Production and fate of faecal pellets during summer in an East Antarctic fjord

The abundance of small faecal pellets is high in marine waters. Little is known, however, about the processes governing their production and fate in the water column. We investigated faecal pellet production and flux in relation to the phytoplankton and copepod assemblages present in Ellis Fjord, Antarctica. Results show that the phytoplankton community shifted from a dominance of diatoms to that of a cryptomonad species during late January. This coincided with an increase in abundance of the small copepods Paralabidocera antarctica and Oithona similis, although Oncaea curvata was still the dominant species. The mean faecal pellet flux was 9943 pellets m^–2 d^–1. Only 37% of the faecal pellet flux at 5 m sedimented to 10 m depth, 15% to 20 m, and 12% to 40 m depth. Our results suggest that recycling of faecal pellets by copepods contributes to this decreased flux with increasing depth, which concurs with results from large scale oceanic studies. Additionally, we propose that the summer ice melt changes the physical characteristics of the water column and the phytoplankton species abundance and distribution; both of which potentially impact on the distribution and abundance of copepods, thereby regulating faecal pellet flux.     

Active Bacterial Populations and Grazing Impact Revealed by an In Situ Experiment in a Shallow Aquifer

To elucidate bacterial population dynamics in an aquifer, we attempted to reveal the impact of protozoan grazing on bacterial productivity and community structure by an in situ incubation experiment using a diffusion chamber. The abundance and vertical distribution of bacteria and protozoa in the aquifer were revealed using wells that were drilled in a sedimentary rock system in Itako, Ibaraki, Japan. The water column in the wells possessed aerobic and anaerobic layers. Active bacterial populations under the grazing pressure of protozoa were revealed through in situ incubation with grazer eliminating experiment by the filtration. On August 19, 2003, the total number of bacteria (TDC) decreased from 1.5 × 10⁶ cells ml^{? 1} at 2.2 m depth to 3.0 × 10⁵ cells ml^{? 1} at 10 m depth. The relative contribution of the domain Bacteria to TDC ranged between 63% and 84%. Protozoa existed at a density of 4.2 × 10⁴ to 1.9 × 10⁵ cells ml^{? 1} in both aerobic and microaerobic conditions. A grazing elimination experiment in situ for 6 days brought about clearly different bacterial community profiles between the 2.2 m and 10 m samples. The bacterial composition of the initial community was predominantly β- and γ -proteobacteria at 2.2 m, while at 10 m β-, α - and γ -proteobacteria represented 56%, 26% and 13% of the community, respectively. The distribution of bacterial abundance, community composition and growth rates in the subsurface were influenced by grazing as well as by geochemical factors (dissolved oxygen and concentrations of organic carbon, methane and sulfate). Results of the in situ incubation experiment suggested that protozoan grazing contributes significantly to bacterial population dynamics.     

Adaptation of the polony technique to quantify Gokushovirinae,a diverse group of single-stranded DNA phage

Advances in metagenomics have revealed the ubiquity of single-stranded DNA (ssDNA) phage belonging to the subfamily Gokushovirinae in the oceans; however, the abundance and ecological roles of this group are unknown. Here, we quantify gokushoviruses through adaptation of the polony method, in which viral template DNA is immobilized in a gel, amplified by PCR, and subsequently detected by hybridization. Primers and probes for this assay were designed based on PCR amplicon diversity of gokushovirus major capsid protein gene sequences from a depth profile in the Gulf of Aqaba, Red Sea sampled in September 2015. At ≥95% identity, these 87 gokushovirus sequences formed 14 discrete clusters with the largest clades showing distinct depth distributions. The application of the polony method enabled the first quantification of gokushoviruses in any environment. The gokushoviruses were most abundant in the upper 40 m of the stratified water column, with a subsurface peak in abundance of 1.26 × 10⁵ viruses ml⁻¹. These findings suggest that discrete gokushovirus genotypes infect bacterial hosts that differentially partition in the water column. Since the designed primers and probe are conserved across marine ecosystems, this polony method can be applied broadly for the quantification of gokushoviruses throughout the global oceans.     

Abundance and size change of Hannaea baicalensis in Lake Baikal

Hannaea baicalensis is a benthic pennate diatom that predominantly grows at depths of less than a metre attached to rocks and filamentous algae in Lake Baikal, Russia. This narrow zone at the edge of the lake is subject to frequent wave action and lake level fluctuations, which combine with other factors to affect seasonal abundance. During ice cover from January to May in 2008, when lake levels decreased from 42 to 14 cm above datum, H. baicalensis cell abundance remained low (0.39 × 10⁶ cells cm^–2). The main period of net cell increase occurred in autumn, when there was a period of stable lake level (±10 cm changes in water depth) that coincided with the return of nutrients during autumn overturn. Cell abundance reached 1.52 × 10⁶ cells cm^–2 on 31 October. Alongside the changes in abundance, cyclic size changes in cell apical lengths were found (40 to 144 µm), which were associated with timing of the length of the life cycle. Size decline occurred in both spring and autumn, with an average decrease in apical length of 36 µm per year. It took two years for the mean apical length of a single cohort to decrease from 128 µm to 56 µm, which was then below the threshold (< 65 µm) for initiation of size regeneration.     

Natural succession of macroalgal-dominated epibenthic assemblages at different water depths and after transplantation from deep to shallow water on Spitsbergen

In the current study, we investigated the primary succession of seaweeds over different time periods at different water depths. Furthermore, we followed the succession of field-grown benthic communities of different successional age, developing on ceramic tiles, prior to and after transplantation from 8 to 0.5 m water depth. The transplantation simulated changes associated with the break up of sea-ice cover, e.g. light regime or wave exposure. For this purpose, we transplanted 12 and 21-month old communities, grown at 8 m water depth, together with a set of sterile tiles, onto rafts, floating in 0.5 m water depth. Our results describe for the first time the succession of macroalgal communities in the Arctic and give important insights into the effect of disturbance of differently aged communities. The primary succession at 0.5 m water depth was mainly driven by Bacillariophyta and filamentous green algae like Urospora sp. and Ulothrix implexa. Twelve-month old communities at 8 m water depth are dominated by members of the Ectocarpales (Phaeophyceae), like Pylaiella littoralis, P. varia, and Ectocarpus siliculosus and the green alga U. implexa, whereas the 21-month old community showed a higher cover of the green algal class Ulvophyceae and sessile invertebrates. After transplantation to near surface conditions, species composition of the communities changed, but this effect was differently strong between communities of different age.     

Epiphytes from a deep-water characean meadow in an oligotrophic New Zealand lake: species composition,biomass and photosynthesis

1. The epiphytic flora of a characean meadow in Lake Coleridge, a deep, oligotrophic lake on the South Island of New Zealand, was dominated by diatoms, particularly Eunotia pectinalis and Achnanthes minutissima. The meadows occupied a depth range from 5 to 30 m. Adnate taxa predominated at all depths below 5 m, while increased taxonomic diversity at 5 m resulted from an increased abundance of erect taxa, including chlorophytes and stalked diatoms. 2. Seasonal changes in epiphyte biomass were followed using artificial substrata and by estimating epiphyte chlorophyll a concentration on host plants. The latter required development of a novel technique utilizing the consistent relationship between fucoxanthin and chlorophyll a concentrations in the epiphyton. Epiphyte chlorophyll a on host plants varied with depth and host species between 0.1 and 0.3 mg g^–1 dry weight. Maximum epiphyte biomass was at 10–15 m depth. At depths of 15 m and less, epiphyte chlorophyll a reached a maximum of ≈ 200–300 mg m^–2 in mid-summer, while at greater depths maximum biomass was less and coincided with a period of clear water in spring. 3. Photosynthetic carbon fixation was estimated from photosynthesis–radiation curves and estimates of radiation flux at sampling depths. At depths greater than 10 m, variability of the vertical extinction coefficient of lake water rather than seasonal fluctuations in incident radiation were responsible for determining the temporal pattern of production. Chlorophyll a-specific photosynthesis was estimated to peak in summer at 5 m (8 mg mg^–1 day^–1), and in spring at all other depths. 4. Annual epiphyte production was estimated as 27 g C m^–2 year^–1 at 5 m depth, falling to 15 g C m^–2 year^–1 at 15 m and 1 g C m^–2 year^–1 at 30 m. Areal biomass changes tended to be temporally but not quantitatively coupled to estimated in situ photosynthesis, and we hypothesize that epiphyte biomass may have been controlled by grazing gastropod snails.     

Microbial community structure and biomass estimates of a methanogenic Antarctic Lake ecosystem as determined by phospholipid analyses

Phospholipid analyses were performed on water column particulate and sediment samples from Ace Lake, a meromictic lake in the Vestfold Hills, Antarctica, to estimate the viable microbial biomass and community structure in the lake. In the water column, methanogenic bacterial phospholipids were present below 17 m in depth at concentrations which converted to a biomass of between 1 and 7×10⁸ cells/liter. Methanogenic biomass in the sediment ranged from 17.7×10⁹ cells/g dry weight of sediment at the surface to 0.1×10⁹ cells/g dry weight at 2 m in depth. This relatively high methanogenic biomass implies that current microbial degradation of organic carbon in Ace Lake sediments may occur at extremely slow rates. Total microbial biomass increased from 4.4×10⁸ cells/ liter at 2 m in depth to 19.4×10⁸ cells/liter at 23 m, near the bottom of the water column. Total nonarchaebacterial biomass decreased from 4.2 ×10⁹ cells/g dry weight in the surface sediment (1/4 the biomass of methanogens) to 0.06×10⁸ cells/g dry weight at 2 m in depth in the sediment. Phospholipid fatty acid profiles showed that microeukaryotes were the major microbial group present in the oxylimnion of the lake, while bacteria dominated the lower, anoxic zone. Sulfate-reducing bacteria (SRB) comprised 25% of the microbial population at 23 m in depth in the water column particulates and were present in the surface sediment but to a lesser extent. Biomass estimates and community structure of the Ace Lake eco-system are discussed in relation to previously measured metabolic rates for this and other antarctic and temperate ecosystems. This is the first instance, to our knowledge, in which the viable biomass of methanogenic and SRB have been estimated for an antarctic microbial community.     

The sub-ice fauna of the Laptev Sea and the adjacent Arctic Ocean in summer 1995

The sub-ice habitat and fauna in the Laptev Sea and the adjacent Arctic Ocean were investigated during the “Polarstern” cruise ARK XI/1 in summer 1995. At the ice-water interface a thin thermo- and halocline developed at many stations due to melting processes. In the lower centi- to decimetres of the ice, an accumulation of organic matter was found (particulate organic carbon: 1.9 mg l⁻¹, chl a: 3.3 μg l⁻¹) that may have provided a food source for the fauna. The water layer directly beneath the ice was inhabited by high numbers of various nauplii (130–23911 ind. m⁻³), and two ecological groups, the pelagic sub-ice fauna that originates from the surface water plankton, and the sympagic sub-ice fauna that migrates into this boundary layer from the ice interior. The pelagic fauna dominated the sub-ice community both in terms of species number and abundance. Both groups mainly comprised small copepods (e.g. Oithona similis, Oncaea borealis, Pseudocalanus spp., Halectinosoma spp., Tisbe spp.), but foraminifers and pteropods, for example, also occurred regularly. Diversity was generally low. Factors influencing the composition and abundance of the sub-ice fauna were most likely water depth, salinity and sea-ice sediments. Accepted: 6 July 1998     

Microbial processes of the carbon and sulfur cycles in an ice‐covered,iron‐rich meromictic lake Svetloe (Arkhangelsk region,Russia)

Biogeochemical, isotope geochemical and microbiological investigation of Lake Svetloe (White Sea basin), a meromictic freshwater was carried out in April 2014, when ice thickness was ～0.5 m, and the ice‐covered water column contained oxygen to 23 m depth. Below, the anoxic water column contained ferrous iron (up to 240 μμM), manganese (60 μM), sulfide (up to 2 μM) and dissolved methane (960 μM). The highest abundance of microbial cells revealed by epifluorescence microscopy was found in the chemocline (redox zone) at 23–24.5 m. Oxygenic photosynthesis exhibited two peaks: the major one (0.43 μmol C L^?1 day^?1) below the ice and the minor one in the chemocline zone, where cyanobacteria related to Synechococcus rubescens were detected. The maximum of anoxygenic photosynthesis (0.69 μmol C L^?1 day^?1) at the oxic/anoxic interface, for which green sulfur bacteria Chlorobium phaeoclathratiforme were probably responsible, exceeded the value for oxygenic photosynthesis. Bacterial sulfate reduction peaked (1.5 μmol S L^?1 day^?1) below the chemocline zone. The rates of methane oxidation were as high as 1.8 μmol CH₄ L^?1 day^?1 at the oxi/anoxic interface and much lower in the oxic zone. Small phycoerythrin‐containing Synechococcus‐related cyanobacteria were probably involved in accumulation of metal oxides in the redox zone.     

Diving and foraging behaviour of Adélie penguins in areas with and without fast sea-ice

The diving and foraging behaviours of Adélie penguins, Pygoscelis adeliae, rearing chiks at Hukuro Cove, Lützow-Holm Bay, where the fast sea-ice remained throughout summer, were compared to those of penguins at Magnetic Island, Prydz Bay, where the fast sea-ice disappeared in early January. Parent penguins at Hukuro Cove made shallower (7.1–11.3 m) but longer (90–111 s) dives than those at Magnetic Island (22.9 m and 62 s). Dive duration correlated with dive depth at both colonies (r ² = 0.001 ∼ 0.90), but the penguins atg Hukuro Cove made longer dives for a given depth. Parents at Hukuro Cove made shorter foraging trips (8.1–14.4 h) with proportionally longer walking/swimming (diving < 1 m) travel time (27–40% of trip duration) and returned with smaller meals (253–293 g) than those at Magnetic Island, which foraged on average for 57.2 h, spent 2% of time walking/swimming ( < 1 m) travel, and with meals averaging 525 g. Trip duration at both colonies correlated to the total time spent diving. Trip duration at Hukuro Cove, but not at Magnetic Island, increased as walking/swimming ( < 1 m) travel time increased. These differences in foraging behaviour between colonies probably reflected differences in sea-ice cover and the availability of foraging sites. Received: 3 November 1995/Accepted: 29 May 1996     

Vertical distribution in and isolation of bacteria from Lake Vanda: an Antarctic lake

Vertical distribution of bacteria in Lake Vanda, an Antarctic meromictic lake, was examined by the acridine orange epifluorescence direct count method. Total bacteria were 10⁴–10⁵ cells · ml^–1 in the water at 55 m depth and above, and increased drastically to 10⁷ cells · ml^–1 in the bottom water. Filamentous or long rodshaped bacteria occurred at a high frequency in the upper layers, but in the bottom layers most bacteria were coccoidal or short rods. Mean bacterial cell volume in water of between 10 m and 60 m deep was fairly large compared with common bacterial populations in seawater and lake water. Aerobic heterotrophic bacteria were recovered from the water of a depth of 30 m and above, and were assumed to belong to Caulobacter. Viable heterotrophic bacteria were not recovered from the high salinity deep water by media prepared with the same deep water. Phototrophic purple non-sulphur bacteria were isolated by enrichment cultures from water at 55 m depth.     

Vertical distribution of organic constituents in an Antarctic lake: Lake Fryxell

Vertical distribution of organic constituents, i.e. total organic carbon (TOC), hydrocarbons, fatty acids and hydroxy acids in water and sediment samples from Lake Fryxell (77° 35 S, 163° 15 E) of southern Victoria Land, Antarctica were studied to elucidate their features in relation to stratification of the lake waters and likely distribution of microorganisms. The TOC content of the surface water (5.0 m; just below the ice cover of 4.50 m thickness) was 1.4 mg l^–1. It increased markedly with depth and attained a maximum value of 21.7 mg C l^–1 at a depth of 17.5 m, but decreased to the bottom (13.3 mg C l^–1). The high TOC content of the anoxic bottom layers (> 15 m) is attributable to the concentration of refractory organic substances over long periods following the degradation of labile organic constituents. Hydrocarbons were not found in the water column, but the major constituent of the bottom sediment was n-C_{29 : 2} alkene. Total concentrations of fatty acids in the oxic layers ( 10 m) were highest at 10.0 m and much higher than those in the anoxic layers (> 10 m), probably reflecting the phytoplankton population. The content of branched (iso and anteiso) fatty acids and 3-hydroxy acids in the anoxic layers were much greater than those in the oxic layers which would seem to reflect the distribution of bacterial abundance. The differences of organic composition between the water column and sediments imply that sinking dead organisms were quickly degraded in the lake bottom. Also, the composition of microorganisms in the water column must be very different from that in the sediments.     

Investigation of the sulfate-reducing bacterial community in the aerobic water and chemocline zone of the Black Sea by the fish technique

Fluorescent in situ hybridization (FISH) was used to analyze the abundance and phylogenetic composition of sulfate-reducing bacteria in the aerobic waters and in the oxic/anoxic transitional zone (chemocline) of the Black Sea, where biogenic formation of reduced sulfur compounds was detected by radioisotope techniques. Numerous sulfate-reducing bacteria of the genera Desulfotomaculum (30.5% of detected bacterial cells), Desulfovibrio (29.6%), and Desulfobacter (6.7%) were revealed in the aerobic zone at a depth of 30 m, while Desulfomicrobium-related bacteria (33.5%) were prevalent in the upper chemocline waters at 150-m depth. Active cells of sulfate-reducing bacteria were much more abundant in the samples collected in summer than in the winter samples from the deep-sea zone. The presence of physiologically active sulfate reducers in oxic and chemocline waters of the Black Sea correlates with the hydrochemical data on the presence of reduced sulfur compounds in the aerobic water column.     

The vertical distribution and abundance of <Emphasis Type="Italic">Gammarus lacustris</Emphasis> in the pelagic zone of the meromictic lakes Shira and Shunet (Khakassia,Russia)

The vertical distribution and abundance of Gammarus lacustris in the pelagic zone of two fishless meromictic lakes, L. Shira and L. Shunet, in Southern Siberia (Russia), was studied with the underwater video recording system and using vertical hauls. In both lakes, during summer stratification, Gammarus was distributed non-homogenously, with a stable peak in the metalimnion. The average depth of Gammarus population in the pelagic zone was significantly correlated with the depth of the thermocline. Gammarus abundances obtained using vertical plankton hauls with net were quite comparable with those obtained from video records. The peak abundance of Gammarus in the pelagic zone of the lakes observed with underwater video amounted up to 400 individuals m⁻², while the peak animal densities in the metalimnion reached 50 ind. m⁻³. The data are compared with previously published abundances of Gammarus in the littoral of Lake Shira. Both littoral and pelagic can be equally important habitats for amphipods in meromictic lakes. The absence of fish in the pelagic zone, high oxygen concentration, low water temperature, increased seston concentration, elevated water density in the metalimnion and the anoxic hypolimnion can be the most probable combination of factors that are responsible for the peak of Gammarus in the metalimnion of these lakes.     

The toxigenic marine dinoflagellate Alexandrium tamarense as the probable cause of mortality of caged salmon in Nova Scotia

The toxins associated with paralytic shellfish poisoning (PSP) are potent neurotoxins produced by natural populations of the marine dinoflagellate Alexandrium tamarense. In early June 2000, a massive bloom (>7×10⁵ cells l⁻¹) of this dinoflagellate coincided with an unusually high mortality of farmed salmon in sea cages in southeastern Nova Scotia. Conditions in the water column in the harbour were characterised by the establishment of a sharp pycnocline after salinity stratification due to abundant freshwater runoff. In situ fluorescence revealed a high sub-surface (2–4 m depth) chlorophyll peak related to the plankton bloom. The intense bloom was virtually monospecific and toxicity was clearly related to the concentration of Alexandrium cells in plankton size fractions. Cultured clonal isolates of A. tamarense from the aquaculture sites were very toxic on a per cell basis and yielded a diversity of PSP toxin profiles, some of which were similar to those from plankton concentrates from the natural bloom population. The toxin profile of plankton concentrates from the 21–56 μm size fraction was complex, dominated by the N-sulfocarbamoyl derivative C2, with levels of other PSP toxins GTX4, NEO, GTX5 (=B1), GTX3, GTX1, STX, C1, and GTX2, in decreasing order of relative abundance. Although no PSP toxin was found systemically in the fish tissues (liver, digestive tract) from this salmon kill event, the detection of Alexandrium cells and low levels of PSP toxins in salmon gills provide evidence that the enhanced mortalities were caused by direct exposure to toxic Alexandrium cells and/or to soluble toxins released during the bloom.     

Variability of the microbial community in the western Antarctic Peninsula from late fall to spring during a low ice cover year

Although winter conditions play a major role in determining the productivity of the western Antarctic Peninsula (WAP) waters for the following spring and summer, a few studies have dealt with the seasonal variability of microorganisms in the WAP in winter. Moreover, because of regional warming, sea-ice retreat is happening earlier in spring, at the onset of the production season. In this context, this study describes the dynamics of the marine microbial community in the Melchior Archipelago (WAP) from fall to spring 2006. Samples were collected monthly to biweekly at four depths from the surface to the aphotic layer. The abundance and carbon content of bacteria, phytoplankton and microzooplankton were analyzed using flow cytometry and inverted microscopy, and bacterial richness was examined by PCR–DGGE. As expected, due to the extreme environmental conditions, the microbial community abundance and biomass were low in fall and winter. Bacterial abundance ranged from 1.2 to 2.8 × 10⁵ cells ml^?1 showing a slight increase in spring. Phytoplankton biomass was low and dominated by small cells (<2 μm) in fall and winter (average chlorophyll a concentration, Chl-a, of, respectively, 0.3 and 0.13 μg l^?1). Phytoplankton biomass increased in spring (Chl-a up to 1.13 μg l^?1), and, despite potentially adequate growth conditions, this rise was small and phytoplankton was still dominated by small cells (2–20 μm). In addition, the early disappearing of sea-ice in spring 2006 let the surface water exposed to ultraviolet B radiations (UVBR, 280–320 nm), which seemed to have a negative impact on the microbial community in surface waters.     

南海北部海域春季浮游细菌和病毒空间分布及其影响因素

应用流式细胞检测技术测定了2014年春季南海北部海域浮游细菌和病毒丰度,研究了其水平和垂直分布特征并对其与环境因子的相关性进行了分析。结果表明,调查海区浮游细菌和病毒丰度分别介于1.28×10~4—9.96×10~5个/m L和4.69×10~5—5.39×10~7个/m L之间,二者丰度随水深的增加基本呈现逐渐下降的趋势,而水平分布趋势不明显。浮游细菌和病毒丰度与温度、p H和溶解氧显著正相关,与水深、盐度、活性磷酸盐、硅酸盐、硝酸盐和总氮则呈显著负相关关系(P0.01),说明该海域细菌和病毒数量受到上述环境因子的共同调控。分析浮游细菌和病毒的相互关系发现,VBR(Virus to bacteria ratio)平均32.23,最小值位于S11站位25m层,最大值则位于S7站位75m层,分别为4.80和264.63,VBR值小于100的站位占到调查站位总数的95.6%。VBR值除与细菌呈显著负相关关系外(P0.01),与其它环境因子相关性不明显(P0.05),说明该海区细菌是病毒的主要寄主,病毒可能主要是以噬菌体的状态存在。     

Artificial destratification of a small tropical reservoir: effects upon the phytoplankton

Seasonal changes in the phytoplankton community of a small tropical reservoir were monitored over a four year period comprising of an initial two seasonal cycles during which the water column stratified strongly for extended periods each year, and two further seasonal cycles after installation of a mechanical aeration system to induce artificial destratification. In the unmanaged reservoir, the concentration of chlorophyll a at 0.5 m reached maximum values (on one occasion > 90 mg m⁻³) when the water column was stratified and the epilimnion was very shallow (ca 2 m depth). The hypolimnion at this time was anoxic (less than 2% oxygen saturation) and had a high concentration of bacteriochlorophyll (100–200 mg m⁻³). The phytoplankton community of the unmanaged reservoir was generally dominated by cyanobacteria (Cylindrospermopsis raciborskii, Anabaena tenericaulis) during the warmer months of the year (November–March) (but replaced by chlorophyta, dinophyceae and euglenophyceae after periods of intense rain) and by bacillariophyceae (Synedra ulna var. chaseana, S. tenera) during the cooler, dry months. In the artificially destratified reservoir (8 h aeration day⁻¹), the phytoplankton community was largely dominated by diatoms except after depletion of the silica content of the water column which caused diatoms to be replaced by cyanobacteria (dominated by A. tenericaulis) and a range of chlorophytes. The changing pattern of stratification and circulation of the water column in the unmanaged reservoir caused repeated disruption of the established phytoplankton assemblage with peaks of high biomass associated with transient cyanobacterial blooms. Continuous aeration and the consequent increase in the ratio mixed: euphotic depth provided conditions suitable for dominance of the phytoplankton by diatoms, as long as silica was available, and resulted in average chlorophyll levels higher than in the unmanaged reservoir (120 ± 10 v. 64 ± 9 mg m⁻²). Hierarchical fusion analysis based on the biomass of species differentiated the phytoplankton samples into cluster groups that could be related primarily to stratification or mixing of the water column.     

蒸发条件下潜水埋深对土壤-柽柳水盐分布的影响

盐水矿化度下模拟设置4个潜水埋深(0.9、1.2、1.5、1.8 m),分析不同土层的土壤相对含水量(RWC)、含盐量(S_C)和土壤溶液绝对浓度(C_S)等水盐参数,及柽柳叶片和新生枝条的含水量及Na~+含量,探讨盐水矿化度下土壤-柽柳水盐参数对潜水埋深的响应规律。结果表明:各土层RWC与潜水埋深呈负相关,0.9 m潜水埋深下各土层的RWC均最高,且各土层RWC随土层深度的增加呈先降低后增加的趋势,其它潜水埋深下各土层RWC均逐渐增加,1.2 m是地下水所能上升且保持柽柳柱体土壤表层湿润的最高高度。各土层S_C和C_S与潜水埋深呈抛物线型,均表现为先增加后降低,潜水埋深1.2 m时,各土层S_C均最高。随土层深度的增加,各潜水埋深下S_C先降低后增加,而C_S呈现减少趋势;潜水埋深越高,土层间C_S变化幅度越激烈。潜水埋深对柽柳叶片和新生枝条的含水量无显著影响(P0.05),而随潜水埋深的增加,柽柳叶片Na~+含量逐渐增加,新生枝条Na~+含量则先增加后降低。从整个柽柳土柱看,随潜水埋深的增加,整个土壤剖面的RWC均值逐渐降低,而S_C和C_S均值先增加后降低,潜水埋深1.2 m是盐分变化的分界点,建议栽植柽柳的潜水埋深大于1.2 m。