Continental Antarctic Lakes Research Articles

Culturable Diversity of Heterotrophic Bacteria in Forlidas Pond (Pensacola Mountains) and Lundström Lake (Shackleton Range), Antarctica

Cultivation techniques were used to study the heterotrophic bacterial diversity in two microbial mat samples originating from the littoral zone of two continental Antarctic lakes (Forlidas Pond and Lundström Lake) in the Dufek Massif (within the Pensacola Mountains group of the Transantarctic Mountains) and Shackleton Range, respectively. Nearly 800 isolates were picked after incubation on several growth media at different temperatures. They were grouped using a whole-genome fingerprinting technique, repetitive element palindromic PCR and partial 16S rRNA gene sequencing. Phylogenetic analysis of the complete 16S rRNA gene sequences of 82 representatives showed that the isolates belonged to four major phylogenetic groups: Actinobacteria, Bacteroidetes, Proteobacteria and Firmicutes. A relatively large difference between the samples was apparent. Forlidas Pond is a completely frozen water body underlain by hypersaline brine, with summer thaw forming a slightly saline littoral moat. This was reflected in the bacterial diversity with a dominance of isolates belonging to Firmicutes, whereas isolates from the freshwater Lundström Lake revealed a dominance of Actinobacteria. A total of 42 different genera were recovered, including first records from Antarctica for Albidiferax, Bosea, Curvibacter, Luteimonas, Ornithinibacillus, Pseudoxanthomonas, Sphingopyxis and Spirosoma. Additionally, a considerable number of potential new species and new genera were recovered distributed over different phylogenetic groups. For several species where previously only the type strain was available in cultivation, we report additional strains. Comparison with public databases showed that overall, 72% of the phylotypes are cosmopolitan whereas 23% are currently only known from Antarctica. However, for the Bacteroidetes, the majority of the phylotypes recovered are at present known only from Antarctica and many of these represent previously unknown species.

Salinity, depth and the structure and composition of microbial mats in continental Antarctic lakes

Summary1. Lakes and ponds in the Larsemann Hills and Bølingen Islands (East‐Antarctica) were characterised by cyanobacteria‐dominated, benthic microbial mats. A 56‐lake dataset representing the limnological diversity among the more than 150 lakes and ponds in the region was developed to identify and quantify the abiotic conditions associated with cyanobacterial and diatom communities.2. Limnological diversity in the lakes of the Larsemann Hills and Bølingen Islands was associated primarily with conductivity and conductivity‐related variables (concentrations of major ions and alkalinity), and variation in lake morphometry (depth, catchment and lake area). Low concentrations of pigments, phosphate, nitrogen, DOC and TOC in the water column of most lakes suggest extremely low water column productivity and hence high water clarity, and may thus contribute to the ecological success of benthic microbial mats in this region.3. Benthic communities consisted of prostrate and sometimes finely laminated mats, flake mats, epilithic and interstitial microbial mats. Mat physiognomy and carotenoid/chlorophyll ratios were strongly related to lake depth, but not to conductivity.4. Morphological‐taxonomic analyses revealed the presence of 26 diatom morphospecies and 33 cyanobacterial morphotypes. Mats of shallow lakes (interstitial and flake mats) and those of deeper lakes (prostrate mats) were characterised by different dominant cyanobacterial morphotypes. No relationship was found between the distribution of these morphotypes and conductivity. In contrast, variation in diatom species composition was strongly related to both lake depth and conductivity. Shallow ponds were mainly characterised by aerial diatoms (e.g. Diadesmis cf. perpusilla and Hantzschia spp.). In deep lakes, communities were dominated by Psammothidium abundans and Stauroforma inermis. Lakes with conductivities higher than ±1.5 mS cm−1 became susceptible to freezing out of salts and hence pronounced conductivity fluctuations. In these lakes P. abundans and S. inermis were replaced by Amphora veneta. Stomatocysts were important only in shallow freshwater lakes.5. Ice cover influenced microbial mat structure and composition both directly by physical disturbance in shallow lakes and by influencing light availability in deeper lakes, as well as indirectly by generating conductivity increases and promoting the development of seasonal anoxia.6. The relationships between diatom species composition and conductivity, and diatom species composition and depth, were statistically significant. Transfer functions based on these data can therefore be used in paleolimnological reconstruction to infer changes in the precipitation–evaporation balance in continental Antarctic lakes.

Modelling diatom responses to climate induced fluctuations in the moisture balance in continental Antarctic lakes

The water chemistry of lake systems on the edge of the Antarctic continent responds quickly to changes in the moisture balance. This is expressed as increasing salinity and decreasing lake water level during dry periods, and the opposite during wet periods. The diatom composition of the lakes also changes with these fluctuations in salinity and lake water depth. This is important, as their siliceous remains become incorporated into lake sediments and can provide long-term records of past salinity using transfer functions. In order to develop transfer functions, diatoms and water chemistry data were inter-calibrated from five different East Antarctic oases, namely the Larsemann Hills, the Bolingen Islands, the Vestfold Hills, the Rauer Islands and the Windmill Islands. Results indicate that salinity is the most important environmental variable explaining the variance in the diatom flora in East Antarctic lakes. In oligo- saline lakes the variance is mainly explained by lake water depth. This dataset was used to construct a weighted averaging transfer function for salinity in order to infer historical changes in the moisture balance. This model has a jack-knifed r2 of 0.83 and a RMSEP of 0.31. The disadvantage of this transfer function is that salinity changes in oligo-saline lakes are reconstructed inaccurately due to the ‘edge effect’ and due to the low species turnover along the salinity gradient at its lower end. In order to infer changes in the moisture balance in these lakes, a second transfer function using weighted averaging partial least squares (with two components) for depth was constructed. This model has a jack-knifed r2 of 0.76 and a RMSEP of 0.22. Both transfer functions can be used to infer climate driven changes in the moisture balance in lake sediment cores from oligo-, hypo-, meso- and hyper-saline lakes in East Antarctic oases between 102–75°E. The transfer function for lake water depth is promising to track trends in the moisture balance of small freshwater lakes, where changes in shallow and deep-water sediments are readily reflected in changing diatom composition.

Life on the edge: the plankton and chemistry of Beaver Lake, an ultra‐oligotrophic epishelf lake, Antarctica

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 μg L–1. Nutrient levels were generally low with soluble reactive phosphorus ranging from undetectable to 8.4 μg L–1, ammonium ranged between 1.8 and 5.0 μg L–1, nitrate from undetectable to 161 μg L–1 and nitrite 1.1–5.3 μg L–1.3. Chlorophyll a concentrations (0.39–4.38 μg L–1) showed an unusual distribution with the highest levels close to the lake bottom at the offshore site (110 m) where the phototrophic nanoflagellates (PNAN) displayed strong autofluorescence.4. Bacterial concentrations were low, with a maximum of 7.60 × 107 L–1, as were the concentrations of heterotrophic nanoflagellates that exploit them.5. Primary production ranged between 19.7 and 25.49 μg C L–1 day–1 and bacterial production from 0.32 to 1.15 μg C L–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.

Grazing rates of a calanoid copepod ( Paralabidocera antarctica ) in a continental Antarctic lake

The presence of crustaceans in some Antarctic lakes adds a level of complexity to the food webs of these environments. The grazing pressure of the crustaceans on phytoplankton and other protists, and the role they play in controlling food web structure, has not yet been examined in any detail. This paper presents the first data on grazing pressure of crustacean zooplankton in continental Antarctic lakes. In this study the grazing rates of Paralabidocera antarctica in saline Ace Lake, Vestfold Hills, were assessed using a radio-tracer method. Clearance rates ranged from approximately 1 to 8 ml ind.−1 day−1, depending on developmental stage. Due to their high abundance, nauplii exhibited the highest daily carbon ingestion rates. The population cleared about 3% of the water column per day, accounting for a significant fraction of primary production. P. antarctica has a considerable impact on the carbon cycle in Ace Lake and, by inference, in the other lakes in which it occurs.

Temporal plankton dynamics in an oligotrophic maritime Antarctic lake

1 The population density, diversity and productivity of the microbial plankton in an oligotrophic maritime Antarctic lake were studied for a 15-month period between December 1994 and February 1996. 2 In the lake, concentrations of nutrients and dissolved organic carbon were uniformly low, temperature varied over a small annual range of 0.1–3 °C, and the surface was ice-covered except during a period of approximately 6 weeks in summer. 3 The total of 57 morphotypes of protozoa observed during the study is a higher taxonomic diversity than previously reported from continental Antarctic lakes, but lower than that found in more eutrophic maritime Antarctic lakes. Likewise, planktonic abundance and productivity were lower than has been reported in other lakes on Signy Island, but generally higher than those of lakes on the Antarctic continent. 4 There were marked seasonal and interannual variations in planktonic population density. 5 Chlorophyll a concentrations ranged from undetectable to 4.2 µg L-1 and the greatest rate of primary productivity measured was 4.5 mg C m-3 h-1. The phytoplankton was dominated by small chlorophytes and chrysophytes, with phototrophic nanoflagellate abundance ranging from 1.1 × 103 to 1.2 × 107 L-1. 6 Bacterial densities of 3.6 × 108 to 1.9 × 1010 L-1 were recorded and bacterial productivity reached a peak of 0.36 µg C L-1 h-1. Numbers of heterotrophic nanoflagellates between 5.0 × 104 and 1.8 × 107 L-1, and of ciliates from undetectable to 1.1 × 104 L-1 were observed. Naked amoebae were usually rare, but occasionally reached peaks of up to 1.5 × 103 L-1.

Seasonal dynamics of the planktonic microbial community in a maritime Antarctic lake undergoing eutrophication

The abundance, biomass and community structure of phytoplankton, bacterioplankton and protozooplankton in a maritime Antarctic lake were determined at approximately monthly intervals from December 1994 to February 1996 and compared with data from earlier studies. Heywood Lake has become significantly eutrophic during the last three decades because of excreta from the expanding fur seal population in its catchment. Marked seasonal variations in the abundance, composition and productivity of biota were correlated with the seasonality of both physical factors and nutrient levels. Protozooplankton were abundant, diverse and usually dominated by heterotrophic nanoflagellates (HNF), with HNF abundance peaking at 2.35 × 107 l–1 in summer. High numbers of naked amoebae were sometimes present, reaching a maximum of 4.8 × 103 l–1 in March. An estimated 89 species of protozoa were observed during the study, indicating substantially more diversity than is found in continental Antarctic lakes. Diversity was highest in spring and lowest in winter, when the entire water column became anoxic and the plankton were dominated by bacteria and a few species of relatively large anaerobic flagellates. The current status of the lake is compared with data for continental Antarctic and lower latitude lakes. Earlier studies of biota and physical/chemical parameters in Heywood Lake are used to examine the effects of eutrophication over three decades. Observed changes include increased microbial abundance and changes in both community structure and seasonal patterns.

Temporal plankton dynamics in a maritime Antarctic lake

Changes in abundance, diversity and productivity of plankton in a maritime Antarctic lake were studied between December 1994 and February 1996. There were large intra- and inter-annual fluctuations in population densities, which were related to changing physical and chemical parameters. The plankton included an abundant protozoan population, comprising at least 66 taxa. This is amongst the highest diversities so far reported for Antarctic lakes. Heterotrophic nanoflagellates (HNF) were the most common, with densities between 4 × 104 and 1.5 × 107 l-1, and were the largest contributors to total protozoan carbon biomass for most of the year. Ciliate abundance varied from 4.0 × 100 to 1.4 × 104 l-1 and included taxa from several trophic levels. Numbers of naked amoebae were usually low but occasional patches of high density occurred. An anoxic sump developed in bottom waters at the end of the winter and this contained a distinct population of anaerobic HNF. There was evidence that the excrement of increasing fur seal numbers in the lake catchment over the last 15 years is having an impact on the lake ecosystem. The Chl-a maximum of 49 μg l-1 and primary productivity of up to 40 mgC m-3 h-1 were significantly higher than those reported by previous studies of this lake and continental Antarctic lakes. Increased summer bacterial abundance and productivity, together with higher winter nutrient concentrations, were also noted.

The microbial plankton of Lake Fryxell, southern Victoria Land, Antarctica during the summers of 1992 and 1994

Samples collected from Lake Fryxell, southern Victoria Land, Antarctica in January 1992 and 1994 were analysed for the abundance of bacterioplankton and the diversity and abundance of protistan plankton. At the times of sampling, 14 ciliate species and 10 species of autotrophic flagellate were recorded. The samples contained two species of rotifer (Philodina spp.), which formed the first record of planktonic metazoans in the Dry Valley lakes of this region of Antarctica. Bacterial concentrations ranged between 1.0 and 3.8×108 l-1 in the upper oxic waters increasing to 20×108 l-1 in the anoxic waters. Heterotrophic flagellates decreased in abundance down the oxygenated water column, disappearing completely at 9 m, and ranged between 0.28 and 7.39×105 l-1 in abundance. Autotrophic flagellates were much more abundant exhibiting a number of distinct peaks down the water column (1.89–25.3×108 l-1). The ciliated protozoa were very abundant (up to 7720 l-1) in relation to flagellate and bacterial numbers, typical of oligotrophic lakes world-wide. The distribution of the protistan plankton showed marked zonation, probably in response to the differing salinity and temperature gradients in the water column. Possible trophic interactions are discussed and comparisons with other continental Antarctic lakes made.

Seasonal dynamics of the planktonic community in Lake Druzhby, Princess Elizabeth Land, Eastern Antarctica

1. The temporal abundance and composition of the plankton of a continental Antarctic lake (Lake Druzhby) situated in the Vestfold Hills, Eastern Antarctica was investigated from December 1992 to December 1993. The system was dominated by microbial plankton (cyanobacteria, heterotrophic bacteria and protozoans) with few metazoans.2. Chlorophyll a concentrations ranged between 0.15 and 1.1 μg l–1 and showed highest levels from late winter to spring.3. Heterotrophic bacteria ranged between 75 and 250 × 106 l–1 with highest abundances in late winter/spring. Mean bacterial biovolumes showed considerable seasonal variation (0.05–0.31 μm3). Largest biovolumes occurred in summer and this was the time of highest community biomass.4. Heterotrophic nanoflagellates reached highest abundances in late summer (maximum 14 × 105 l–1). Their mean biovolume also exhibited considerable seasonal variation, ranging between 1.77 and 27.0 μm3, with largest size resulting in community biomass peaking in early summer. Ciliated protozoa were poorly represented and sparse. Phototrophic nanoflagellates were sparse in this lake; instead the phototrophic plankton was dominated by a small rod‐shaped cyanobacterium which constituted the largest carbon pool in the system. It was common throughout the year, its biomass peaking in autumn. Its presence is discussed in relation to lake morphometry and light climate.5. Heterotrophic flagellate grazing rates ranged from 6.78 bacteria cell–1 day–1 at 2 °C to 11.8 bacteria cell–1 day–1 at 4 °C. They remove around 2% of the bacterial carbon pool per day during summer and winter.6. Nutrient levels were low and recorded in pulses. Dissolved and particulate organic carbon were also low, usually less than 3 mg l–1 and 600 μg l–1, respectively. The carbon pools were derived from autochthonous sources. This lake system is driven by bottom‐up forces and lacks top‐down control, which fits into the picture currently seen for continental Antarctic lakes.