Prosopis glandulosa and the nitrogen balance of rangelands: extent and occurrence of nodulation

Summary We report the recovery of root nodules from P. glandulosa var. glandulosa in the eastern portion of its range, where the species reaches its greatest vegetational development. Single cores 4.7 cm in diameter and up to 250 cm deep yielded from 0 to over 250 nodules. Nodules were found at all depths below 10 cm, with the highest concentration often around 100 cm. Detailed studies of three trees revealed relatively small volume densities of about 0.02 nodules cm^–3, high surface area densities of 2–4 nodules cm^–2, and high nodule biomass of 8–23 g m^–2, when compared to cultivated legumes. Nodules are small, weakly attached to roots that are seldom over 0.5 mm in diameter, and not easily observed under field conditions. No nodules were recovered from cores from the more arid western portion of P.glandulosa's range, although seedlings nodulated readily in these soils in the glasshouse as well as in most unamended soils from throughout mesquite's geographical range. Local differences in nodulating potential of soils included a negative association with mesquite canopies and a positive association with depth. These results suggest a significant role for biological fixation in the nitrogen regime and vegetation dynamics of Prosopis-dominated ecosystems.     

Translocation and plant availability of radio caesium in an organic soil

¹³⁴Caesium chloride solution was injected into the surface of a peaty podzol at 3 cm depth with 5 cm spacing over a M² at two upland sites, one of which had been fertilized and reseeded. The movement of radio caesium in the soil was subsequently followed by coring and/or taking out 10-cm square blocks at monthly intervals over a period of 2 years. There was very little movement of the caesium down the profile, with more than 95% remaining at the point of application. Lateral movement was also minimal. The caesium did not concentrate in the roots. Samples of herbage collected from the surface during the growing season showed a progressively decreasing concentration of caesium over the period. Total caesium removed in herbage amounted to 3.5 and 0.6% of the original present for the improved and umimproved soils, respectively.The extractability of the caesium from the ¹³⁴Cs-amended peaty soil was compared with that from a low-ash peat which had been treated with the radionuclide for 2 months. Initially 1 M ammonium acetate (pH 7.2) extracted 30 and 56% of the Cs from 0–3 and 3–6 horizous of the improved peaty podzol, respectively. After 5 months only about 12 and 7% of the Cs was extracted by 1 M ammonium acetate and 4 and 1% by 0.1 M sodium pyrophosphate from the 0–3 and 3–6 cm horizons, respectively. After 24 months 1 M ammonium acetate extracted 1.6 and 3.3% of the Cs from the 0–3 and 3–6 cm horizons, respectively. In contrast, the values for the peat after 2 months were 100 and 80% for 1 M ammonium acetate and 0.1 M pyrophosphate, respectively.     

Activity of North Jordan soil streptomycete isolates against Candida albicans

Eighteen percent of 116 different isolates of Streptomyces recovered from soils of northern Jordan showed activity against Candida albicans. The recovered isolates were distributed into three groups according to the diameter of the inhibition zone on the agar plate: group 1 (5–10 mm, slightly active); group 2 (11–15 mm, moderately active); and group 3 (16–35 mm, highly active). Isolates of group 3 were further grouped into four sub-groups and were culturally and morphologically identified. The u.v. spectra of the fermentation broth for the isolates in sub-group 4 were determined, and showed absorbance peaks ranging between 230 and 300 nm.     

Alley cropping with Senna siamea in South-western Nigeria: I. Recovery of 15N labeled urea by the alley cropping system

Improved cropping systems with in-situ production of organic matter require the input of additional inorganic N to maintain crop production in a sustainable way. For proper management of this fertilizer-N, it is necessary to quantify how the applied fertilizer N is used by the various components of the system and by the system as a whole. The fate of a single application of ¹⁵N labeled urea-N through the different components (crop, hedgerow, surface litter, soil profile up to 150 cm) of a Senna siamea alley cropping system, intercropped with maize in the first and cowpea in the second season, was followed for a period of 1.5 years (1994–1995), equivalent to 2 maize and 1 cowpea crop. Special attention was given to the role of the particulate organic matter (POM) in the cycling of urea-N through the soil organic matter (SOM). The maize crop recovered 26.5 and 1.7% of the applied urea-N at harvest in 1994 and 1995, respectively. The cowpea pods recovered only 0.7% of the applied urea-N at harvest. The highest proportion of applied urea-N recovered by the hedgerow occurred at 38 days after 1994 maize planting (DAP) (3.8%), while at later dates, recoveries of applied urea-N were always below 1%. This indicates that the Senna hedge is not a strong competitor for the applied urea-N during crop growth, i.e. while the Senna canopy is pruned at regular intervals. At 21 DAP, 12.7% of the applied urea-N was recovered in the surface litter and this value dropped significantly to 1.6% at 107 DAP and remained below 1% up to 480 DAP. The top 10 cm of soil contained 21% of the applied urea-N at 21 DAP and this value dropped to 9% at 480 DAP. Significantly more urea-N was recovered in the top 10 cm of soil than in the deeper soil layers at all sampling times. At 21 DAP, 11% of the applied urea-N was recovered in the 120–150 cm layer. This fast movement of urea-derived N to deep soil layers must have happened by preferential flow in macropores as the rainfall between urea application and the first sampling (74.2 mm) was not high enough to explain downward movement of N with the mobile water. Significant linear relationships between the proportion of urea-N in the different soil layers (excluding 0–10 cm) and the anion exchange capacity (AEC) and silt+clay content of the respective layers were found at 67, 107, 347 and 480 DAP. The total N content of the POM fraction increased significantly between 0 and 101 DAP from 127 to 171 mg N kg^–1 and decreased to 92 mg N kg^–1 at 480 DAP. The highest recovery of applied urea-N in the POM pool was measured at 101 DAP (3.6%) and this value decreased to 1.8% at 480 DAP. The total recovery of applied urea-N was 81% at 21 DAP, and decreased to values varying between 53 and 60% up from 38 to 347 DAP. At 480 DAP, the recovery decreased further to 47%. The fast movement of a substantial amount of urea-N may be responsible for this incomplete recovery, already at 21 DAP. Although the soil N status in the fertilized alley cropping system appears to be favourable for plant growth, this may be short-lived in the absence of further urea additions, as the soil-derived maize uptake in 1995 was already significantly lower than in 1994, and as the labile POM pool decreased significantly between the maize harvest in 1994 and 1995.     

Alley cropping with Senna siamea in South-western Nigeria: II. Dry matter,total N,and urea-derived N dynamics of the Senna and maize roots

Crop and tree roots are crucial in the nutrient recycling hypotheses related to alley cropping systems. At the same time, they are the least understood components of these systems. The biomass, total N content and urea-derived N content of the Senna and maize roots in a Senna-maize alley cropping system were followed for a period of 1.5 years (1 maize-cowpea rotation followed by 1 maize season) to a depth of 90 cm, after the application of ¹⁵N labeled urea. The highest maize root biomass was found in the 0–10 cm layer and this biomass peaked at 38 and 67 days after planting the 1994 maize (DAP) between the maize rows (112 kg ha^–1, on average) and at 38, 67 and 107 DAP under the maize plants (4101 kg ha^–1, on average). Almost no maize roots were found below 60 cm at any sampling date. Senna root biomass decreased with time in all soil layers (from 512 to 68 kg ha^–1 for the 0–10 cm layer between 0 and 480 DAP). Below 10 cm, at least 62% of the total root biomass consisted of Senna roots and this value increased to 87% between 60 and 90 cm. Although these observations support the existence of a Senna root `safety net' between the alleys which could reduce nutrient leaching losses, the depth of such a net may be limited as the root biomass of the Senna trees in the 60–90 cm layer was below 100 kg ha^–1, equivalent to a root length density of only < 0.05 cm cm^–3. The proportion of maize root N derived from the applied urea (%Ndfu) decreased significantly with time (from 21% at 21 DAP to 8% at 107 DAP), while %Ndfu of the maize roots at the second harvest (480 DAP) was only 0.6%. The %Ndfu of the Senna roots never exceeded 4% at any depth or sampling time, but decreased less rapidly compared to the %Ndfu of the maize roots. The higher %Ndfu of the maize roots indicates that maize is more efficient in retrieving urea-derived N. The differences in dynamics of the %Ndfu also indicate that the turnover of N through the maize roots is much faster than the turnover of N through the Senna roots. The recovery of applied urea-N by the maize roots was highest in the top 0–10 cm of soil and never exceeded 0.4% (at 38 DAP) between the rows and 7.1% (at 67 DAP) under the rows. Total urea N recovery by the maize roots increased from 1.8 to 3.2% during the 1994 maize season, while the Senna roots never recovered more than 0.8% of the applied urea-N at any time during the experimental period. These values are low and signify that the roots of both plants will only marginally affect the total recovery of the applied urea-N. Measurement of the dynamics of the biomass and N content of the maize and Senna roots helps to explain the observed recovery of applied urea-N in the aboveground compartments of the alley cropping system.     

The influence of micro-algae on the oxygen dynamics in a brackish ditch

Summary Seasonal changes in the activity of phytoplankton and benthic algae in relation to diurnal oxygen pulses were investigated in a 120 cm deep, brackish hypertrophic ditch. A vertical chloride gradient was built up by saline seepage and drain-water effluent. The stable chloride gradient could lead to oxygen stratification near the sediment, and to oxygen gradients towards the water surface. The oxygen gradients were rather unstable, depending on the chloride gradient and the wind velocity.Light was limiting photosynthesis both in summer and in winter. Surface oxygen maxima increased with solar radiation during summer.In summer the diatomCyclotella caused surface oxygen maxima at light saturation in the late afternoon. Simultaneously, the dominant flagellatesPeridinium andChlamydomonas produced oxygen in dim light, probably choosing their favourite light energy level by vertical migration. Oxygen fluctuations ranged from 0 to 34 mg O₂.l^–1 in a 100 cm vertical profile above a 20 cm anoxic layer. The amplitude of the diurnal oxygen maxima varied from 10 to 34 mg O₂.l^–1.In winter the water became very clear. The oxygen gradient was inverted during the day showing a characteristic oxygen maximum above the bottom, produced by benthicAchnanthes colonies.Communication no. 193.     

Disturbance and population dynamics of rotifers in bed sediments

Rotifers inhabiting the sediment surface and the hyporheic interstitial of a gravel stream were investigated between October 1991 and October 1992. At the sediment surface, samples were taken with a modified Hess sampler at five randomly chosen sites and at roughly weekly intervals. In the hyporheic interstitial, samples were collected using standpipe traps permanently installed at two sites at four sediment depths (0–40 cm), and every fifteen days. The distribution of five rotifer species revealed different temporal and spatial patterns. Two species, Notholca foliacea (Ehrb.) and Notholca squamula (O.F.M.) co-occurred having their temporal peaks at the beginning of summer and in autumn. Cephalodella gibba (Ehrb.) and Proales theodora (Gosse) occurred throughout the year, while Parencentrum longipes (Wulf.) showed a clear peak during the spring only in the hyporheic interstitial. The effect of variables such as water level, surface discharge and variation of the groundwater level was tested upon the densities of each species. Despite the spring spate, the species are able to persist due to the presence of low-flow refuge habitats or the dispersion into deeper layer of the hyporheic interstitial.     

Spatial distribution of bacterivorous nematodes in a Chinese Ecosystem Research Network (CERN) site

Geostatistics was applied to assess the spatial distribution of bacterivorous nematodes in a Chinese Ecosystem Research Network (CERN) site in northeast China. A 42×30 m plot was divided into grids with 7×5 m spacing that included 49 sampling points. Soil samples were collected from depths of 0–10 cm and 10–20 cm. The results showed that Cephalobus and Panagrolaimus were the most dominant bacterivores, which together represented 74 and 82% of total bacterivorous nematodes on average at depths of 0–10 cm and 10–20 cm, respectively. Within-site variability was high for these nematode genera. Geostatistical analysis indicated that a major part of this variability was spatially dependent; variograms suggest that 55–100% of sample population variance was related to spatial autocorrelation over ranges of 9–91 m. The mean numbers of total bacterivores and Panagrolaimus populations were higher at 0–10 cm depth than at 10–20 cm depth. Between the two depths, different distribution patterns were found for Panagrolaimus, whereas similar patterns were found for Cephalobus. The maps obtained by kriging well illustrated horizontal distribution patterns of bacterivorous nematodes at two depths at field scale.     

The fate of15N-labelled organic nitrogen in sheep manure applied to soils of different texture under field conditions

The fate of nitrogen from¹⁵N-labelled sheep manure and ammonium sulfate in small lysimeters and plots in the field was studied during two growth seasons. In April 1991,¹⁵N-labelled sheep faeces (87 kg N ha^–1) plus unlabelled (NH₄)₂SO₄ (90 kg N ha^–1), and (¹⁵NH₄)₂SO₄ (90 kg N ha^–1) were each applied to three soils; soil 1 (100% soil + 0% quartz sand), soil 2 (50% soil + 50% quartz sand) and soil 3 (25% soil + 75% quartz sand). The lysimeters were cropped with spring barley (Hordeum vulgare L.) and undersown ryegrass (Lolium perenne L.). The barley crop recovered 16–17% of the labelled manure N and 56% of the labelled (NH₄)₂SO₄-N. After 18 months 30% of the labelled manure N and 65% of the labelled (NH₄)₂SO₄-N were accumulated in barley, the succeeding ryegrass crop and in leachate collected below 45 cm of soil, irrespective of the soil-sand mixture. Calculating the barley uptake of manure N by difference of N uptake between manured and unmanured soils, indicated that 4%, 10% and 14% of the applied manure N was recovered in barley grown on soil-sand mixtures with 16%, 8% and 4% clay, respectively. The results indicated that the mineralization of labelled manure N was similar in the three soil-sand mixtures, but that the manure caused a higher immobilization of unlabelled ammonium-N in the soil with the highest clay content. Some of the immobilized N apparently was remineralized during the autumn and the subsequent growth season. After 18 months, 11–19% of the labelled manure N was found in the subsoil (10–45 cm) of the lysimeters, most of this labelled N probably transported to depth as organic forms by leaching or through the activities of soil fauna. In unplanted soils 67–74% of the labelled manure N was recovered in organic form in the 0–10 cm soil layer after 4 months, declining to 55–64% after 18 months. The lowest recovery of labelled N in top-soil was found in the soil-sand mixture with the lowest clay content. The mass balance of¹⁵N showed that the total recovery of labelled N was close to 100%. Thus, no significant gaseous losses of labelled N occurred during the experiment.     

Discrimination of Aspergillus isolates at the species and strain level by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry fingerprinting

Matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI–TOF MS) was used to generate highly reproducible mass spectral fingerprints for 12 species of fungi of the genus Aspergillus and 5 different strains of Aspergillus flavus. Prior to MALDI–TOF MS analysis, the fungi were subjected to three 1-min bead beating cycles in an acetonitrile/trifluoroacetic acid solvent. The mass spectra contain abundant peaks in the range of 5 to 20 kDa and may be used to discriminate between species unambiguously. A discriminant analysis using all peaks from the MALDI–TOF MS data yielded error rates for classification of 0 and 18.75% for resubstitution and cross-validation methods, respectively. If a subset of 28 significant peaks is chosen, resubstitution and cross-validation error rates are 0%. Discriminant analysis of the MALDI–TOF MS data for 5 strains of A. flavus using all peaks yielded error rates for classification of 0 and 5% for resubstitution and cross-validation methods, respectively. These data indicate that MALDI–TOF MS data may be used for unambiguous identification of members of the genus Aspergillus at both the species and strain levels.     

Quantitation of mitomycin C in human ocular tissues by high-performance liquid chromatography–photo-diode array detection

A chromatographic method, which can quantitate mitomycin C (MMC) along with two antiglaucoma drugs, is described. The separation of MMC, alphagan and timolol was performed on a reversed-phase C₁₈ column with water–methanol–trifluoroacetic acid (65:35:0.01, v/v) as the mobile phase. By monitoring at 360, 248 and 296 nm, the lower limits of detection for MMC, alphagan and timolol are, respectively, 1.0, 2.0 and 5.0 ng (injection amount) at three-time S/N ratio. The dynamic ranges of quantitation for the three drugs are, respectively, 1.0 ng–10.0 μg, 2.0 ng–10.0 μg and 5.0 ng–10.0 μg with linearity being larger than 0.9960. This method was applied to the determination of MMC levels in Tenon’s and trabeculum tissues of 10 glaucoma patients. MMC levels in these tissues, which were obtained from glaucoma filtering surgery, were determined following a multiple extraction with methanol. The recovery of MMC for a two-batch extraction was better than 91.2%. The reproducibility of measurement for the MMC levels in these tissues is 2.5–6.0% RSD for triplicate injections. The intra-day variation of retention times for the MMC peaks was less than 1.6% RSD (n=3). The inter-day variation of retention times for the MMC peaks was less than 4.8% RSD (n=3). MMC was detectable in three trabeculum tissues out of 10 cases (ranging from 0.8 to 25.5 ng/mg specimen), while MMC was detected in nine Tenon’s tissues out of 10 cases (ranging from 0.3 to 21.1 ng/mg specimen). The results obtained show that the method is sensitive and selective for the quantitation of MMC.     

The soil C pool in different agroecosystems derived from the dry tropical forest of Guanacaste, Costa Rica

Inventories of soil C pools are still lacking from tropical sites. Our objective was to assess total C and N concentrations in the different mineral soil fractions down to 50 cm depth in relation to selected physical and chemical properties of 5 ecosystems at La Flor Sustainable Center in Guanacaste, Costa Rica. The ecosystems studied were a derived savanna with scattered trees, a gallery forest, an abandoned Mango indigofera L. plantation, a Citrus sp. plantation, and a Saccharum officinarum L. (sugarcane) plantation. Significant differences were found for the main fixed factor ecosystem for all variables analyzed (ANOVA). The TSC concentration was significantly higher in the sugarcane plantation compared to the rest of land use systems. The TSC concentration decreased significantly with increase in depth in all ecosystems and ranged from 20.3–38.3 to 4.3–20.9 g kg⁻¹ in the 0–10 and 40–50 cm depth, respectively. In all cases, the clay + silt fraction (<50 μm) contained the highest C concentration. N concentration (0–10 cm depth) at La Flor ranged from 0.32 to 0.19%, and decreased in the order sugarcane > Curatella savanna > Mango and Citrus plantations > gallery forest. A principal component analysis (PCA) performed with all variables studied showed that the ordination of land uses (ecosystems) in the factorial plane defined by the first two axes was significant (Monte Carlo permutation test, P < 0.0001). The highest TSC pool down to 50 cm depth was obtained in the sugarcane plantation (160 Mg C ha⁻¹) while less C was found in the rest of ecosystems, i.e. from 66 (gallery forest) to 80 Mg C ha⁻¹ (Curatella savanna). The TSC concentration obtained in the sugarcane plot is likely the result of the incorporation of surface residues into the soil that would have otherwise been lost through burning, which is the current practice in the region. Further studies on C stabilization in the clay fraction are thus needed to test the hypothesis of soil C enrichment due to residue management. Finally, trade-offs are to be considered for both preservation of the fragile dTf and the productivity of derived land uses that increases soil C at the same time.     

Decomposition of roots and twigs: Effects of wood type (beech and ash), diameter,site of exposure and macrofauna exclusion

Carbon loss and nitrogen dynamics in beech roots (Fagus sylvatica L.), beech twigs and ash roots (Fraxinus excelsior L.) of 0–3, 3–10 and 10–40 mm diameter were investigated during 36 months of exposure in litter bags of 1 and 4 mm mesh. Four experiments were set up: (1) Beech and ash roots (three size classes) were placed in a soil depth of ca 5 cm in a beechwood on limestone; (2) beech twigs (three size classes) were placed on the soil surface of the beechwood; (3) beech roots (3–10 mm) were placed on the soil surface of the beechwood: (4) beech twigs (3–10 mm) were placed on the soil surface of four sites representing different stages of secondary succession (wheat field, 13 year old fallow, ca 50 year old fallow, beechwood). Ash roots generally lost more C than beech roots. Loss in C of ash roots was similar for each of the size classes, whereas in beech roots and beech twigs C loss was in the order large roots > medium roots > small roots. Beech roots (3–10 mm) placed on the soil surface lost considerably less C than beech twigs (3–10 mm). Decomposition of beech twigs varied among ecosystems but generally did not follow clear patterns with successional stages. The fit of linear vs exponential models of decay is compared and in most materials exponential models fitted the data better. In each of the wood materials an accumulation of N occurred. Irrespective of wood type, root and twig diameter, mineralization of N of wood materials placed in the beechwood started uniformly after 12 months. Multiple regression analysis indicated a negative relationship between initial N content and C loss in beech roots and twigs but not in ash roots. The analysis also indicated a significant influence of the degree of white rot and of the amount of mineral soil deposited in the litter bags on C loss of certain wood materials. Generally, mesh size affected C loss and N dynamics only slightly, which is attributed to the comparatively short exposure time.     

Development of nitrogen mineralisation gradients through surface soil depth and their influence on surface soil pH

Following mixing of the surface soil to about 7.5 cm depth in the field, soil layers (0–2.5, 2.5–5, 5–10 and 10–15 cm) were separately incubated in the laboratory to determine the rate of development of net N mineralisation gradients through surface soil depth under fallow, wheat and subterranean clover plots. Gradients in net N mineralisation were compared with those observed in the field, and their contribution to the observed pH changes was investigated.Heterotrophic activity, and thus net N mineralisation, decreased only slightly with depth immediately after soil mixing. This pattern persisted over time in soil layers sampled from fallow plots. In contrast, within 1 growing season after soil mixing, heterotrophic activity and net N mineralisation decreased significantly with depth in soil sampled from wheat and clover plots. In 0–15 cm soil sampled from under senescing plants, 32–38% of CO₂-C produced and net N mineralised originated from the surface 2.5 cm, while 52–56% originated from the surface 5 cm of soil. This resulted from an increase of pH and organic substrate concentration within the surface 2.5 cm of soil following plant residue return. Limitations of the in situ measurement of net N mineralisation in fallow soil was identified.Laboratory incubation studies showed that since most net N mineralisation occurred within the surface 2.5 cm of soil under senescing plants, nitrification and acidification were also concentrated at this depth. Despite this, compared to fallow soil, high potential acidification rates of 0–2.5 cm soil under senescing plants were not realised in the field due to the exposure to prolonged dry periods and moist-dry cycles. As a consequence, in the field the large magnitude of surface soil pH gradient which resulted from the return of alkaline plant residues was maintained over summer and autumn.     

Hydrology of winter–spring “red tides” in Bahía de Mazatlán, Sinaloa, México

“Red tide” events are frequent and periodical in Bahía de Mazatlán, Sinaloa, México. Yet, the ones observed from 4 February to 4 June 2000, showed some distinctive features: First, the dinoflagellates Prorocentrum balticum (85%), P. mexicanum (5%), and Ceratium furca (5%), dominated the composition of the blooms; Second, the average cell abundance by date was 1.3×10⁶ cells l⁻¹, with a range of 3.5×10³ to 24,500 × 10³ cells l⁻¹, well above previous records; Third, the temperature registered at 10–20 m deep was unusually cold (19 °C), below the normal average of 21.5 °C observed over the last 10 years. Salinity was high (35.9 psu) and showed very little influence on the water density gradient. A mean thermal stratification index (TSI), of 3.4, with a maximum of 7 °C, was observed throughout the period, in spite of a weak upwelling activity and intermittent strong NW winds which were unable to break up water column stratification. Temperature fluctuations at the surface and at the bottom layers showed a 30-day periodicity, suggesting some association with the lunar cycle. To explain the characteristics of the “red tides” registered in Bahía de Mazatlán during the winter–spring period of year 2000, it is proposed that the temperature and density stratification, stabilized further by internal waves that compensated for the weak upwelling activity and provided the necessary nutrients to the surface layer, favored the persistence and intensity of the harmful algal bloom events then observed.     

新薛河溪流生境底栖动物群落结构季节动态

明确底栖动物在时空尺度上的自然变化规律对有效评估和保护水生态系统十分重要。基于2012年季节调查数据,对新薛河典型生境底栖动物群落季节动态进行了研究。共鉴定底栖动物108种,隶属5门10纲74属。短脉纹石蛾、长钝直突摇蚊、拟长跗摇蚊、Neozarelia sp.为研究区域优势分类单元,相对丰度分别为25.1%、9.8%、9.0%、8.6%;各季节间优势分类单元组成差异较大,3个季度共同优势物种仅短脉纹石蛾1种。密度、物种丰度、香农指数表现为4月和12月显著高于10月;均匀度指数10月份最高。非度量多维标度排序表明,不同季节底栖动物群落结构差异显著;对照河段(D河段)群落结构季节间差异相对较小。指示物种法分离出了新薛河及各河段代表性物种41种,但B河段未分离出指示物种;双向聚类分析进一步明确了指示物种的指示属性(特有或者相对丰度最高)。     

Nile perch,Lates niloticus,predation on the freshwater prawn,Caridina nilotica,in the Nyanza Gulf,Lake Victoria,East Africa

Synopsis The percentage of Nile perch, Lates niloticus, containing the prawn, Caridina nilotica, declined from 60–85%, for fish between 5 and 40 cm total length, to less than 10% for fish larger than 80 cm in length. The maximum number of prawns eaten by a Nile perch increased steadily, from 10 prawns for a fish in the 5–9.9 cm length group, to 913 prawns in a fish in the 60–69.9 cm length group, and then fell sharply for larger fish. The mean number of Caridina eaten by fish that were foraging on this animal followed a similar trend. Gill raker spacing increased in direct proportion to fish length and this may prevent fish larger than 70 cm in length from being effective prawn predators.     

A Short-Term Study of Vertical and Horizontal Distribution of Zooplankton During Thermal Stratification in Lake Kinneret,Israel

This study documents for the first time both vertical and horizontal distribution patterns of the zooplankton community in Lake Kinneret during the period of thermal stratification. The zooplankton distribution patterns were explored in relation to abiotic (temperature, oxygen) and biotic (picocyanobacteria, ciliates, flagellates, phytoplankton, fish) environmental gradients. Sampling was carried out on 6–7 July 1992 at five stations and six depths from nearshore to offshore. Zooplankton abundance and biomass varied from 5 to 267 ind. l^–1(mean: 95 ind. l^–1), and from 0.1 to 65 d.w. mg m^–3(mean: 24 d.w. mg m^–3). Zooplankton taxonomic groups (Rotifera, Cladocera, Cyclopoida, Calanoida) and size classes (micro-, meso- and macrozooplankton) showed peaks of maximal density and biomass in the epilimnetic and metalimnetic strata (5 and 14 m). Depth, accounting for 31–39% of total spatial variation, reflected the vertical distribution of zooplankton in relation to temperature and oxygen declines, and the higher concentration of food resources (protists and phytoplankton) in the epilimnion and metalimnion. Onshore–offshore distance, accounting for 17–22% of the total spatial variance, reflected different distribution patterns shown among zooplankton groups and size classes. The macrozooplankton (Copepoda, Cladocera) was more abundant offshore, whereas microzooplankton (Rotifera and nauplii) predominated nearshore. These horizontal distribution patterns were related to small increases in temperature and phytoplankton biomass, and higher concentrations of fish in the littoral zone. Although limited to a short temporal scale, our study indicated that zooplankton spatial distribution in Lake Kinneret during the period of thermal stratification was related to physicochemical, food and predation factors, manifested differently along the vertical and nearshore–offshore gradients.     

Oxygen consumption and ammonia accumulation in the hypolimnion of Walker Lake, Nevada

Walker Lake (area = 140 km², Z _mean = 19.3 m) is a large, terminal lake in western Nevada. As a result of anthropogenic desiccation, the lake has decreased in volume by 75% since the 1880s. The hypolimnion of the lake, now too small to meet the oxygen demand exerted by decaying matter, rapidly goes anoxic after thermal stratification. Field and laboratory studies were conducted to examine the feasibility of using oxygenation to avoid hypolimnetic anoxia and subsequent accumulation of ammonia in the hypolimnion, and to estimate the required DO capacity of an oxygenation system for the lake. The accumulation of inorganic nitrogen in water overlaying sediment was measured in laboratory chambers under various DO levels. Rates of ammonia accumulation ranged from 16.8 to 23.5 mg-N m^–2 d^–1 in chambers with 0, 2.5 and 4.8 mg L^–1 DO, and ammonia release was not significantly different between treatments. Beggiatoa sp. on the sediment surface of the moderately aerated chambers (2.5 and 4.8 mg L^–1 DO) indicated that oxygen penetration into sediment was minimal. In contrast, ammonia accumulation was reversed in chambers with 10 mg L^–1 DO, where oxygen penetration into sediment stimulated nitrification and denitrification. Ammonia accumulation in anoxic chambers (18.1 and 20.6 mg-N m^–2 d^–1) was similar to ammonia accumulation in the hypolimnion from July through September of 1998 (16.5 mg-N m^–2 d^–1). Areal hypolimnetic oxygen demand averaged 1.2 g O₂ m^–2 d^–1 for 1994–1996 and 1998. Sediment oxygen demand (SOD) determined in experimental chambers averaged approximately 0.14 g O₂ m^–2 d^–1. Continuous water currents at the sediment-water interface of 5–6 cm s^–1 resulted in a substantial increase in SOD (0.38 g O₂ m^–2 d^–1). The recommended oxygen delivery capacity of an oxygenation system, taking into account increased SOD due to mixing in the hypolimnion after system start-up, is 215 Mg d^–1. Experimental results suggest that the system should maintain high levels of DO at the sediment-water interface (10 mg L^–1) to insure adequate oxygen penetration into the sediments, and a subsequent inhibition of ammonia accumulation in the hypolimnion of the lake.     

Seasonal changes in composition and biomass of epiphytic algae on the macrophyte Ranunculus penicillatus in a chalk stream,with estimates of production,and observations on the epiphytes of Cladophora glomerata

The epiphytic algae on surfaces of the macrophyte Ranunculus penicillatus (Dumort.) Bab. var. calcareus (R. W. Butcher) C. D. K. Cook and on Cladophora glomerata (L.) Kütz growing in the River Itchen at Otterbourne near Southampton were studied between February 1984 and June 1985. The river at this site has a mean flow rate of 0.33 m s^–1, and is about 16 m wide and on average 20 cm deep, with a discharge ranging through the year between 0.34 and 2.46 m³ s^–1. The pH of the river varies little around 8.2, with a mean alkalinity of 236 mg HCO _inf3 ^sup1 l^–1, because of its origin from chalk springs. Ranunculus grows throughout the year, with peaks of biomass in spring and autumn. It forms a very large surface for attachment of epiphytes, and covers on average 40% of the stream bed. Numbers of epiphyte cells removed from Ranunculus ranged through the year between 52 × 10³ and 271 × 10³ cells mm^–2 stream floor, with maximum numbers in April, and a secondary peak in October. This pattern partly reflects fluctuations in the biomass of Ranunculus; the number of cells per unit area of plant surface showed a broader spring peak and lower fluctuations in other seasons. Diatoms formed 65 to 98% of these epiphyte cells, with chlorophytes reaching their peak (10%) in summer and cyanophytes (25%) in autumn. Estimates of biomass of these epiphytic forms, derived from measurements of chlorophyll c, indicate a range between 30 and 100 g dry weight m^–2 of weed bed. Colonisation studies showed that the algae grow and reproduce throughout the year, with a mean generation time of about 5 days, suggesting an annual production of about 3 kg dry weight m^–2 of weed bed, which makes epiphytic algae the principal primary producers in the stream. The numbers and biomass of epiphytic algae on Cladophora are considerably less.The species of epiphytic algae found on Ranunculus were generally similar to those growing on Cladophora, and to epilithic algae on pebbles of the stream bed, but different species were dominant on the different substrata. Algal cells in the water column were all derived from benthic habitats, although their relative abundance was very different.