Abundance and distribution of picoplankton in tropical, oligotrophic Lake Kivu, eastern Africa

1. We used flow cytometry to characterize freshwater photosynthetic picoplankton (PPP) and heterotrophic bacteria (HB) in Lake Kivu, one of the East‐African great lakes. Throughout three cruises run in different seasons, covering the four major basins, phycoerythrin‐rich cells dominated the PPP. Heterotrophic bacteria and PPP cell numbers were always high and spatial variations were modest. This represents an important difference from temperate and high latitude lakes that show high fluctuations in cell abundance over an annual cycle. 2. Three populations of picocyanobacteria were identified: one corresponded to single‐cells (identified as Synechococcus by epifluorescence microscopy, molecular methods and pigment content), and the two other that most probably correspond to two and four celled colonies of the same taxon. The proportion of these two subpopulations was greater under stratified conditions, with stronger nutrient limitation. 3. High PPP concentrations (c. 10⁵ cell mL^?1) relative to HB (c. 10⁶ cell mL^?1) were always found. Lake Kivu supports relatively less bacteria than phytoplankton biomass than temperate systems, probably as a consequence of factors such as temperature, oligotrophy, nutrient limitation and trophic structure. 4. A review of PPP concentration across aquatic systems suggests that the abundance of Synechococcus‐like cyanobacteria in large, oligotrophic, tropical lakes is very high. 5. Photosynthetic picoplankton cell abundances in the oligotrophic tropical lakes Kivu and Tanganyika are comparable to those of eutrophic temperate lakes. This apparently contradicts the view that PPP abundance increases with increasing eutrophy. More data on PPP in tropical lakes are needed to explore further this particular pattern.     

Picophytoplankton predominance in hypersaline lakes (Transylvanian Basin,Romania)

The occurrence and importance of photoautotrophic picoplankton (PPP, cells with a diameter <2 μm) was studied along a trophic and salinity gradient in hypersaline lakes of the Transylvanian Basin (Romania). The studied lakes were found to be rich in PPP, with abundances (maximum 7.6 × 10⁶ cells mL^?1) higher than in freshwater and marine environments of similar trophic conditions. The contribution of PPP to the total phytoplankton biovolume did not decrease with increasing trophic state as it was generally found in other aquatic environments. Regardless of the trophic conditions, the contribution of PPP could reach 90–100 % in these hypersaline lakes. We hypothesized that the PPP predominance might be the result of the low grazing pressure, since heterotrophic nanoflagellates (the main grazers of PPP) were absent in the studied samples. There were significant differences in community composition among the lakes along the salinity gradient. CyPPP predominated in less saline waters (mainly below 5 %), while EuPPP were present along the entire salinity range (up to 18.7 %), dominating the phytoplankton between 3 and 13 % salinity. Above 13 % salinity, the phytoplankton was composed mainly of Dunaliella species.     

Systematics and Ecology of Chlorophyte Picoplankton in German Inland Waters along a Nutrient Gradient

The community structure and succession of autotrophic picoplankton in several oligotrophic to hypertrophic German freshwater ecosystems were studied with emphasis on the occurrence and characterization of chlorophyte picoplankton. Depending on the trophic status and the time of the year, the relation of green eukaryotic picoplankton to picocyanobacteria, the contribution of the picoplankton to the total phytoplankton biomass, and the succession and dominance of picoplankton groups changed considerably. A significant correlation between the picoplankton abundances, their biomass and their biomass contribution could not be found. Although the chlorophyte picoplankton were similar with respect to their ultrastructure, phylogenetic analyses of the rbcL genes revealed that these organisms evolved independently within several green algal lineages. The most common picoplanktonic green algae in the lakes that were studied belong to the genera Choricystis and Pseudodictyosphaerium. Considering the new molecular biological findings, the systematics of picoplanktonic green algae from freshwater and marine habitats are discussed.     

Transition zones in small lakes: the importance of dilution and biological uptake on lake-wide heterogeneity

Community structure and diversity patterns of pelagic copepods were investigated for the coastal areas and in the marine lakes of the Palau islands in the West Pacific. We conducted field surveys during 2004–2007 and collected zooplankton samples from eight coastal areas and 16 marine lakes. The marine lakes in the islands of Palau are limnologically classified into two types, meromictic and holomictic lakes. Species diversity indices (Margalef’s species richness d′, Shannon–Wiener index H′, and Simpson’s dominance) were measured at each sampling site. The copepod community structure was analyzed using multivariate analyses, hierarchical cluster analysis, and non-metric multidimensional scaling (MDS) from the PRIMER package. A total of 36 copepod taxa were identified to the genus or species level from Calanoida, Cyclopoida, Harpacticoida, and Poecilostomatoida. Multivariate analysis based on the Bray–Curtis similarity index revealed that copepod assemblages could be classified into three different groups according to their habitat: (I) meromictic lakes, (II) holomictic lakes with an exceptionally shallow and flat basin (type A holomictic lakes), (III) holomictic lakes with a deep basin (type B holomictic lakes), and all coastal areas. The meromictic lakes were characterized by markedly decreased species diversity (d′ = 0.15, H′ = 0.41) in which only two brackish-water species, Bestiolina similis and Oithona dissimilis, were dominant. Type A holomictic lakes were characterized by relatively low levels of species diversity (d′ = 1.25, H′ = 1.35). In contrast, type B holomictic lakes were characterized by relatively high levels of species diversity, which was comparable to that of coastal areas. This result indicated that zooplankton assemblages in the type B holomictic lakes were closely related to a coastal community. The present study showed that the species diversity of pelagic copepods varies according to the level of isolation and the local environmental conditions for each marine lake.     

Pico- and Nanoplankton in Aquatic Ecosystems in the Valley of the Lakes and Great Lakes Depression (Mongolia)

The abundance and biomass of bacterioplankton, phototrophic picoplankton, and heterotrophic nanoflagellates has been determined in lakes, rivers, and reservoirs located in the Valley of the Lakes and Great Lakes Depression (Mongolia). The species richness of the heterotrophic flagellates and their consumption of bacteria are estimated. Pico- and nanoplankton are the most abundant in shallow mineral lakes Orog and Tatsyn and in the freshwater Durgun Reservoir. Heterotrophic nanoflagellates consume 26–92% (on average 66%) of the daily bacterioplankton production. Thus, flagellates are important in the transfer of bacterial carbon to the higher levels of planktonic trophic webs. A total of 30 species and their forms of heterotrophic flagellates from 14 large taxa are identified. The highest species diversity of these protists are found in the Durgun and Taishyr reservoirs.     

Eukaryotic picoplankton communities of the Mediterranean Sea in summer assessed by molecular approaches (DGGE, TTGE, QPCR)

The composition and abundance of eukaryotic picoplankton (defined here as cells smaller than 3 mum) was investigated in the Morocco upwelling and throughout the Mediterranean Sea in late summer using flow cytometry and molecular methods (gradient gel electrophoresis and quantitative PCR). The picoplankton displayed characteristics typical of oligotrophic oceanic areas with concentrations down to 1000 cells mL(-1) in the Eastern Basin. The most abundant eukaryotic sequences recovered by gradient gel electrophoresis were related to uncultivated marine groups: alveolates I (16%) and II (26%) and a newly discovered group (env Nansha, 17%) for which sequences have been recently obtained from the South China Sea and that could be related to Acantharians. Prasinophyceae (photosynthetic green algae) accounted for 10% of the sequences, whereas Cercozoa, Stramenopiles, Polycystinea, dinoflagellates and ciliates provided minor contributions. The use of quantitative PCR coupled with taxon-specific primers allowed us to estimate the relative abundance of several taxa belonging to the Prasinophyceae. Of the three genera assessed, Bathycoccus appeared as the most abundant, forming localized maxima at depth.     

Analysis of a marine picoplankton community by 16S rRNA gene cloning and sequencing.

The phylogenetic diversity of an oligotrophic marine picoplankton community was examined by analyzing the sequences of cloned ribosomal genes. This strategy does not rely on cultivation of the resident microorganisms. Bulk genomic DNA was isolated from picoplankton collected in the north central Pacific Ocean by tangential flow filtration. The mixed-population DNA was fragmented, size fractionated, and cloned into bacteriophage lambda. Thirty-eight clones containing 16S rRNA genes were identified in a screen of 3.2 x 10(4) recombinant phage, and portions of the rRNA gene were amplified by polymerase chain reaction and sequenced. The resulting sequences were used to establish the identities of the picoplankton by comparison with an established data base of rRNA sequences. Fifteen unique eubacterial sequences were obtained, including four from cyanobacteria and eleven from proteobacteria. A single eucaryote related to dinoflagellates was identified; no archaebacterial sequences were detected. The cyanobacterial sequences are all closely related to sequences from cultivated marine Synechococcus strains and with cyanobacterial sequences obtained from the Atlantic Ocean (Sargasso Sea). Several sequences were related to common marine isolates of the gamma subdivision of proteobacteria. In addition to sequences closely related to those of described bacteria, sequences were obtained from two phylogenetic groups of organisms that are not closely related to any known rRNA sequences from cultivated organisms. Both of these novel phylogenetic clusters are proteobacteria, one group within the alpha subdivision and the other distinct from known proteobacterial subdivisions. The rRNA sequences of the alpha-related group are nearly identical to those of some Sargasso Sea picoplankton, suggesting a global distribution of these organisms.     

Picoplankton in Six New Zealand Lakes: Abundance in Relation to Season and Trophic State

The abundance of picoplankton (0.2-2 μm) was measured seasonally in the surface waters of six New Zealand lakes that represent a range of trophic states. The lakes were: Wakatipu, Te Anau, Manapouri, Hayes, Mahinerangi and Ross Creek Reservoir. Among the lakes, picoplankton abundance was associated positively with temperature; picoplankton were most abundant in summer and autumn when they attained densities of 108,000-270,000 cells/ml in the oligotrophic lakes. In these lakes, prokaryotic picoplankton was generally an order of magnitude more abundant than eukaryotic picoplankton. Consistent with the hypothesis that picoplankton are more important in oligotrophic than eutrophic ecosystems, there was a weak negative correlation between the density of prokaryotic picoplankton in the lakes and the level of chlorophyll a. The presence of large numbers of chroococcoid cyanobacteria in the guts of Ceriodaphnia dubia and Bosmina meridionalis implies that prokaryotic picoplankton are collected, but not digested, by these species.     

Seasonal Changes in Chlorophyll-containing Picoplankton Populations of Ten Lakes in Northern England

Key features of photosynthetic picoplankton populations were compared during 1988 in ten lakes in northern England ranging from oligotrophic to slightly eutrophic; two of the three eutrophic lakes were shallow and lacked a thermocline. Measurements were made at 0.5 m depth of temperature, total chlorophyll a, chlorophyll-containing picoplankton cell density, mean picoplankton cell volume and percentage of phycoerythrin-rich cells in the total picoplankton population. All lakes showed maxima for total chlorophyll concentration and picoplankton cell density in mid- to late summer. The maximum value for picoplankton density ranged from 3.4 × 10³ (Esthwaite Water) to 1.3 × 10⁶ cells ml⁻¹ (Ennerdale Water). There was a significant negative relationship (p < 0.05) between log₁₀ of maximum picoplankton cell density and maximum total chlorophyll, the latter being taken as an indicator of lake trophic status. The ratio of maximum to minimum picoplankton density during the year in a particular lake ranged from 39 to 2360 and showed no obvious relationship to lake type. Overall, the seasonal range in picoplankton density was about one order of magnitude greater than the range in total chlorophyll a, but there were considerable differences between lakes. Phycoerythrin-rich picoplankton as a percentage of total picoplankton reached a maximum in summer in all lakes. Values were always very low (<5%) in the two shallow eutrophic lakes, but reached 97% and over in the four most oligotrophic lakes. In two of the oligotrophic lakes, Wast-water and Ennerdale Water, phycoerythrin-rich picoplankton was a major component of the summer phytoplankton biomass.     

Archaeal nucleic acids in picoplankton from great lakes on three continents

Phylogenetic analysis of PCR-amplified 16S rRNA genes revealed the presence of archaea in picoplankton collected from the Laurentian Great Lakes in North America, Africas Lake Victoria, and Lakes Ladoga and Onega in northeastern Eurasia. From 1 to 10% of the rRNA extracted from size-fractionated picoplankton (>0.2 µm but <1.2 µm) collected in the epilimnion and hypolimnion of these lakes was specific to the Archaea, whereas the majority of rRNA was derived from Bacteria. Analysis of the 16S rRNA genes cloned from these samples indicated they were closely related to crenarchaeal sequences that have been widely characterized from marine environments. The presence of nearly identical 16S rDNA clones in several of these geographically disparate lakes suggests a cosmopolitan distribution of specific subgroups of these Archaea in freshwater environments. Despite their abundance in the water column of freshwater lakes, we have no representatives of these crenarchaea in pure culture, and so their physiological characteristics and ecological role remain unknown. Present address (B.P. Keough): Department of Microbiology and Molecular Genetics, Bio-Physical Sciences Building, Michigan State University, East Lansing, MI 48823, Phone: (517) 355-6463; Fax: (517) 353-8957     

Freshwater diatom biogeography in the Canadian Arctic Archipelago

Aim Document the biogeographical distributions of diatoms in the Canadian Arctic in relation to environmental factors. Location The Canadian Arctic Archipelago. Methods Diatoms were extracted from lake sediments and treated using standard methods. Rarefaction‐estimated species richness, diatom concentrations (valves cc^?1), and diatom abundance were mapped using a Geographic Information System. The physical and chemical parameters of the lakes were measured. Results A total of 326 taxa from 63 genera were found in 62 lakes of the study area. Up to 85 and as low as eight taxa were identified in any one lake, and rarefaction‐estimated species richness correlated with lake size. Nearby lakes could have greatly different diatom assemblages. Many taxa showed limited distributions. Response surfaces and ordination techniques indicate that a large number of taxa prefer colder regions of the Arctic while the abundance of others was influenced by lake water chemical parameters such as TKN and SiO₂. Main conclusions Although many taxa were common and found across the study area, diatom assemblages showed regional differences in the Arctic. Some taxa were not found in either the northern or southern parts of the Archipelago and others were restricted to particular regions. Newly delineated genera showed interpretable geographical patterns and could be related to environmental factors, suggesting that this more natural grouping may enhance our understanding of diatom ecology. Geographical, physical, and chemical factors are needed to explain diatom distributions in the Arctic.     

Cladocera assemblages from the surface sediments of south-central Ontario (Canada) lakes and their relationships to measured environmental variables

Fossil cladoceran remains preserved in surface sediment samples from 44 oligotrophic lakes in south-central Ontario were examined to evaluate the relationships between species assemblages and measured environmental variables. Differences in cladoceran assemblages were related to physical and chemical variables using multivariate techniques. Redundancy Analysis (RDA) identified five environmental variables as significantly influencing assemblage composition: sulphate (SO₄²⁻), calcium (Ca²⁺), pH, maximum lake depth (Z _max) and dissolved organic carbon (DOC). There was a distinct separation of lakes and taxa along the ion gradient based on SO₄, Ca and pH. Additionally, cladoceran communities in coloured, shallow lakes had relatively higher abundances of littoral chydorid species and the pelagic taxa Holopedium spp., and the Daphnia pulex complex. Deep, clear lakes had relatively higher abundances of other pelagic taxa. Predation by fish (measured as presence–absence) and Chaoborus (measured as density) were less significant than some of the physico-chemical variables in influencing cladoceran assemblage structure. However, this could be due to the limited resolution of the predation data that was available at the time of this study. The distribution of cladocerans in the surface sediment, and their relation to these important environmental variables, suggests that there is considerable potential for the use of sedimentary cladoceran remains as environmental indicators in south-central Ontario lakes. Handling editor: J. Saros     

Ecological Characteristics of Autotrophic Picoplankton in a Prealpine Lake

The seasonal distribution of autotrophic picoplankton in Lake Constance was investigated over four consecutive years. Cell numbers varied seasonally and vertically over four orders of magnitude (10² to 10⁶ cells ml⁻¹). A horizontal variation by a factor of 3 in abundance and biomass across the different parts of the lake was found during summer stratification. Picoplankton peaks occurred during the phytoplankton spring bloom and in late summer. Low values were characteristic for the clear-water phase in early summer and for autumn-winter. This seasonal pattern differed from that of larger phytoplankton in Lake Constance and from the seasonal distribution of picoplankton known from other lakes and marine environments. Picoplankton was predominated by chroococcoid cyanobacteria of about 0.6 μ³ biovolume. The average cell size increased from winter until early summer. Using HPLC pigment analysis, we identified zeaxanthin and β-carotene as typical picoplankton pigments. Results of the pigment analyses suggest that algae others than picocyano-bacteria may be more prominent in the picoplankton size class than derived from routine epifluorescence counting.     

Patterns in the Distribution of Cladocerans (Crustacea: Branchiopoda) in Lakes Across a North–south Transect in Alaska, USA

The remains of cladocerans were examined from the surface sediments of 51 freshwater sites along a north–south transect spanning Alaska. We identified 27 cladoceran taxa from the sediments, consisting primarily of littoral chydorid species. Variations in cladoceran assemblages were related to measured physical and chemical variables using multivariate techniques. Redundancy analysis (RDA) indicated that lake depth, total phosphorus (TP), and altitude all had a significant influence in determining the composition of cladoceran assemblages. Cladoceran communities in tundra and forest-tundra lakes, which were relatively shallow and nutrient-poor, had relatively low abundances of pelagic Cladocera, and were primarily composed of several littoral chydorid species. Among pelagic cladoceran species, there was a distinct shift in dominance from the Bosminidae in lakes in the southern boreal forest region to Daphniidae in lakes in the northern boreal forest. Daphnia dominated lakes had significantly higher total phosphorus, specific conductivity, and calcium concentrations than lakes dominated by Eubosmina. Overall, the relative importance of physical and chemical factors in structuring cladocerans is similar to other previously studied regions, and suggests the Cladocera may be useful as ecological and paleoenvironmental indicators in this region.     

Classification of the Tatra Mountain lakes (Slovakia) using chironomids (Diptera,Chironomidae)

Chironomid assemblages in thirty-three mountain lakes situated above tree line in the Slovakian part of the Tatra Mountains were studied during 2000–2002. Chironomid species/taxa, collected as pupal exuviae, were correlated with physical, chemical, and lake morphometry variables of 22 lakes. Two-way indicator species analysis (TWINSPAN) was used to classify the lakes into four distinct groups: higher situated alpine lakes, lower situated alpine lakes, subalpine lakes and acidified lakes. Presence/absence of eight taxa was identified as indicative for this classification. In discriminant function analysis, pH, dissolved organic carbon, altitude and lake area were the most significant variables reflecting differences among groups of lakes. This model of four variables allowed 77% success in the prediction of group membership. A multiple regression model with lake area, concentration of magnesium and total phosphorus accounted for 37% of the variance in taxa richness. Lakes with greater area contained more chironomid taxa than smaller ones. Lakes with higher alkalinity and higher trophic status tend to support more taxa. Canonical correspondence analysis (CCA) indicated that most variation in the composition of chironomid assemblages was related to pH and to altitude. The results can be used as reference data for long-term monitoring of the Tatra lakes, especially in connection with a recovery from acidification and global climatic change.     

The diversity of small eukaryotic phytoplankton (< or =3 microm) in marine ecosystems

Small cells dominate photosynthetic biomass and primary production in many marine ecosystems. Traditionally, picoplankton refers to cells ≤2 μm. Here we extend the size range of the organisms considered to 3 μm, a threshold often used operationally in field studies. While the prokaryotic component of picophytoplankton is dominated by two genera, Prochlorococcus and Synechococcus , the eukaryotic fraction is much more diverse. Since the discovery of the ubiquitous Micromonas pusilla in the early 1950s, just over 70 species that can be <3 μm have been described. In fact, most algal classes contain such species. Less than a decade ago, culture-independent approaches (in particular, cloning and sequencing, denaturing gradient gel electrophoresis, FISH) have demonstrated that the diversity of eukaryotic picoplankton is much more extensive than could be assumed from described taxa alone. These approaches revealed the importance of certain classes such as the Prasinophyceae but also unearthed novel divisions such as the recently described picobiliphytes. In the last couple of years, the first genomes of photosynthetic picoplankton have become available, providing key information on their physiological capabilities. In this paper, we discuss the range of methods that can be used to assess small phytoplankton diversity, present the species described to date, review the existing molecular data obtained on field populations, and end up by looking at the promises offered by genomics.     

Evolutionary history of an adaptive radiation in species pairs of threespine sticklebacks (Gasterosteus): insights from mitochondrial DNA

Species pairs of threespine stickleback, Gasterosteus aculeatus, co-exist in several lakes in the Strait of Georgia, southwestern British Columbia. One species, ‘benthics’ is robust-bodied and is morphologically and behaviourally specialized for benthivory. The other species, ‘limnetics’ is specialized for planktivory in open-water habitats of the lakes. We examined mitochondrial DNA restriction site variation in benthic and limnetic sticklebacks as well as in solitary freshwater, anadromous (sea-run), and marine populations to test: (i) if benthic and limnetic pairs have evolved only once or multiple times (parallel evolution) and (ii) if the species have evolved sympatrically, or allopatrically from ‘double invasions’ of lakes by ancestral anadromous/marine sticklebacks. Stickleback mtDNA comprised a single clade with a low (mean = 0.40%) degree of sequence divergence among the 77 haplotypes resolved. Most nucleotide diversity (97%) was found within (rather than among) populations of anadromous/marine sticklebacks whereas most diversity (77%) was found among populations in freshwater sticklebacks. Significant differences in haplotype frequencies were found between benthics and limnetics in three of the four species pair lakes examined, but in all cases the pairs within lakes were characterized by unique assemblages of closely related haplotypes. Hierarchical clustering of divergence estimates suggested that comparable species from different lakes have originated independently in all lakes because in no case did comparable species from different lakes cluster together. Divergent species within lakes tended to be more closely related to one another than to species in other lakes and there were two cases were benthics and limnetics within a particular lake were monophyletic. In two of the four two-species lakes, limnetics were less divergent from putative ancestral anadromous/marine stickleback as predicted by the double invasion hypothesis, but in the two other lakes benthics were less divergent. Our data argue strongly that the species pairs have evolved independently in each lake were they now co-exist. Further, in two lakes our data are consistent with the species having evolved by sympatric divergence, but allopatric divergence followed by introgression of mtDNA that has obscured ancestral relationships cannot be discounted completely. Finally, despite remaining uncertainty about the geography of speciation, the species appear to have evolved in the face of gene flow arguing that natural selection acting on trophic ecology has been a major component of ecological speciation in sticklebacks.     

The insight into diatom diversity,ecology, and biogeography of an extreme cold ultraoligotrophic Lake Labynkyr at the Pole of Cold in the northern hemisphere

The freshwater ultraoligotrophic Lake Labynkyr is located near the Pole of Cold in the northern hemisphere (Yakutia, Russia). The lake is covered by ice during 240 days a year. We undertook several expeditions to the lake during the ice and open water periods for sampling ice fouling, plankton and periphyton that were then analyzed by means of scanning electron microscopy. As a result, we identified a high biodiversity of diatoms—123 species and intraspecific taxa from 53 genera, among them 3 species were new for Russia and 26 taxa were new for the algal flora of Yakutia. The oligo- and xenosaprobionts and their variations dominate—71 taxa. 18 Species were evaluated as tolerant to cold oligotrophic waters, 12 occurred on the ice bottom, and 62 in the water column under ice (0–25 m). 104 taxa were found during the open water period, 70 taxa were identified in the periphyton. We showed the diatom flora of Lake Labynkyr to be unique compared with other lakes of Yakutia and to share taxa with the diatom flora of Lake Baikal. The diatoms being indicators of the global climate changes and ecological status of lakes, our data can be used as an evidence of such changes as well as to be useful studies of biogeography and history of formation of flora in Arctic and Subarctic waters.

     

Autotrophic Picoplankton: Community Composition,Abundance and Distribution across a Gradient of Oligotrophic British Columbia and Yukon Territory Lakes

Autotrophic picoplankton communities were examined in eleven oligotrophic lakes from a broad geographic region of western Canada, representing a variety of physico-chemical and biological conditions. During our study, several of the lakes were treated with additions of inorganic nitrogen and phosphorus fertilizers. Picoplankton communities in most lakes were dominated (>70%) by unicellular or colonial coccoid cyanobacteria, provisionally identified by morphological and autofluorescence properties as Synechococcus. Also common in some lakes were red-fluorescing cyanobacteria and Chlorella-like eucaryotes. Autotrophic picoplankters contributed from 36-63% to total chlorophyll, from >2-26% to total phytoplankton carbon, and from 29–53% to total photosynthesis. Average populations ranged from >5-10,000 cells·ml⁻¹ in winter and early spring to 65-75,000 cells · ml⁻¹ in summer and fall. Peak densities in most lakes occurred in August-September and most populations were within the epilimnion or metalimnion/hypolimnion boundary. Subsurface peaks were prevalent only in untreated, strongly stratified lakes. Eucaryotic picoplankters became dominant in acidic (pH < 6.2), humic lakes. Colonial picoplankters were more common in more productive interior lakes in August, and though present, were uncommon in coastal systems. Picoplankton populations exhibited large increases under ice in a Yukon lake, and their abundance and seasonal distribution showed little relation to temperature or to light. Fertilization of lakes resulted in picoplankton population increases (>2x) and the elimination of subsurface peaks. Nutrients were considered to be one of the major factors controlling population abundance in these oligotrophic lakes with average pH < 6.5.     

吉林省白城、松原地区部分湖泡藻类植物的初步研究

2008年6～7月间对吉林省白城及松原地区4个大型湖泡中的藻类植物进行了调查,共鉴定藻类植物86个分类单位,包括76种10变种,分别隶属于5门7纲12目20科37属,初步分析了白城及松原地区4个大型湖泡中藻类植物群落的种类组成及生态类型,在该区域出现的大部分藻类属于普生种类,同时出现了一些碱水种类,此外还有少量的污染指示种出现。根据种类组成及分布初步推断,夏季白城、松原地区部分湖泡的藻类植物群落组成主要为硅藻—绿藻类型。