentedly high abundances of microbially produced CH 4 in the anoxic deep waters of Lake Untersee, an oligotrophic, perennially ice-covered Antarctic freshwater lake. The maximum CH 4 concentration (approaching 21.8 Ϯ 1.4 mmol L Ϫ1 ) is one of the highest observed so far in a natural aquatic ecosystem. Although surficial lake sediments are the predominant source of CH 4 in Lake Untersee, methanogenesis occurs also within the anoxic waters. Radiocarbon labeling experiments show that H 2 /CO 2 reduction is the predominant methanogenic pathway (90-100%) both in the sediments and the water column, whereas acetate is only a minor CH 4 precursor. This result is consistent with the stable carbon isotope fractionation between coexisting CH 4 and CO 2 . In the water column, CH 4 is partly consumed by both aerobic and anaerobic microbial oxidation as evidenced by CH 4 concentration patterns, stable isotope analyses ( 13 C, 2 H), and 14 C-CH 4 assays. Dissimilatory sulfate reduction also occurs and peaks at 84 m water depth (1.83 mol SO 4 L Ϫ1 d Ϫ1 ). Intense methanogenesis in surficial lake sediments, diffusion of CH 4 from sediments to the water column, additional CH 4 production in the water column, gross CH 4 production higher than CH 4 consumption, and lack of mixing because of the permanent ice cover cause the exceptionally high CH 4 concentration in the lake. Our studies demonstrate that H 2 /CO 2 reduction may sometimes be the major pathway of methanogenesis in low-sulfate freshwater environments even at low temperatures. This pathway is obviously more important in Antarctic lakes than hitherto assumed.As an environmentally important greenhouse gas, methane (CH 4 ) plays a significant role in the global climatic system. Studies of CH 4 cycling in various environments is therefore of fundamental interest. Biological processes are the primary source (ca. 80-90%) of atmospheric CH 4 (e.g., Cicerone and AcknowledgmentsWe thank M. Schwab, D. Schachtschneider, and G. Müller for assistance in the field; K. Weingart, W. Städter, G. Schäfer, and L. Schönicke for technical assistance. H. Kämpf kindly provided SO 4 , NH 4 , and DIC analyses. We thank A. Mackensen for providing the ␦ 13 C-DIC analyses, and P. Harting for providing solubility data. The acetate analyses were done by L. Dulov, Institute of Microbiology, Moscow. The manuscript benefited from critical comments made

Lake Untersee is the largest freshwater lake in the interior of East Antarctica. It is a perennially ice-covered, max. 169 m deep, ultra-oligotrophic lake. In contrast to earlier studies, we found clear evidence for physical and chemical stratification in the summer of 1991-92. However, the stratification was restricted to a trough, c. 500 m wide and up to 105 m deep, in the south-western part of the lake. There, the water body was distinctly stratified as indicated by sharp vertical gradients of temperature, pH, dissolved oxygen, and electrical conductivity. The water column was anoxic below 80 m. The chemical stratification is also indicated by changes of ionic ratios. Moreover, there was some evidence for methanogenesis and bacterial sulphate reduction in Lake Untersee.

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Large glacial boulders, up to several metres in diameter, resting on the lake ice are a remarkable feature of Lake Untersee (71°21'S, 13°28'E), an ice-dammed, perennially frozen freshwater lake in the Ottovon-Gruber-Gebirge (Gruber Mountains) of central Queen Maud Land, East Antarctica. A geodetic survey of such ice-rafted boulders was made over two summer seasons to determine the direction and velocity of their movement. They are transported between 3.9 and 11.1 m annually and the residence time of the boulders is estimated at approximately 500 years. Lake Untersee must have been permanently covered with lake ice for at least that long.