The importance of nitrogen in Pyramid Lake (Nevada,USA), a saline,desert lake

The increase in human development in the downstream portion of the Pyramid Lake drainage basin has resulted in increased nutrient loading to the lake. Since this is a deep, terminal lake, concern over nutrient build up and change in trophic status exists. On the basis of lake chemistry which shows consistently high concentrations of total reactive-P (mean = 55 µg P l^–1) relative to dissolved inorganic-N (DIN) (mean = 15 µg N 1^–1), it has been hypothesized that Pyramid is N-limited. However, no systematic study of nutrient limitation had been undertaken. Nutrient enrichment bioassays conducted throughout an entire year clearly showed that additions of DIN resulted in a 350–600% stimulation of chlorophyll production. Phosphate, when added singly or in combination with DIN, had no effect. This positive response to N-addition was significant at all times of the year except, (1) immediately after complete lake mixing in February when a large pool of hypolimnetic nitrate was injected into the euphotic zone, and (2) during a fall bloom of the nitrogen fixing species Nodularia spumigena. The positive response to N-addition in the bioassay experiments was strong between March and November. However, the seston exhibited only a gradual depletion of nitrogen relative to carbon over this same period. PN:PC ratios suggested no N-deficiency in phytoplankton biomass in February, March and April, moderate N-deficiency in May, June and July and, severe N-deficiency from August until winter turnover. The appearance of nitrogen fixing blue-green algae in September supports the hypothesis of N-limitation in the summer-autumn. In evaluating the nutrient status of a lake, the concepts of nutrient stimulation versus nutrient deficiency versus nutrient limitation must clearly be defined.This paper is dedicated to G. Evelyn Hutchinson who first visited Pyramid Lake in 1933.     

Community structure of Archaea and Bacteria in a profundal lake sediment Lake Kinneret (Israel)

The microbial community structure of an anoxic profundal lake sediment, i.e., subtropical Lake Kinneret, was analysed with respect to its composition by culture-independent molecular methods including terminal restriction fragment length polymorphism (T-RFLP) analysis, comparative sequence analysis, and quantitative real-time PCR. In particular we were interested in the structure, species composition, and relative abundance of the overall microbial community in the methanogenic sediment layer (0-10 cm depth). Pairwise comparison of archaeal and bacterial 16S rRNA gene T-RFLP profiles obtained from three independent samplings indicated stability of the microbial community. The numbers of Archaea and Bacteria, quantified by real-time PCR, amounted to about 10(8) and 10(10) 16S rRNA gene copies cm(-3) sediment, respectively, suggesting that Archaea may account for only a minor fraction (approximately 1%) of the total prokaryotic community. Hydrogenotrophic Methanomicrobiales and acetoclastic Methanosaeta spp. dominated T-RFLP profiles of the archaeal community. T-RFLP profiles of the bacterial community were dominated by Deltaproteobacteria, sulphate reducers and syntrophs in particular. The second most abundant group was assigned to the Bacteroidetes-Chlorobi-group. Only one bacterial group, which was affiliated with halorespiring bacteria of subphylum II of the Chloroflexi, showed variation in abundance within the sediment samples investigated. Our study gives a comprehensive insight into the structure of the bacterial and archaeal community of a profundal lake sediment, indicating that sulphate reducers, syntrophs, bacteroidetes, halorespirers and methanogens are of particular importance in Lake Kinneret sediment.     

A sulfate-reducing bacterium from the oxic layer of a microbial mat from Solar Lake (Sinai), Desulfovibrio oxyclinae sp. nov.

In an investigation on the oxygen tolerance of sulfate-reducing bacteria, a strain was isolated from a 10⁷-fold dilution of the upper 3-mm layer of a hypersaline cyanobacterial mat (transferred from Solar Lake, Sinai). The isolate, designated P1B, appeared to be well-adapted to the varying concentrations of oxygen and sulfide that occur in this environment. In the presence of oxygen strain P1B respired aerobically with the highest rates [260 nmol O₂ min^–1 (mg protein)^–1] found so far among marine sulfate-reducing bacteria. Besides H₂ and lactate, even sulfide or sulfite could be oxidized with oxygen. The sulfur compounds were completely oxidized to sulfate. Under anoxic conditions, it grew with sulfate, sulfite, or thiosulfate as the electron acceptor using H₂, lactate, pyruvate, ethanol, propanol, or butanol as the electron donor. Furthermore, in the absence of electron donors the isolate grew by disproportionation of sulfite or thiosulfate to sulfate and sulfide. The highest respiration rates with oxygen were obtained with H₂ at low oxygen concentrations. Aerobic growth of homogeneous suspensions was not obtained. Additions of 1% oxygen to the gas phase of a continuous culture resulted in the formation of cell clumps wherein the cells remained viable for at least 200 h. It is concluded that strain P1B is oxygen-tolerant but does not carry out sulfate reduction in the presence of oxygen under the conditions tested. Analysis of the 16S rDNA sequence indicated that strain P1B belongs to the genus Desulfovibrio, with Desulfovibrio halophilus as its closest relative. Based on physiological properties strain P1B could not be assigned to this species. Therefore, a new species, Desulfovibrio oxyclinae, is proposed. Received: 7 August 1996 / Accepted: 29 January 1997     

Species composition,distribution and seasonal dynamics of Rotifera in a Rift Valley lake in Ethiopia (Lake Awasa)

The species composition of rotifers in Lake Awasa was studied and 40 species recorded. Seven species appeared in large numbers in the plankton seasonally. Brachionus and Keratella species made up more than 50% of the rotifer community by numbers during the observation period (1983–1987). The standing stock numbers of rotifer species are low with a maximum of 50 individuals L^–1, and some possible reasons for this observation are discussed. Most rotifer species are distributed randomly in the lake and show 3-fold fluctuations between consecutive days.The seasonal dynamics of most rotifer species are correlated with mixing periods in the lake, and the amplitude of seasonal fluctuation may be as high as 50-fold. Clear-cut seasonal succession of rotifer species was observed during the study period, but no consistent seasonal pattern for individual species was obvious. Also, observations indicate that rotifer biomass is partly sustained by availability of carbon through the bacterial pathway and that competitive exclusion for food by nauplii and ciliates probably keeps rotifer abundance low.     

富营养化城市湖泊武汉东湖沉积物微生物群体结构及空间变化研究(英文)北大核心CSCD

【目的】城市湖泊沉积物中的微生物群体是富营养环境生物修复的重要因素。本研究拟对中型富营养化湖泊——武汉东湖沉积物细菌群体结构及空间变化规律进行研究。加深对富营养湖泊微生物群体结构的理解,并为城市湖泊的保护和污染控制提供参考。【方法】采用16S r RNA基因RFLP分析和序列分析技术获得细菌群体系统发育信息。采用PCoA法分析武汉东湖沉积物细菌群体内以及与其它湖泊沉积物细菌群体间的相似性。采用CCA法分析环境因素对细菌群体的影响。【结果】武汉东湖沉积物细菌群体包含13个门和2个未知种群。PCoA分析进一步表明来自东湖3个子湖的细菌群体与其它地域富营养化湖泊的微生物种群结构相似,并区别于重度富营养化子湖庙湖的细菌群体。在庙湖中,我们发现了Thermogymnomonas类古菌的含量显著偏高。CCA分析表明细菌的分布与沉积物中碳、氮和磷元素的含量显著相关。【结论】本研究进一步确证了环境因素对细菌群体组成和分布的调节作用,加深了对富营养水体微生物群体结构的理解,并为城市湖泊的保护和污染控制提供了参考。     

Effects of the halocline on water quality and phytoplankton composition in a shallow brackish lake (Lake Obuchi, Japan)

The relationships of the halocline to both water quality and phytoplankton composition in Lake Obuchi, a shallow brackish lake in northern Japan, were investigated from April 2001 to December 2004. The halocline in this lake became stronger in summer (July–September, mean maximum density gradient 4.3–5.8 ρ_tm⁻¹) but weaker in spring, fall, and winter (1.9–3.3 ρ_tm⁻¹). Although the difference in water quality between the upper and lower layers separated by the halocline was high in summer, nutrients (PO₄³⁻-P and NH₄⁺-N) were eluted from the bottom sediment as levels of dissolved oxygen decreased in the bottom layer because of the strong stratification caused by the halocline formed over the long term. Moreover, phytoplankton taxa composition also differed between the upper and lower layers in summer, but was similar in other seasons. The dominant phytoplankton taxa in the upper layer in summer were Skeletonema costatum and Cyclotella spp., whereas in the lower layer, Gymnodinium spp. (Dinophyceae) and Chlorophyceae, which prefer eutrophic and low dissolved oxygen conditions, dominated. This suggests that the halocline was related to differentiations in both water quality and ecosystem components between the upper and lower layers in the brackish lake water.     

Afrocymbella barkeri sp. nov. (Bacillariophyta), a common phytoplankton component of Lake Challa,a deep crater lake in East Africa

A new freshwater diatom, Afrocymbella barkeri Cocquyt & Ryken sp. nov., is described from Lake Challa, a deep and unproductive crater lake near Mt Kilimanjaro in equatorial East Africa, based on light and electron microscopic observations. This tropical diatom is a representative of the rather small genus Afrocymbella with only 12 known species and with a distribution restricted to the African Rift. Taxa belonging to this genus are heteropolar and characterized by dorsiventral valves curved along the pervalvar axis and the presence of small pseudosepta and septa on the open girdle bands. Afrocymbella barkeri was common only towards the end of the dry and windy season corresponding to northern hemisphere summer, when deep water-column mixing caused upwelling of nutrient-rich water from the hypolimnion. This taxon was observed free living in the water column during mixing, but the presence of a small apical pore field at the foot pole, along with some cells with mucilage stalks, suggests that its primary habitat probably involves attachment to a substrate.     

A multivariate statistical evaluation of the “acolla-complex” of Corythionella species, including a description of C. darwini n. sp. (Rhizopoda: Filosea or Rhizaria: Cercozoa)

The testate amoebid genus Corythionella inhabits beach sands and includes three marine and two freshwater species of the “acolla-complex”, all characterized by a poorly developed flared collar surrounding the pseudostomal aperture. Multivariate statistical methods (cluster analysis, principal coordinates analysis, non-metric multidimensional scaling ordination and discriminant analyses) were applied to morphologic characters of the test (shell) of the members of this complex in order to visualize and assess the degree of similarity among them. Graphical displays of the results of these analyses clearly revealed distinct separations between all pairs of species, including a sixth previously un-named taxon described here as a new species (Corythionella darwini n. sp.). Discriminant functions were developed to assist in identification of Corythionella with reduced oral collars.C. darwini was discovered in Pacific Ocean beach sand samples from western Canada and is characterized by a test with a median length, width, height and pseudostome width of 53, 20, 19 and 12 μm, respectively. It presents the least variability in test morphology of all known species in the “acolla-complex” (coefficients of variation ranging between 3 and 5.6% for the four test variables) and is further distinguished from the other members of the complex by its distinctive near-cylindrical shape.     

Recruitment, early survival and growth of the Mediterranean red coral Corallium rubrum (L 1758), a 4-year study

Recruitment, early survival and growth rates of the precious Mediterranean red coral Corallium rubrum (L 1758) are poorly known. We examined these life history traits by means of artificial long-term settlement plates. Eighteen marble tiles placed off the coast of Leghorn (Tuscany, Italy) at two depths (25 and 35 m) were photographed monthly over the 4-year period from 1998 to 2002. Overall, 864 transparencies were examined to follow the individual life histories of red coral colonies belonging to four successive cohorts. Red coral planulae settled on tiles each year between July and September. Overall, 388 settlers colonized the tiles (244 at 25 m and 144 at 35 m), and their respective densities varied between 12.37±6.1 and 2.75±2.4 dm⁻². Heavy mortality affected these colonies (−24.35±9.12 colonies % y⁻¹), but, after 4 years, the tiles still harboured a persistent population (19±4.97 and 9.75±2.87 colonies dm⁻², respectively, at 25 and 35 m) with positive net recruitment rates. Only in 1999 did the net recruitment rate show a negative trend, although only at the shallower depth. At the same time (late summer 1999), a thermal anomaly affected several epibenthic communities in the Ligurian Sea. After 4 years, the tiles were removed, and the colonies that settled on them were measured. The average annual growth rate of colonies was low (0.62±0.19 mm y⁻¹ in diameter), and a marked reduction in growth with age was observed. Our findings suggest that the populations of this slow-growing long-lived octocoral exhibit a high capacity for colonization and seem to be quite resilient to environmental variability.     

Seasonal variation in the composition and rate of change of the phytoplankton community in a deep subalpine lake (Lake Garda, Northern Italy). An application of nonmetric multidimensional scaling and cluster analysis

Seasonal variations and spatial homogeneity of the phytoplankton community were followed, from spring 1991 to spring 1992, in four pelagic stations of a large deep subalpine lake (Lake Garda, Northern Italy). Both cluster analysis and nonmetric multidimensional scaling (NMDS) applied to Bray & Curtis' dissimilarity matrices computed on density and biovolume data were employed; the Bray & Curtis index, calculated between pairs of chronologically contiguous samples, was also used as a measure of the community change rate over the temporal succession. In the tree diagrams obtained both from density and biovolume data, six groups of different size have been identified, with ordered sequences of samples within them. Superimposition of the results of cluster analysis on the NMDS configurations has allowed interpretations of the trajectories of the samples as a chronological and cyclical succession of compositionally homogeneous groups. A clear relationship between the community change rate and stability of the water column has been assessed. The specific composition of the six groups has been discussed in relation to environmental variables and in terms of adaptive strategies. During winter (winter-spring group) the turbulence of the water and the availability of nutrients favour the development of colonial Bacillariophyceae (Fragilaria crotonensis Kitton, Tabellaria fenestrata (Lyngb.) Kütz., Aulacoseira spp.), Cyanophyceae (Planktothrix agardhii (Gom.) Anagn. et Kom. and Planktolyngbya limnetica (Lemur.) Kom.-Legn. et Cronb.) and Cryptophyceae. In late spring group, with the stabilisation of the water column and silicon depletion, the diatoms give way to small, opportunistic species (Ankyra judayi (G. M. Smith) Fott, Cyclotella spp., Chroomonas acuta Utermhöl) and larger species (Ceratium hirundinella (O. F. Müll.) Dujardin). The three summer groups are characterised by a development of Chlorophyceae (chiefly Chlorococcales), Cyanophyceae (mainly Chroococcales), Cyclotella spp. and Dinophyceae. With the autumn destratification the summer community undergoes a rearrangement principally in favour of Cyanophyceae and Cryptophyceae.     

The first European stand of Paramecium sonneborni (P. aurelia complex), a species known only from North America (Texas,USA)

P. aurelia is currently defined as a complex of 15 sibling species including 14 species designated by Sonneborn (1975) and one, P. sonneborni, by Aufderheide et al. (1983). The latter was known from only one stand (Texas, USA). The main reason for the present study was a new stand of Paramecium in Cyprus, with strains recognized as P. sonneborni based on the results of strain crosses, cytological slides, and molecular analyses of three loci (ITS1-5.8S-ITS2-5′LSU rDNA, COI, CytB). The new stand of P. sonneborni in Europe shows that the species, previously considered endemic, may have a wider range. This demonstrates the impact of under-sampling on the knowledge of the biogeography of microbial eukaryotes. Phylogenetic trees based on all the studied fragments revealed that P. sonneborni forms a separate cluster that is closer to P. jenningsi and P. schewiakoffi than to the other members of the P. aurelia complex.     

Genetic impoverishment and cross-incompatibility in remnant genotypes of Ziziphus celata (Rhamnaceae), a rare shrub endemic to the Lake Wales Ridge, Florida

The loss of genetic diversity in fragmented populations ofself-incompatible plant species may result in sexual reproductive failure andlocal extinctions. Florida ziziphus (Ziziphus celata) is aself-incompatible clonal shrub known only from five genetically depauperatepopulations on the Lake Wales Ridge, Florida, USA. Recovery of this speciesrequires identification of cross-compatible genotypes that can be used to createviable (i.e., sexually reproducing) populations. To further development of arecovery program for this highly imperiled species, we investigated its geneticstructure and sexual reproductive viability. We used random amplifiedpolymorphic DNAs (RAPDs) to investigate genetic variability within remnantpopulations and we conducted experimental compatibility trials to determine thecross-compatibility of remnant genotypes. One hundred and ninety-nine uniquestem samples collected from one ex situ and fivein situ populations were assayed for the presence orabsence of a band for 32 RAPD markers. Based on unweighted pair-group meancluster analysis (UPGMA), only 11 multi-locus genotypes (MLGs) were identified.Eight of these MLGs correspond to MLGs identified in an earlier allozyme study.In addition, we identified three new RAPD-based MLGs. Three of the five naturalpopulations consisted of only one MLG, while the largest and most geneticallydiverse population comprised only four MLGs. Coefficients of similarity rangedfrom 96.6% for the most closely related MLGs to 20.7% for the most distantlyrelated. The compatibility trials demonstrated that most MLGs arecross-incompatible. With 69% of all possible one-way crosses tested (38/55), wehave identified only eight compatible crosses via germination trials. Based onthe results of the compatibility trials, we assigned MLGs toself-incompatibility (SI) mating types. On present evidence, the currentbreeding population of Florida ziziphus may comprise as few as two SI matingtypes. These SI mating types will be used to guide future breeding efforts andan experimental introduction.     

Spatial Analysis of Twaite Shad,Alosa fallax (Lacepède, 1803), in the Southern North Sea: Application of Non‐Linear Geostatistics as a Tool to Search for Special Areas of Conservation

This study aims to evaluate the suitability of non‐linear geostatistics and indicator kriging (IK) as a tool in environmental impact assessment and nature conservation, in particular to search for potential Special Areas of Conservation (SAC) for the endangered fish species twaite shad, Alosa fallax (Lacepède , 1803) within the German Exclusive Economical Zone (EEZ) of the North Sea. To analyse the spatial distribution of this fish species, data on standardised biomass index (catch per unit effort, c.p.u.e., kg × 30 min^–1) from 1996 to 2001 were used, regarding the third and fourth quarters of each year, respectively. Thereby we assume that the spatial distribution can be described as a time‐invariant process. This assumption is supported by information on annual sampling effort, allocation of hauls and spatial distribution of the positive catches. All indicator variograms obtained for different c.p.u.e. cut‐off values displayed distinct spatial structures, clearly indicating that the indicator variables were spatially autocorrelated. Gaussian models were fitted by least‐squares methods and were evaluated with a goodness‐of‐fit statistic. Subsequently, IK was employed to estimate the probability of exceeding the c.p.u.e. cut‐off values for the twaite shad in the investigation area. These were highest in the Weser‐ and Elbe‐estuary, probably because of migrations of twaite shad to and from estuaries at the time of investigation due to spawning, while within the German EEZ of the North Sea no such areas with increased probabilities could be discerned. Thus, although available data did not allow to identify and implement any SAC in the German EEZ, the methods employed here can be regarded as a promising management tool in biological conservation issues. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)