Life on the edge: the plankton and chemistry of Beaver Lake, an ultra-oligotrophic epishelf lake, Antarctica JOHANNA LAYBOURN-PARRY,* WENDY C. QUAYLE,² TRACEY HENSHAW,*² ANDREW RUDDELL³ and HARVEY J. MARCHANT§ *Institute of Environmental Sciences, University of Nottingham, Nottingham, UK ²The British Antarctic Survey, High Cross, Cambridge, UK ³Antarctic Cooperative Research Centre, University of Tasmania, Hobart, Tasmania, Australia §Australian Antarctic Division, Channel Highway, Kingston, Tasmania, Australia SUMMARY 1. Beaver Lake, a large epishelf lake in eastern Antarctica was sampled on two occasions during the austral summer of 2000. Two sites, one 1 km offshore and another 6 km offshore were sampled at intervals to depths of 40 and 110 m, respectively. 2. The lake is an end member of ultra-oligotrophic lake systems with a very low carbon pool. Dissolved organic carbon concentrations ranged between 95 and 652 lgL ±1 . Nutrient levels were generally low with soluble reactive phosphorus ranging from undetectable to 8.4 lgL ±1 , ammonium ranged between 1.8 and 5.0 lgL ±1 , nitrate from undetectable to 161 lgL ±1 and nitrite 1.1±5.3 lgL ±1 . 3. Chlorophyll a concentrations (0.39±4.38 lgL ±1 ) showed an unusual distribution with the highest levels close to the lake bottom at the offshore site (110 m) where the phototrophic nano¯agellates (PNAN) displayed strong auto¯uorescence. 4. Bacterial concentrations were low, with a maximum of 7.60 ´ 10 7 L ±1 , as were the concentrations of heterotrophic nano¯agellates that exploit them. 5. Primary production ranged between 19.7 and 25.49 lgCL ±1 day ±1 and bacterial production from 0.32 to 1.15 lgCL ±1 day ±1 . 6. In common with other continental Antarctic lakes, the system was dominated by a microbial plankton. However, a dwarf variety of the calanoid copepod, Boeckella poppei, occurred below 25 m at concentrations of 3±5 L ±1 . 7. The data suggest that primary production and bacterial production were not limited by nutrient availability, but by other factors, e.g. in the case of bacterial production by organic carbon concentrations and primary production by low temperatures. 1 Introduction Most Antarctic lakes rank as oligotrophic when placed in a global context. They are subject to continuous low temperatures, low nutrient inputs, long or continuous ice-cover and low annual levels of photosynthetically active radiation (PAR). The paucity of organic carbon in these systems has resulted in truncated food chains with few or no zooplankton and no ®sh (Laybourn- Parry et al., 1995; Laybourn-Parry et al., 1997; James et al., 1998; Priscu et al., 1999). These systems are dominated by a microbial plankton which comprises the major or sole component of the microbial loop proposed by Azam et al. (1983), rather than being an integral component of a complex food web which includes numerous invertebrate and vertebrate meta- zoans. In such extreme lacustrine environments, micro-organisms and a few invertebrate species are the only life which can survive. Antarctica possesses an extraordinary array of lake types including hypersaline monomictic lakes, saline meromictic lakes, amictic and monomictic freshwater Correspondence: J. Laybourn-Parry, Institute of Environmental Sciences, University of Nottingham, University Park, Notting- ham, NG7 2RD, UK. E-mail: j.laybourn-parry@nottingham.ac.uk Freshwater Biology (2001) 46, 1205±1217 Ó 2001 Blackwell Science Ltd 1205