Abundance and diversity of soil invertebrates in the Windmill Islands region,East Antarctica

The harsh climate and patchy distribution of habitable terrestrial ecosystems constrain soil invertebrate communities in continental Antarctica. The Windmill Islands in East Antarctica have a relatively gentle climate by Antarctic standards, and the region supports some of the most well-developed moss beds on the continent. These moss beds and soils are known to sustain invertebrate communities dominated by nematodes, rotifers and tardigrades, but our knowledge of the diversity and composition of these communities remains limited. We extracted soil fauna from 74 soil samples representing a wide range of microhabitats, and 24 moss samples, collected at Clark Peninsula, Bailey Peninsula and Robinson Ridge in the Windmill Islands during the 2012–2013 austral summer. Invertebrates were present in all samples, but densities varied considerably both within and between sites with limited correlation with edaphic variables or cover type. Taxa found included two species of nematodes (Plectus murrayi; Plectus frigophilus), one mite (Nanorchestes antarcticus) as well as tardigrades and rotifers (enumerated only). No springtails were found in this study, but individuals of the genus Cryptopygus were later recovered from moss collected near Casey Station. The Windmill Islands soils and moss beds support dense populations of soil fauna. However, despite the relatively mild climate conditions and favorable soil properties, species diversity is low. The diversity is possibly limited by recent deglaciation and limited dispersal opportunities to the region. Given favorable local conditions, it is likely that colonizing species will perform well, whether these arrive by natural means or are accidentally introduced by humans.     

Depth distribution of euryhaline halophilic bacteria in Suribati Ike, a meromictic lake in East Antarctica

Euryhaline halophiles grow over a wide range of salinity, from <3% NaCl (seawater equivalent) to >15% NaCl and even saturation level (about 30% NaCl). Several species of euryhaline halophiles occur worldwide, especially in marine environments and also in aquatic and terrestrial habitats of the Antarctic ice-free areas. A biogeographic view of Antarctic halophiles is that their migration among lakes on land is more difficult than in marine setting. Ponds and lakes on land may thus serve as “islands” which facilitate the selection and separation of unique species. We isolated euryhaline halophiles from the saline lake, Suribati Ike, near Syowa Station and placed them into seven groups, each demonstrating a clear depth-related distribution. Six of the seven groups probably represent new species of the genera Halomonas and Marinobacter. This result suggests that Antarctic saline lakes exhibit high selectivity of unique euryhaline halophiles and possibly of other microbial groups.     

Sulfate distribution in a multi-basin, saline lake

Devils Lake in northeastern North Dakota (USA) is a terminal lake of glacial origin with a substantial sulfate content. Since 1940, the lake water level has risen over 13.5m and salinity has decreased. Devils Lake consists of a series of interconnected basins with sulfate concentration increasing in an eastward direction from 450mg/l in West Bay to 3000mg/l in East Devils Lake. Using sulfate as a tracer and linking measurements of specific conductance with observed time series of water surface elevations, wind, and water velocity, this paper illustrates the magnitude and dynamics of mass transport and exchange processes between the multiple basins of Devils Lake. Measurements show small horizontal salinity gradients within the lake basins, but large salinity gradients in some of the exchange zones connecting basins. Vertical salinity gradients were observed in small basins and near exchange zones with strong horizontal salinity differentials. These instances of vertical stratification were associated with buoyancy driven exchange flows and wind sheltering. Exchange flow velocities between selected basins were correlated with wind direction and speed. Interbasin exchange flows were found to reverse direction frequently in response to the prevailing wind direction. Flow direction frequencies in one specific exchange zone were 31% west to east, 48% east to west, and 21% bidirectional (stratified). At the lake level observed in 2000, time averaged, measured interbasin exchange flow rates were 30--40m³/s compared to tributary inflow rates to the lake of 3--9m³/s.     

Standing crop densities and distribution of Spirulina and benthic diatoms in East African alkaline saline lakes

SUMMARY. The diversity of Eastern Rift Valley alkaline saline lakes is illustrated and their temporal fluctuations in ionic concentration are emphasized.
The standing crop densities of phytoplanktonic blue-green algae of the genus Spirulina and benthic diatoms were measured. Spirulina abundance was measured as dry mass density and the validity of the gravimatric estimates was checked by chlorophyll analysis and microscopic counting. Benthic diatom standing crops were assessed by chlorophyll determinations. Spirulina density is extremely variable in time and may fluctuate over more than an order of magnitude. High density blooms of Spirulina are not a common characteristic feature of these lakes. The causes of changes in Spirulina standing crop and the associated alterations in the structure of the primary producer community are discussed and three qualitative states which may occur are described.
Similar standing crops of benthic diatoms (mean 45 mg m⁻² of chlorophyll-α) were found at all lakes wherever the water depth was shallow enough to allow light penetration to the sediment surface.     

An integrative study of a meromictic lake ecosystem in Antarctica

In nature, the complexity and structure of microbial communities varies widely, ranging from a few species to thousands of species, and from highly structured to highly unstructured communities. Here, we describe the identity and functional capacity of microbial populations within distinct layers of a pristine, marine-derived, meromictic (stratified) lake (Ace Lake) in Antarctica. Nine million open reading frames were analyzed, representing microbial samples taken from six depths of the lake size fractionated on sequential 3.0, 0.8 and 0.1 μm filters, and including metaproteome data from matching 0.1 μm filters. We determine how the interactions of members of this highly structured and moderately complex community define the biogeochemical fluxes throughout the entire lake. Our view is that the health of this delicate ecosystem is dictated by the effects of the polar light cycle on the dominant role of green sulfur bacteria in primary production and nutrient cycling, and the influence of viruses/phage and phage resistance on the cooperation between members of the microbial community right throughout the lake. To test our assertions, and develop a framework applicable to other microbially driven ecosystems, we developed a mathematical model that describes how cooperation within a microbial system is impacted by periodic fluctuations in environmental parameters on key populations of microorganisms. Our study reveals a mutualistic structure within the microbial community throughout the lake that has arisen as the result of mechanistic interactions between the physico-chemical parameters and the selection of individual members of the community. By exhaustively describing and modelling interactions in Ace Lake, we have developed an approach that may be applicable to learning how environmental perturbations affect the microbial dynamics in more complex aquatic systems.     

Limnological study of a Pantanal saline lake.

Limnological studies involving physical, chemical and biological aspects, with emphasis on the zooplankton community, were carried out in a saline lake of the Pantanal floodplain, in the subregion of Nhecolandia (Mato Grosso do Sul, Brazil), during the dry and wet seasons of the local hydrological cycle. The results show that the lake is not directly affected by the flood-pulse through superficial coalescence, probably receiving groundwater instead. Most of the measured variables showed extremely high values, particularly dissolved salts, pH, conductivity, nutrients and chlorophyll a. These values were highest during the dry period due to the system's low water volume. The zooplankton community was basically represented by Metacyclops mendocinus, which displayed high population densities throughout the periods studied. The low diversity recorded is congruent with other studies on continental saline water-bodies.     

Experimental investigations of benthic reentry by migrating meiobenthic copepods

The resettlement behavior of meiobenthic copepods, which actively migrated from sediments in a seagrass bed, was investigated in a shallow subtidal area in Tampa Bay, Florida, U.S.A. Experimental studies were conducted to determine whether meiobenthic copepods after emerging from sediments at sunset reenter the sedimentary substratum or select other subhabitats, water and seagrass blades. Migrating copepods were collected with emergence traps and transferred to experimental aquaria in the field which contained sediment, seagrass-blade and water treatments. Settlement into each type of treatment was measured in separate 2-h and 9-h experiments. Differences in densities of copepod taxa retrieved from emergence traps and introduced into experimental aquaria were recorded as were differing relative proportions of each copepod species returning to the substratum treatments. Settlement patterns of total copepods and three dominant copepod species, Zausodes arenicolus, Halicyclops sp. and Robertsonia hamata, departed from those expected by chance. The populations of R. hamata and Halicyclops sp. which settled were generally skewed towards males and a close matching of males and copepodites within treatment dishes was evident. Similar to nighttime-emergence patterns, timing and magnitude of postmigration reentry differs among copepod taxa and such reentry may be linked to reproductive events. Complex behavioral processes previously noted for fish and macrofaunal organisms in seagrass beds may also be important in recruitment and reassortment of meiobenthic copepods.     

Production and decomposition processes in a saline meromictic lake

Bacterial and phytoplankton cell number and productivity were measured in the mixolimnion and chemocline of saline meromictic Mahoney Lake during the spring (Apr.–May) and fall (Oct.) between 1982 and 1987. High levels of bacterial productivity (methyl ³H-thymidine incorporation), cell numbers, and heterotrophic assimilation of ¹⁴C-glucose and ¹⁴C-acetate in the mixolimnion shifted from near surface (1.5 m), at a secondary chemocline, to deeper water (4–7 m) as this zone of microstratification gradually weakened during a several year drying trend in the watershed. In the mixolimnion, bacterial carbon (13–261 µgC 1^–1) was often similar to phytoplankton carbon (44–300 µgC 1^–1) and represented between 14–57% of the total microbial (phytoplankton + bacteria) carbon depending on the depth interval. Phototrophic purple sulphur bacteria were stratified at the permanent primary chemocline (7.5–8.3 m) in a dense layer (POC 250 mg 1^–1, bacteriochlorophyll a 1500–70001µ 1^–1), where H₂S changed from 0.1 to 2.5 mM over a 0.2 m depth interval. This phototrophic bacterial layer contributed between 17–66% of the total primary production (115–476 mgC m^–2 d^–1) in the vertical water column. Microorganisms in the phototrophic bacterial layer showed a higher uptake rate for acetate (0.5–3.7 µC 1^–1 h^–1) than for glucose (0.3–1.4 µgC 1^–1 h^–1) and this heterotrophic activity as well as bacterial productivity were 1 to 2 orders of magnitude higher in the dense plate than in the mixolimnetic waters above. Primary phytoplanktonic production in the mixolimnion was limited by phosphorus while light penetration appeared to regulate phototrophic productivity of the purple sulphur bacteria.     

Exopolysaccharide production by a new Halomonas strain CRSS isolated from saline lake Cape Russell in Antarctica growing on complex and defined media

A haloalkalophilic Halomonas strain CRSS, isolated from salt sediments in Antarctica, produced exocellular polysaccharides (EPS) up to 2.9 g g(-1) dry cells. Acetate was the most efficient carbon source for EPS production. The composition of media strongly affected the nature of the polymers; a mannan and a xylo-mannan, were obtained when cells were grown on complex media. Acetate was the most efficient carbon source for EPS production and in presence of this substrate, a new polysaccharide, a fructo-glucan, was produced. The EPS fraction was composed by glucose, fructose, glucosamine and galactosamine in relative proportions of 1:0.7:0.3:trace.     

Depth-related changes in benthic diatom assemblages of a maritime Antarctic lake

Summary The epiphytic diatoms of filamentous matforming algae in Sombre Lake, Signy Island, South Orkneys, Antarctica were examined in a field study from January to March 1987. Results reveal 1) a shallow shelf assemblage, dominated by Fragilaria spp, with high percentages of empty frustules, and low numbers of viable cells and, 2) a mid-depth zone containing the most luxuriant epiphytic growth dominated by Achnanthes minutisima Kütz., Synedra rumpens var. familiaris (Kütz) Hust., and Cymbella minuta Hilse ex Rabenh., and 3) a deep water assemblage with low numbers of actively growing cells, dominated by Stauroneis anceps var. hyalina M. Perag. et Brun. in Hérib. The comparison of methods and their impact on ecological interpretation is discussed together with a consideration of the principal factors potentially influencing community structure.     

Abundance and feeding of microheterotrophic flagellates from a eutrophic lake

The growth of planktonic bacteria from a eutrophic lake was evaluated with microflagellate predators present and absent. Differential filtration (50 and 8 µm filters) was used to exclude ciliates and larger zoo-and phytoplankton from replicate experimental cultures. Additional filtration (1 µm filter) excluded heterotrophic microflagellates from a second set of experimental cultures, producing cultures that contained either bacteria and microflagellates or only bacteria. Growth of bacteria and microflagellates was evaluated by epifluorescent microscopy from repeated sampling over approximately 200 h. Bacterial numbers were reduced in the presence of microflagellates, and microflagellates were observed to contain bacterial prey. However, microflagellate numbers were high (about 10⁶ cells ml^-1) and were less than an order of magnitude lower than bacterial numbers. Bacteria growing in the presence of microflagellates did not show predator-prey population oscillations but had in-phase oscillations in numbers, suggesting that microflagellate predation in freshwater may not control numbers of planktonic bacteria. Clearance rates of heterotrophic microflagellates, estimated to be only 30 body volumes hr^-1, were insufficient to maintain flagellate growth, suggesting that other energy sources may be needed to maintain microflagellates in eutrophic freshwater ecosystems.     

Isabela Crater-Lake: a Mexican insular saline lake

Opinion This is a series of contributions, expressing explicit opinions with regard to contemporary topics in limnology. These Opinions are mostly based on new books with a large impact, but can also refer to other currently debated topics. Documented reactions to these contributions should be sent to the editor-in-chief; they will be considered for publication in subsequent issues.     

Eukaryotic phylotypes in aquatic moss pillars inhabiting a freshwater lake in East Antarctica, based on 18S rRNA gene analysis

Aquatic mosses of Leptobryum species form unique tower-like pillars of vegetation termed “moss pillars” in Antarctic lakes. Moss pillars have distinct redox-affected sections: oxidative exteriors and reductive interiors. We have proposed that a “pillar” is a community and habitat of functionally interdependent organisms and may represent a mini-biosphere. Batteries of 16S rRNA genotypes, or phylotypes, of eubacteria and cyanobacteria, but no archaea, have been identified in moss pillars. However, detailed identification or phylogenetic analyses of the moss and their associated eukaryotic microbiota have not been performed. This study analyzed near-full-length 18S rRNA gene sequences obtained from two whole moss pillars. In total, 28 PCR clone libraries from two whole moss pillars were constructed, and 96 clones from each library (total 2,688 clones) were randomly selected and sequenced. Molecular phylogenetic analysis revealed that the phylotype belonging to Bryophyta, considered to be derived from moss, was closely related (99.9?%) to the 18S rRNA gene sequence from Leptobryum pyriforme. Unexpectedly, phylotypes belonging to a novel clade of fungi dominated (approximately 27–75?%) the moss pillar libraries. This suggests that fungi may contribute to carbon cycling in the moss pillar as parasites or decomposers. In addition, phylotypes related to ciliates and tardigrades were subdominant in the exterior, while the phylotype of the ameba-like, single-celled eukaryote, Cercomonas (Cercozoa), was detected only in the interior. These features were shared by both moss pillars. The 18S rRNA gene-based profiles demonstrated that redox-related factors may control distribution of some eukaryotic microbes in a whole moss pillar.     

Nutrient limitation in a tropical saline lake: a microcosm experiment

There is increasing evidence that nitrogen limitation is of widespread occurrence in tropical lakes. Nonetheless, data on the deep tropical Lake Alchichica (Mexico) show that dissolved inorganic nitrogen (DIN) to soluble reactive phosphorus (SRP) ratio fluctuates widely. To elucidate further the role of nitrogen and phosphorus limitation on the phytoplankton growth in tropical saline lakes, we present the results of a series of nutrient enrichment experiments with natural assemblages of Lake Alchichica phytoplankton conducted monthly for a year. Our assays indicate that phosphorus and nitrogen alternate in limiting Lake Alchichica phytoplankton biomass. Phosphorous limited phytoplankton growth most (41.7%) of the time, followed by nitrogen (33.3% of the time), and both nutrients for the rest of the time (25.0%). This alternation in nitrogen and phosphorus responsible for phytoplankton growth limitation in Lake Alchichica is attributed to the combination of natural conditions (e.g., young volcanic terrain rich in phosphorus) that would favor nitrogen limitation and anthropogenic impacts (e.g., agricultural nitrogen fertilization) which would cause phosphorus limitation. Guest Editors: J. John & B. Timms Salt Lake Research: Biodiversity and Conservation—Selected papers from the 9th Conference of the International Society for Salt Lake Research     

Distribution and seasonal abundance of benthic macroinvertebrates in a subtropical Florida lake

We studied the distribution and seasonal abundance of benthic macroinvertebrates from July 1975 through September 1976 in a hypereutrophic lake in subtropical Florida. The benthic community was comprised principally of oligochaetes (56.1%), chironomids (37.1%), and chaoborids (5.7%). Numbers of taxa and mean densities correlated negatively with depth and positively with mean grain size of the substratum and dissolved oxygen concentration at the mud-water interface. Seasonal abundances and life history information obtained for the predominant species of Chironomidae (Polypedilum halterale, Glyptotendipes paripes, Chironomus crassicaudatus, Cryptochironomus fulvus, C. blarina, Cladotanytarsus sp., Procladius culiciformis, and Coelotanypus concinnus) indicated that all of these species are multivoltine with rapid generation times. Larval lengths of life at summer temperatures, 27–31 °C, ranged from 14–22 days indicating that sampling in subtropical lakes should be at short intervals (approximately 3 days) if the life cycles, ecology, and function of the components of the benthic community is to be understood.     

Planktivory in benthic nototheniid fish in McMurdo Sound,Antarctica

Synopsis Four species of nototheniid fish were sampled from below the sea ice near Cape Armitage, McMurdo Sound:Pagothenia borchgrevinki from just below the ice 1.5 km offshore,Trematomus bernacchii, Trematomus hansoni andTrematomus centronotus from off the bottom in about 20 m of water near the shore. Scale worms and isopods were conspicuous non-planktonic prey, and present in the three benthic fish species. The planktonic pteropod molluscLimacina helicina was numerically common in all four species of fish. The planktonic hyperiid amphipodHyperiella dilatata was also found in all fish species. WhereasP. borchgrevinki is planktivorous in accord with its pelagic habit, theTrematomus spp. clearly also feed on plankton from the water column.T. hansoni is particularly planktivorous, taking smaller copepods thanP. borchgrevinki.     

Sedimentation in McMurdo Sound, Antarctica: a disturbance mechanism for benthic invertebrates

A slumping event that occurred on permanent transect lines from 12- to 30- m depth located at Arrival Heights, McMurdo Sound, Antarctica in 1993, provided an opportunity to examine the effects of sediment-mediated disturbance on the benthic invertebrate fauna. The disturbance had a particularly significant impact on the soft coral Alcyonium paessleri, which resulted in 84% colony mortality downslope from the slump site compared to an average annual mortality rate of 14% on control transects. In contrast, anchor ice at the same site accounted for removal of 5% of the population in 1992. Laboratory experiments with A. paessleri colonies under conditions of periodic sediment resuspension indicate that the soft corals are susceptible to this form of disturbance. Our observations suggest they are capable of shedding fine silt in the laboratory, which might explain the presence of A. paessleri in soft-sediment sites around McMurdo Sound. However, scarring by larger gravel in laboratory assays was slow to heal and may account for much of the colony mortality we observed. Several invertebrate-barren rocky benthic regions in McMurdo Sound were suggestive of historical slumping events. Given the removal of the smaller grain size sediments from these areas – a typically slow process – it appears these communities are slow to recover. The long-term effects of sedimentation on the benthic communities are unknown, but the impact on A. paessleri, one of the most common and fastest growing species, suggests this disturbance mechanism could lead to significant restructuring of these communities. Received: 29 November 1996 / Accepted: 23 February 1997