Growth of modern branched columnar stromatolites in Lake Joyce,Antarctica

Modern decimeter‐scale columnar stromatolites from Lake Joyce, Antarctica, show a change in branching pattern during a period of lake level rise. Branching patterns correspond to a change in cyanobacterial community composition as preserved in authigenic calcite crystals. The transition in stromatolite morphology is preserved by mineralized layers that contain microfossils and cylindrical molds of cyanobacterial filaments. The molds are composed of two populations with different diameters. Large diameter molds (>2.8 μm) are abundant in calcite forming the oldest stromatolite layers, but are absent from younger layers. In contrast, <2.3 μm diameter molds are common in all stromatolites layers. Loss of large diameter molds corresponds to the transition from smooth‐sided stromatolitic columns to branched and irregular columns. Mold diameters are similar to trichome diameters of the four most abundant living cyanobacteria morphotypes in Lake Joyce: Phormidium autumnale morphotypes have trichome diameters >3.5   μm, whereas Leptolyngbya antarctica, L. fragilis, and Pseudanabaena frigida morphotypes have diameters <2.3   μm. P. autumnale morphotypes were only common in mats at <12 m depth. Mats containing abundant P. autumnale morphotypes were smooth, whereas mats with few P. autumnale morphotypes contained small peaks and protruding bundles of filaments, suggesting that the absence of P. autumnale morphotypes allowed small‐scale topography to develop on mats. Comparisons of living filaments and mold diameters suggest that P. autumnale morphotypes were present early in stromatolite growth, but disappeared from the community through time. We hypothesize that the mat‐smoothing behavior of P. autumnale morphotypes inhibited nucleation of stromatolite branches. When P. autumnale morphotypes were excluded from the community, potentially reflecting a rise in lake level, short‐wavelength roughness provided nuclei for stromatolite branches. This growth history provides a conceptual model for initiation of branched stromatolite growth resulting from a change in microbial community composition.     

On the use of high‐throughput sequencing for the study of cyanobacterial diversity in Antarctic aquatic mats

The study of Antarctic cyanobacterial diversity has been mostly limited to morphological identification and traditional molecular techniques. High‐throughput sequencing (HTS) allows a much better understanding of microbial distribution in the environment, but its application is hampered by several methodological and analytical challenges. In this work, we explored the use of HTS as a tool for the study of cyanobacterial diversity in Antarctic aquatic mats. Our results highlight the importance of using artificial communities to validate the parameters of the bioinformatics procedure used to analyze natural communities, since pipeline‐dependent biases had a strong effect on the observed community structures. Analysis of microbial mats from five Antarctic lakes and an aquatic biofilm from the Sub‐Antarctic showed that HTS is a valuable tool for the assessment of cyanobacterial diversity. The majority of the operational taxonomic units retrieved were related to filamentous taxa such as Leptolyngbya and Phormidium, which are common genera in Antarctic lacustrine microbial mats. However, other phylotypes related to different taxa such as Geitlerinema, Pseudanabaena, Synechococcus, Chamaesiphon, Calothrix, and Coleodesmium were also found. Results revealed a much higher diversity than what had been reported using traditional methods and also highlighted remarkable differences between the cyanobacterial communities of the studied lakes. The aquatic biofilm from the Sub‐Antarctic had a distinct cyanobacterial community from the Antarctic lakes, which in turn displayed a salinity‐dependent community structure at the phylotype level.     

Biogeographical distribution and ecological ranges of benthic cyanobacteria in East Antarctic lakes

For the first time, the cyanobacterial diversity from microbial mats in lakes of Eastern Antarctica was investigated using microscopic and molecular approaches. The present study assessed the biogeographical distribution of cyanobacteria in Antarctica. Five samples were taken from four lakes spanning a range of different ecological environments in Larsemann Hills, Vestfold Hills and Rauer Islands to evaluate the influence of lake characteristics on the cyanobacterial diversity. Seventeen morphospecies and 28 16S rRNA gene-based operational taxonomic units belonging to the Oscillatoriales, Nostocales and Chroococcales were identified. The internal transcribed spacer was evaluated to complement the 16S rRNA gene data and showed similar but more clear-cut tendencies. The molecular approach suggested that potential Antarctic endemic species, including a previously undiscovered diversity, are more abundant than has been estimated by morphological methods. Moreover, operational taxonomic units, also found outside Antarctica, were more widespread over the continent than potential endemics. The cyanobacterial diversity of the most saline lakes was found to differ from the others, and correlations between the sampling depth and the cyanobacterial communities can also be drawn. Comparison with database sequences illustrated the ubiquity of several cyanobacterial operational taxonomic units and their remarkable range of tolerance to harsh environmental conditions.     

Consistent patterns in diatom assemblages and diversity measures across water‐depth gradients from eight Boreal lakes from north‐western Ontario (Canada)

1. Until recently, the distribution of diatom species assemblages and their attributes (e.g. species richness and evenness) in relation to water depth have been identified but not quantified, especially across several lakes in a region. Here, we examined diatom assemblages in the surface sediment across a water‐depth gradient in eight small, boreal lakes in north‐western Ontario, minimally disturbed by human activities. 2. Surface‐sediment diatom assemblages were collected within each lake along a gentle slope from near‐shore to the centre deep basin of the lake, at a resolution of ～1 m water depth. Analysis of sedimentary samples provided an integrated view of assemblages that were living in the lake over several years and enabled a high‐resolution analysis of many lakes. The study lakes ranged in water chemistry, morphology and size and are located along an east–west transect approximately 250 km long in north‐western Ontario (Canada). 3. The majority of diatom species were distributed along a continuum of depth, with those taxa having similar habitat requirements forming distinct, though overlapping, assemblages. Three major zones of diatom assemblages in each lake were consistently identified: (i) a near‐shore assemblage of Achnanthes (sensu lato), Nitzschia, Cymbella (sensu lato) and other benthic species; (ii) a mid‐depth assemblage of small Fragilaria (sensu lato)/small Aulacoseira and various Navicula taxa; and (iii) a deep‐water assemblage of planktonic origin (mainly Discotella spp.). 4. The depth of the transition between assemblage zones varied between the eight lakes. The boundary between the deep‐water planktonic zone and the mid‐depth benthic zone varied according to water chemistry and was probably related to light attenuation. The boundary was deeper in lakes with the lower dissolved organic carbon and total phosphorus (TP) (i.e. less light attenuation) and vice versa. 5. Generally, species richness, species evenness and turnover rate of species as a function of depth were significantly lower in the planktonic assemblage zone in comparison with the two zones nearer the shore. Reproducibility of species and assemblage distributions across the depth gradient of the lakes illustrated that, despite potential for sediment transport, detailed ecological characterisation of diatom species can be gleaned from sedimentary data. Such data are often lacking, particularly for near‐shore benthic species.     

LIPOPHILIC PIGMENTS FROM CYANOBACTERIAL (BLUE-GREEN ALGAL) AND DIATOM MATS IN HAMELIN POOL,SHARK BAY,WESTERN AUSTRALIA1

Lipophilic pigments were examined in microbial mat communities dominated by cyanobacteria in the intertidal zone and by diatoms in the subtidal and sublittoral zones of Hamelin Pool, Shark Bay, Western Australia. These microbial mats have evolutionary significance because of their similarity to lithified stromatolites from the Proterozoic and Early Paleozoic eras. Fucoxanthin, diatoxanthin, diadinoxanthin, β-carotene, and chlorophylls a and c characterized the diatom mats, whereas cyanobacterial mats contained myxoxanthophyll zeaxanthin, echinenone, β-carotene, chlorophyll a and, in some cases, sheath pigment. The presence of bacteriochlorophyll a with in the mats suggest a close association of photosynthetic bacteria with diatoms and cyanobacteria. The high carotenoids: chlorophyll a ratios (0.84–2.44 wt/wt) in the diatom mats suggest that carotenoids served a photoprotective function in this high light environment. By contrast, cyanobacterial sheath pigment may have largely supplanted the photoprotective role of carotenoids in the intertidal mats.     

Groundwater shapes sediment biogeochemistry and microbial diversity in a submerged Great Lake sinkhole

For a large part of earth's history, cyanobacterial mats thrived in low‐oxygen conditions, yet our understanding of their ecological functioning is limited. Extant cyanobacterial mats provide windows into the putative functioning of ancient ecosystems, and they continue to mediate biogeochemical transformations and nutrient transport across the sediment–water interface in modern ecosystems. The structure and function of benthic mats are shaped by biogeochemical processes in underlying sediments. A modern cyanobacterial mat system in a submerged sinkhole of Lake Huron (LH) provides a unique opportunity to explore such sediment–mat interactions. In the Middle Island Sinkhole (MIS), seeping groundwater establishes a low‐oxygen, sulfidic environment in which a microbial mat dominated by Phormidium and Planktothrix that is capable of both anoxygenic and oxygenic photosynthesis, as well as chemosynthesis, thrives. We explored the coupled microbial community composition and biogeochemical functioning of organic‐rich, sulfidic sediments underlying the surface mat. Microbial communities were diverse and vertically stratified to 12 cm sediment depth. In contrast to previous studies, which used low‐throughput or shotgun metagenomic approaches, our high‐throughput 16S rRNA gene sequencing approach revealed extensive diversity. This diversity was present within microbial groups, including putative sulfate‐reducing taxa of Deltaproteobacteria, some of which exhibited differential abundance patterns in the mats and with depth in the underlying sediments. The biological and geochemical conditions in the MIS were distinctly different from those in typical LH sediments of comparable depth. We found evidence for active cycling of sulfur, methane, and nutrients leading to high concentrations of sulfide, ammonium, and phosphorus in sediments underlying cyanobacterial mats. Indicators of nutrient availability were significantly related to MIS microbial community composition, while LH communities were also shaped by indicators of subsurface groundwater influence. These results show that interactions between the mats and sediments are crucial for sustaining this hot spot of biological diversity and biogeochemical cycling.     

Microbial mats in tidal channels at San Carlos,Baja California Sur,Mexico

Microbial communities of stratified phototrophic bacteria in laminated intertidal sediments north of Estuary El Puente, near San Carlos, Baja California Sur, Mexico,‐were studied. This study describes the macroscopic and microscopic characteristics of the mats, including their annual growth. The mats were located in and along meandering mangrove‐lined tidal channels. Their thickness ranged from 0.5 to 25 cm. Square‐meter areas of polygonal mats were detected in several ponds infiltrated by sea water. The principal microbial community of the upper surface of various morphotypes of microbial mats was identified as cyanobacteria belonging to the genera Microcoleus, Lyngbya, Phormidium, and Oscillatoria. Other cyanobacte‐rial genera such as Pseudanabaena, Spirulina, Synechococcus, and Gloeocapsa, as well as many unidentified diatoms, were also present but at lower population densities. The second inward reddish layers of the microbial mats contained similar cyano‐bacterial genera plus anoxygenic phototrophic bacteria belonging to the genera Chloroflexus, Thiocapsa, Chromatium, Prosthecochloris, Rhodopseudomonas, and Chlorobium, as well as several unidentified bacteria. In situ measurements on the growth of the mats, from intermittent tide sites, showed an annual buildup of two layers: green and reddish. These layers corresponded to a vertical growth of 1.4 ± 0.27 mm/year. Permanently submerged mats did not show vertical growth during the same period of time.     

Distribution and diversity of diatom assemblages in surficial sediments of shallow lakes in Wapusk National Park (Manitoba,Canada) region of the Hudson Bay Lowlands

The hydrology of shallow lakes (and ponds) located in the western Hudson Bay Lowlands (HBL) is sensitive to climate warming and associated permafrost thaw. However, their biological characteristics are poorly known, which hampers effective aquatic ecosystem monitoring. Located in northern Manitoba along the southwestern coast of Hudson Bay, Wapusk National Park (WNP) encompasses numerous shallow lakes representative of the subarctic zone. We analyzed the distribution and diversity of diatom (microscopic algae; class Bacillariophyceae) assemblages in surficial sediments of 33 lakes located in three different ecozones spanning a vegetation gradient, from NE to SW: the Coastal Fen (CF), the Interior Peat Plateau (IPP), and the Boreal Spruce Forest (BSF). We found significant differences (P < 0.05) in diatom community composition between CF and IPP lakes, and CF and BSF lakes, but not between IPP and BSF lakes. These results are consistent with water chemistry measurements, which indicated distinct limnological conditions for CF lakes. Diatom communities in CF lakes were generally dominated by alkaliphilous taxa typical of waters with medium to high conductivity, such as Nitzschia denticula. In contrast, several IPP and BSF lakes were dominated by acidophilous and circumneutral diatom taxa with preference for low conductivity (e.g., Tabellaria flocculosa, Eunotia mucophila, E. necompacta var. vixcompacta). This exploratory survey provides a first detailed inventory of the diatom assemblages in the WNP region needed for monitoring programs to detect changes in shallow lake ecosystems and ecozonal shifts in response to climate variations.     

Mechanisms influencing changes in lake area in Alaskan boreal forest

During the past ～50 years, the number and area of lakes have declined in several regions in boreal forests. However, there has been substantial finer‐scale heterogeneity; some lakes decreased in area, some showed no trend, and others increased. The objective of this study was to identify the primary mechanisms underlying heterogeneous trends in closed‐basin lake area. Eight lake characteristics (δ¹⁸O, electrical conductivity, surface : volume index, bank slope, floating mat width, peat depth, thaw depth at shoreline, and thaw depth at the forest boundary) were compared for 15 lake pairs in Alaskan boreal forest where one lake had decreased in area since ～1950, and the other had not. Mean differences in characteristics between paired lakes were used to identify the most likely of nine mechanistic scenarios that combined three potential mechanisms for decreasing lake area (talik drainage, surface water evaporation, and terrestrialization) with three potential mechanisms for nondecreasing lake area (subpermafrost groundwater recharge through an open talik, stable permafrost, and thermokarst). A priori expectations of the direction of mean differences between decreasing and nondecreasing paired lakes were generated for each scenario. Decreasing lakes had significantly greater electrical conductivity, greater surface : volume indices, shallower bank slopes, wider floating mats, greater peat depths, and shallower thaw depths at the forest boundary. These results indicated that the most likely scenario was terrestrialization as the mechanism for lake area reduction combined with thermokarst as the mechanism for nondecreasing lake area. Terrestrialization and thermokarst may have been enhanced by recent warming which has both accelerated permafrost thawing and lengthened the growing season, thereby increasing plant growth, floating mat encroachment, transpiration rates, and the accumulation of organic matter in lake basins. The transition to peatlands associated with terrestrialization may provide a transient increase in carbon storage enhancing the role of northern ecosystems as major stores of global carbon.     

Community structure and pigment organisation of cyanobacteria-dominated microbial mats in Antarctica

Benthic microbial mat communities were sampled from 20 lakes, ponds and streams of the McMurdo Sound region, Antarctica. At least five distinct assemblages could be differentiated by their cyanobacterial species composition, pigment content and vertical structure. The most widely occurring freshwater communities were dominated by thin-trichome (0·5–3 µm) oscillatoriacean species that formed benthic films up to several millimetres thick. ‘Lift-off mats’ produced mucilaginous mats 1–5 cm thick at the surface and edge of certain ponds. Another group of oscillatoriacean communities was characteristic of hypersaline pond environments; these communities were dominated by species with thicker trichomes such as Oscillatoria priestleyi. Black mucilaginous layers of Nostoc commune were widely distributed in aquatic and semi-aquatic habitats. Dark brown sheath pigmentation was also characteristic of less cohesive mats and crusts dominated by Pleurocapsa, Gloeocapsa and Calothrix. High performance liquid chromatography analysis of the lipophilic pigments showed that the upper region of most of the Antarctic mats was enriched in sheath pigments (scytonemin) and/or certain carotenoids such as myxoxanthophyll and canthaxanthin. Most of the chlorophyll a (Chla), as well as phycocyanin, β-carotene and echinenone, was located in the lower strata of the mat profiles. In many of these communities most of the photosynthetic biomass occurred in a ‘deep Chla maximum’ that was well protected from short-wavelength radiation by the surface layer of light-screening pigments.     

Comparison of Freshwater Diatom Assemblages from a High Arctic Oasis to Nearby Polar Desert Sites and Their Application to Environmental Inference Models

Arctic oases are regions of atypical warmth and relatively high biological production and diversity. They are small in area (<5 km²) and uncommon in occurrence, yet they are relatively well studied due to the abundance of plant and animal life contained within them. A notable exception is the lack of research on freshwater ecosystems within polar oases. Here, we aim to increase our understanding of freshwater diatom ecology in polar oases. Diatoms were identified and enumerated from modern sediments collected in 23 lakes and ponds contained within the Lake Hazen oasis on Ellesmere Island, and compared with diatom assemblages from 29 sites located outside of the oasis across the northern portion of the island. There were significant differences in water chemistry variables between oasis and northern sites, with oasis sites having higher conductivity and greater concentrations of nutrients and related variables such as dissolved organic carbon (DOC). Taxa across all sites were typical of those recorded in Arctic freshwaters, with species from the genera Achnanthes sensu lato, Fragilaria sensu lato, and Nitzschia dominating the assemblages. A correspondence analysis (CA) ordination showed that oasis sites generally plotted separately from the northern sites, although the sites also appear to plot separately based on whether they were lakes or ponds. Canonical correspondence analysis (CCA) identified specific conductivity, DOC, and SiO₂ as explaining significant (P < 0.05) and additional amounts of variation in the diatom data set. The most robust diatom‐based inference model was generated for DOC, which will provide useful reconstructions on long‐term changes in paleo‐optics of high Arctic lakes.     

Hydrology and Diatom Phytoplankton of High Arctic Lakes and Ponds on Store Koldewey,Northeast Greenland

We document hydrological and phytoplankton characteristics of nine lakes and two ponds on Store Koldewey, a culturally undisturbed island off Northeast Greenland. The limnological survey included the recording of temperature, conductivity, oxygen concentration and saturation, pH, ionic composition, transparency, and the diatom phytoplankton community. In summer 2003, the lakes were cold, monomictic, thermally unstratified, alkaline and likely oligotrophic water bodies. Diatom phytoplankton was present in six lakes and consisted of four dominant species (Aulacoseira tethera, Cyclotella pseudostelligera, C. rossii, and Fragilaria tenera). The concentration of planktonic diatoms varied distinctly between the lakes. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Benthic and pelagic food resources for zooplankton in shallow high-latitude lakes and ponds

1. Shallow lakes and ponds are a major component of the northern landscape and often contain a high zooplankton biomass despite clear waters that are poor in phytoplankton. 2. In this study we quantified zooplankton food sources and feeding rates in the shallow waters of two contrasting high‐latitude biomes: subarctic forest tundra (Kuujjuarapik, Quebec) and high arctic polar desert (Resolute, Nunavut). Five substrate types were tested (beads, bacteria, picophytoplankton, filamentous plankton and microbial mats). Special attention was given to the role of benthos, a component that is usually poorly integrated into models of aquatic foodwebs. 3. Consistent with observations elsewhere in the circumpolar region, high concentrations of adult macrozooplankton occurred in all sites (up to 17 100 crustaceans m^?3) while phytoplankton concentrations and primary productivity were low. The communities were composed of multiple species, including Daphnia middendorfiana, Hesperodiaptomus arcticus, Leptodiaptomus minutus, Artemiopsis stefanssoni and Branchinecta paludosa. 4. Detritus made 89–98% of the planktonic resource pool and bacteria contributed the highest biomass (up to 29 mg C m^?3) of the planktonic food particles available to zooplankton. Benthic resources were dominated by microbial mats that grew in nutrient‐rich conditions at the base of the ponds and which dominated overall ecosystem biomass and productivity. 5. All species were flexible in their feeding but there were large, order of magnitude differences in clearance rates among taxa. These differences likely resulted from different grazing strategies among cladocerans, copepods and fairy shrimps, and possibly also from adaptation to specific food types and size ranges that occur locally in these waters. 6. The subarctic cladocerans Ceriodaphnia quadrangula and D. middendorfiana, and the arctic fairy shrimp B. paludosa were observed to graze directly on the microbial mats and the feeding experiments confirmed their assimilation of benthic substrates. The other zooplankton species showed a more pelagic feeding mode but were capable of using microbial mat filaments, thus may be indirectly linked to benthic processes via resuspension. 7. Our study indicates that the classical aquatic food web in which phytoplankton provide the sole production base for grazers does not apply to northern shallow lakes and ponds. Instead, microbial mats increase the physical complexity of these high latitude ecosystems and likely play a role in sustaining their high zooplankton biomass.     

Benthic algal mats of some lakes of Inexpressible Island (northern Victoria Land,Antarctica)

 Benthic algal mats from four lakes located on Inexpressible Island (northern Victoria Land, Antarctica) were studied, analysing their structure and floristic composition. Thirty taxa of algae were identified: 11 Cyanophyta, 15 Bacillariophyta and 4 Chlorophyta. The lake mats contained from 15 (lake C) to 26 species (lake D). The number of diatom frustules in the mats varied from 771×10³ (lake C) to 9428×10³ frustules g^-1 (lake B). In terms of floristic composition and macromorphology, these mats were observed to be very similar to moat mats described from Lake Gondwana (northern Victoria Land) and the Dry Valleys lakes of southern Victoria Land. Received: 9 July 1995/Accepted: 12 February 1996     

THE INFLUENCE OF SUBMERGED MACROPHYTES ON SEDIMENTARY DIATOM ASSEMBLAGES1

Submerged macrophytes are a central component of lake ecosystems; however, little is known regarding their long‐term response to environmental change. We have examined the potential of diatoms as indicators of past macrophyte biomass. We first sampled periphyton to determine whether habitat was a predictor of diatom assemblage. We then sampled 41 lakes in Quebec, Canada, to evaluate whether whole‐lake submerged macrophyte biomass (BiomEpiV) influenced surface sediment diatom assemblages. A multivariate regression tree (MRT) was used to construct a semiquantitative model to reconstruct past macrophyte biomass. We determined that periphytic diatom assemblages on macrophytes were significantly different from those on wood and rocks (ANOSIM R = 0.63, P < 0.01). A redundancy analysis (RDA) of the 41‐lake data set identified BiomEpiV as a significant (P < 0.05) variable in structuring sedimentary diatom assemblages. The MRT analysis classified the lakes into three groups. These groups were (A) high‐macrophyte, nutrient‐limited lakes (BiomEpiV ≥525 μg · L^?1; total phosphorus [TP] <35 μg · L^?1; 23 lakes); (B) low‐macrophyte, nutrient‐limited lakes (BiomEpiV <525 μg · L^?1; TP <35 μg · L^?1; 12 lakes); and (C) eutrophic lakes (TP ≥35 μg · L^?1; six lakes). A semiquantitative model correctly predicted the MRT group of the lake 71% of the time (P < 0.001). These results suggest that submerged macrophytes have a significant influence on diatom community structure and that sedimentary diatom assemblages can be used to infer past macrophyte abundance.     

The diatom flora and limnology of lakes in the Amery Oasis,East Antarctica

The diatom flora of three lakes in the ice-free Amery Oasis, East Antarctica, was studied. Two of the lakes are meltwater reservoirs, Terrasovoje Lake (31 m depth) and Radok Lake (362 m depth), while Beaver Lake (>435 m depth) is an epishelf lake. The lakes can be characterized as cold, ultra-oligotrophic and alkaline, displaying moderate (Radok and Terrasovoje lakes) to high (Beaver Lake) conductivities. There was no diatom phytoplankton present in any of the three lakes. While 34 benthic diatom taxa were identified from modern and Holocene sediments of Terrasovoje and Radok lakes, a 30-cm long sediment core recovered in Beaver Lake was barren. Five species (Luticola muticopsis, Muelleria peraustralis, Pinnularia cymatopleura, Psammothidium metakryophilum, P. stauroneioides) are endemic to the Antarctic region. All identified taxa are photographically documented and brief notes on their taxonomy, biogeography and ecology are provided. The most abundant diatom taxa are Amphora veneta, Craticula cf. molesta, Diadesmis spp, M. peraustralis and Stauroneis anceps. This is the first report on the diatom flora in lakes of the Amery Oasis.This revised version was published online in May 2004 with corrections to Figure 1.     

Lake depth and geographical position modify lake fish assemblages of the European 'Central Plains' ecoregion

1. Classification of European lake fish assemblages can be based on fish‐assemblage structure or morphological, geographical, physical and chemical lake attributes. However, substantial gaps in knowledge exist with respect to the correspondence between both classification approaches. 2. Here, we compiled fish assemblage data from 165 lakes situated in the European ‘Central Plains’ ecoregion. Cluster analysis of fish abundances was performed to compare fish assemblage types of the entire ecoregion with those from previous country‐specific studies. Nonparametric group comparisons, classification trees and partial canonical ordinations were used to infer the correspondence between fish assemblage types and morphology, geographical position and nutrient concentration of the lakes. 3. Three distinct fish assemblages were revealed: vendace (Coregonus albula), ruffe (Gymnocephalus cernuus) and roach (Rutilus rutilus) lake types. Both latitude and lake depth were the best determinants of lake type, but total phosphorus (TP) concentrations were also important. Vendace lakes were deep and had low TP concentrations, whereas the shallower ruffe and roach lakes had higher TP values. Roach lakes were more frequent in the north‐west area of the ecoregion, whereas ruffe lakes were more often found south of the Baltic Sea. 4. Controlling for the influence of nutrient concentration showed that lake morphology and geographical position were important determinants of fish assemblages. However, the variance explained was low (<20%), implying that biological interactions may also be important in forming the lake‐specific fish assemblages. 5. The results suggest that fish assemblages differ between deep and shallow lakes, and between the north‐west and south‐east locations within the Central Plains ecoregion. Accordingly, establishment of depth‐related lake morphotypes is needed, and the European ecoregions recommended to be used in evaluation systems according to the Water Framework Directive seem to be too coarse to reflect the subtle differences of fish species richness along geographical gradients.     

Effects of small‐scale environmental variation on metaphyton condition and community composition

1. Metaphyton mats typically originate as benthic algal biofilms that experience higher solar radiation and temperatures, and reduced access to nutrients, once they reach the water surface, but the impacts of these physicochemical changes on metaphyton condition and community composition have received little attention. 2. Using microprobes positioned at 0, 2, 4 and 6 cm depth, we recorded small‐scale differences in water chemistry within metaphyton mats constrained in floating nets during an 8‐week period. Concurrent weekly samples of filamentous algae and their diatom epiphytes were collected from the top, middle and bottom of the mats and were analysed for changes in ash‐free dry mass (AFDM) and chlorophyll‐a, nutrient (N, P, C, Si) content and taxonomic composition. 3. Light intensity, temperature and dissolved oxygen declined both with increasing depth in the mat and over the study period. The autotrophic index (=AFDM/chlorophyll‐a) was greatest at the top of the mats and increased over time; samples also had higher C/P and C/N ratios than samples deeper within the mat. 4. Pithophora was consistently the dominant algal filament throughout the study (representing 85% of all filaments averaged over time and depth); epiphytic diatom cover on Pithophora (calculated as epiphyte area index) declined over time, particularly at the top of the mat. 5. Densities of the diatom epiphytes Gomphonema, Cocconeis and Fragilaria increased with increasing depth within the mat, whereas Cymbella/Encyonema was more common in surface samples. 6. Our results indicate that metaphyton mats are highly dynamic communities, spatially organised in part by small‐scale environmental variation and subject to changes in taxonomic composition following their arrival at the water surface.     

Origin and Phylogeny of Microbes Living in Permanent Antarctic Lake Ice

Abstract The phylogenetic diversity of bacteria and cyanobacteria colonizing sediment particles in the permanent ice cover of an Antarctic lake was characterized by analyses of 16S rRNA genes amplified from environmental DNA. Samples of mineral particles were collected from a depth of 2.5 m in the 4-m-thick ice cover of Lake Bonney, McMurdo Dry Valleys, Antarctica. A rRNA gene clone library of 198 clones was made and characterized by sequencing and oligonucleotide probe hybridization. The library was dominated by representatives of the cyanobacteria, proteobacteria, and Planctomycetales, but also contained diverse clones representing many other microbial groups, including the Acidobacterium/Holophaga division, the Green Non-Sulfur division, and the Actinobacteria. Six oligonucleotide probes were made for the most abundant clades recovered in the library. To determine whether the ice microbial community might originate from wind dispersal of the algal mats found elsewhere in Taylor Valley, the probes were hybridized to 16S rDNAs amplified from three samples of terrestrial cyanobacterial mats collected at nearby sites, as well as to bacterial 16S rDNAs from the lake ice community. The results demonstrate the presence of a diverse microbial community dominated by cyanobacteria in the lake ice, and also show that the dominant members of the lake ice microbial community are found in terrestrial mats elsewhere in the area. The lake ice microbial community appears to be dominated by organisms that are not uniquely adapted to the lake ice ecosystem, but instead are species that originate elsewhere in the surrounding region and opportunistically colonize the unusual habitat provided by the sediments suspended in lake ice. Received: 16 August 1999; Accepted: 28 December 1999; Online Publication: 28 April 2000