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31.
Bjrn Grüneberg Jacqueline Rücker Brigitte Nixdorf Horst Behrendt 《International Review of Hydrobiology》2011,96(5):599-621
Based on measured and calculated long‐term data on external phosphorus (P) load (1920–2009), hypolimnetic P accumulation and trophic parameters for the dimictic Lake Scharmützelsee, we aimed to identify factors which cause variable P net sedimentation and the importance of internal P loading for different time periods especially focusing on non‐steady state after abrupt external load reduction in 1988. P retention (R) decreased from 0.85 during the high external P loading (0.37 g m–2 a–1) phase (1950–1988) to 0.71 during the following transient phase, and increased to 0.81 for the present recovery phase (0.17 g m–2 a–1) beginning in 2003. Mean net sedimentation coefficients for the same periods were 0.47, 0.22 and 0.30. Our results show that a) empirical models overestimate R during the high loading phase and underestimate R during the transient phase after load reduction, and b) the application of simple one‐box models which assume that a portion of in‐lake P stock is retained requires the consideration of the variability of the net sedimentation coefficient. We identified multiple reasons for variable gross sedimentation (e.g., particle trapping in the elongated lake; efficient accumulation of sewage P) as well as release of P (delayed release of mobile P from sediment; changes in plant colonization and food webs), so that their relation (net sedimentation) varies. Despite a new equilibrium reached in 2003, it is still unclear when the lake will reach mesotrophic reference conditions and a good ecological status. Historical data and elevated Cl– concentration (22 mg L–1) indicate that P import from sewage contaminated groundwater still continues, so that the mean in‐lake P concentration is still too high (53 µg L–1), and biological structures have not fully recovered yet. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
32.
Samantha Jane Hughes 《Hydrobiologia》2006,553(1):27-41
Temporal and spatial distribution patterns of lotic larval trichopteran assemblages in relation to environmental variables
were investigated in Madeiran streams using multivariate analyses. TWINSPAN classification detected distinct faunal assemblages
related to spatial factors between non-polluted high altitude sites and lower lying enriched sites where tolerant taxa were
predominant but showed strong seasonal shifts in species composition and abundance. The 15 TWINSPAN end groups were grouped
into five arbitrary clusters based upon the seasonal and spatial changes in the trichopteran assemblages detected by the analysis.
Significant differences between environmental variables (distance from source, altitude, temperature, conductivity, alkalinity
and nitrate) and the trichopteran assemblages (using trichopteran based metrics) of these clusters were confirmed by the Kruskal-Wallis
test (H) and Dunn’s test. Chemical classification of samples within the clusters revealed a strong association between trichopteran
assemblages and water quality. Canonical Correspondence Analysis and Monte Carlo global permutation tests also identified
significant associations between the larval assemblages and physicochemical variables such as temperature and conductivity
along a strong physical gradient (altitude, slope) and nitrate along a weaker seasonal gradient. Analysis of functional feeding
group distribution patterns clearly showed that mid to high altitude indigenous woodland sites were trophically diverse whilst
the lower reaches of the islands streams are trophically impoverished with strong seasonal shifts between two feeding groups
of enrichment tolerant taxa. Trichopteran shredders are exclusive to indigenous woodland sites, indicating a limited distribution
associated with land use, allochthonous input and habitat destruction. The results indicate that several ‘environmental filters’
operate at different levels upon the islands trichopteran fauna, producing temporally and spatially distinct ‘subsets’ of
species best able to exploit conditions and resources at a given site or time, confounding the direct comparison of these
insular systems with the findings of the River Continuum Concept, traditionally associated with unaffected continental lotic
systems. 相似文献
33.
Type-specific reference conditions that describe sites with no or only very minor anthropogenic disturbances are a basic requirement
of the European Water Framework Directive. The reference condition approach implies a previous determination of criteria acceptable
for the definition of near-natural stretches. In this paper, a methodology based on selection and validation procedures is
applied to a Portuguese watershed to guide the identification of reference sites. The methodology consisted of three phases:
(1) a preliminary site inspection through the use of maps, available data and an extensive screening field campaign that resulted
in the selection of 52 potentially undisturbed small- and medium-sized rivers stretches, ranging from 43 to 1069 m in altitude,
with catchment areas between 4 and 641 km2 and representing siliceous and calcareous riverbeds; (2) a detailed site selection based on ten pre-defined criteria involving
physical/social/biological attributes that indicated catchment land use changes and alien vegetal riparian species as major
human impacts; (3) a site validation procedure involving detailed investigation of benthic macroinvertebrate communities,
riparian vegetation, in-stream habitat quality and chemical parameters that showed riparian wood-related problems as a major
restriction to validate a stretch as a near-natural site. Only about 12% of all investigated river sites, accounting for 600 m
of all 5200 m studied, could be considered as near-reference stretches or sites retaining essential natural functions. Selection
and validation procedures can identify different sites as reference, which reinforces the need for applying both procedures.
The results presented can help to accomplish the requirements of the EU – Water Framework Directive by selecting reference
sites as the first step to establish biological reference conditions and, simultaneously, to form a basis for nature conservation
strategies.
Electronic supplementary material Electronic supplementary material is available for this article at
and accessible for authorised users. 相似文献
34.
35.
To develop more targeted intervention strategies, an important research goal is to identify markers predictive of clinical events. A crucial step toward this goal is to characterize the clinical performance of a marker for predicting different types of events. In this article, we present statistical methods for evaluating the performance of a prognostic marker in predicting multiple competing events. To capture the potential time-varying predictive performance of the marker and incorporate competing risks, we define time- and cause-specific accuracy summaries by stratifying cases based on causes of failure. Such definition would allow one to evaluate the predictive accuracy of a marker for each type of event and compare its predictiveness across event types. Extending the nonparametric crude cause-specific receiver operating characteristics curve estimators by Saha and Heagerty (2010), we develop inference procedures for a range of cause-specific accuracy summaries. To estimate the accuracy measures and assess how covariates may affect the accuracy of a marker under the competing risk setting, we consider two forms of semiparametric models through the cause-specific hazard framework. These approaches enable a flexible modeling of the relationships between the marker and failure times for each cause, while efficiently accommodating additional covariates. We investigate the asymptotic property of the proposed accuracy estimators and demonstrate the finite sample performance of these estimators through simulation studies. The proposed procedures are illustrated with data from a prostate cancer prognostic study. 相似文献
36.
Assessment of ecological status in U.K. rivers using diatoms 总被引:6,自引:0,他引:6
MARTYN KELLY STEVE JUGGINS ROBIN GUTHRIE SARAH PRITCHARD JANE JAMIESON BRIAN RIPPEY HEIKE HIRST MARIAN YALLOP 《Freshwater Biology》2008,53(2):403-422
1. The European Union's Water Framework Directive requires all water bodies to achieve ‘good ecological status’ by 2015 and this paper describes a rationale for defining ‘good ecological status’ based on diatoms, a significant component of the biological quality element ‘macrophyte and phytobenthos’. 2. A database of benthic diatom samples collected over the past 20 years was assembled. New sampling, specifically for this project, was undertaken during 2004 to supplement these data. In total 1051 samples were included in the database with matching environmental data. 3. ‘Reference sites’, relatively unimpacted by human activity, were selected from this database by a series of screening steps and these sites were used to develop a site‐specific reference typology. 4. Environmental variables not related to the pressure gradient were used to predict the ‘expected’ Trophic Diatom Index (TDI) values at each site. Site‐specific TDI predictions were used to generate ecological quality ratios (EQRs) ranging from ≥1, where the diatom assemblage showed no impact, to (theoretically) 0, when the diatom assemblage was indicative of major anthropogenic activities. 5. The boundary between ‘high’ and ‘good’ status was defined as the 25th percentile of EQRs of all reference sites. The boundary between ‘good’ and ‘moderate’ status was set at the point at which nutrient‐sensitive and nutrient‐tolerant taxa were present in equal relative abundance. An ecological rationale for this threshold is outlined in the paper. 相似文献
37.
Use of marker pigments and functional groups for assessing the status of phytoplankton assemblages in lakes 总被引:2,自引:0,他引:2
Phytoplankton is a key biological quality element for the establishment of the Water Framework Directive (WFD) ecological
status in reservoirs and lakes. In freshwaters, inverted microscope examination is the traditional standard method for estimating
phytoplankton and assessing taxonomic composition. Based on the enumeration of algal units and measurements for biovolume
calculation, this technique is cumbersome and time-consuming. In large monitoring programmes, such as the application of the
WFD in lakes and reservoirs, chemotaxonomy (HPLC pigment analysis and CHEMTAX treatment) is ideally suited as an alternative
method because it allows the rapid processing of large numbers of samples from numerous locations and depths, thereby providing
ideal temporal and spatial resolution. The low taxonomical detail obtained by HPLC and CHEMTAX (phytoplankton classes or phyla)
can easily be overcome by a rapid inverted microscope screening with identification of the dominant species. Combining HPLC
and microscopy provides a useful method for monitoring phytoplankton assemblages, which can be used to implement the WFD with
respect to phytoplankton. Here, we present the application of a method combining marker pigments and microscopy to phytoplankton
samples from 12 Belgian reservoirs. This method substantially reduced the workload and enabled us to assess the status of
the phytoplankton assemblage in these lakes. The method complies with the WFD, as it takes into account taxonomic composition,
assesses abundance and biomass of the phytoplankton taxa, and easily detects blooms. Additionally, a set of templates of probability
of occurrence of phytoplankton functional groups at the maximal ecological potential for reservoirs from the Central/Baltic
region is presented, based on reference conditions defined for natural lakes from other regions. 相似文献
38.
Summary . We develop methods for competing risks analysis when individual event times are correlated within clusters. Clustering arises naturally in clinical genetic studies and other settings. We develop a nonparametric estimator of cumulative incidence, and obtain robust pointwise standard errors that account for within-cluster correlation. We modify the two-sample Gray and Pepe–Mori tests for correlated competing risks data, and propose a simple two-sample test of the difference in cumulative incidence at a landmark time. In simulation studies, our estimators are asymptotically unbiased, and the modified test statistics control the type I error. The power of the respective two-sample tests is differentially sensitive to the degree of correlation; the optimal test depends on the alternative hypothesis of interest and the within-cluster correlation. For purposes of illustration, we apply our methods to a family-based prospective cohort study of hereditary breast/ovarian cancer families. For women with BRCA1 mutations, we estimate the cumulative incidence of breast cancer in the presence of competing mortality from ovarian cancer, accounting for significant within-family correlation. 相似文献
39.
The usefulness of benthic multi-metric indices when assessing seafloor integrity across broad environmental gradients should be deliberated, as their lack of transparency might hide important sources of variation and fail to identify environmental change. This study compares the performance of two multi-metric indices; the Benthic Quality Index (BQI) and the Brackish water Benthic Index (BBI) between three sub-basins in the Baltic Sea. Both indices reflect the salinity-driven gradient in macroinvertebrate diversity and composition as well as changes in bottom water oxygen concentrations. The relative contribution of predictor variables for explaining index variation does, however, differ between sub-basins, resulting in the indices representing different aspects of the benthic community along the environmental gradient. This context-dependency is caused by inherent differences in benthic community characteristics between the sub-basins of the Baltic Sea, and how the communities are portrayed by the indices. An increased transparency of the importance of the different predictors for directing index values is needed for coherent classifications over broad environmental gradients, such as those occurring in large estuarine water bodies. Use of a weight of evidence table to combine multiple indicators would preserve transparency and be more likely to provide a robust assessment method that would detect seafloor degradation at an early stage. 相似文献
40.
P. Manolaki 《Plant biosystems》2013,147(4):1064-1077
Abstract In Greece, as in many other Mediterranean countries, there is a lack of information enabling the use of aquatic macrophytes as biological indicators of the water trophic status. This research aimed to investigate the trophic preferences of the aquatic macrophyte assemblages encountered in selected standing water bodies of a lowland river basin of western Greece. It also aimed to assess the response of the macrophyte species that occur in the studied water bodies to different nutrient concentrations. A total of 56 macrophyte species were identified. TWINSPAN analysis distinguished four main vegetation groups potentially related to three trophic types. The Trophic Ranking Score developed in the UK was applied in order to investigate the trend in aquatic macrophytes in response to the different trophic conditions. The results indicated that TRSUK was not in accordance to the water trophic indices. After the addition of 14 new species and the re-scoring of the plants from the UK list, the re-calculation of TRS enabled a better classification of the trophic status convergent with the information from the water trophic indices. The current study constitutes a reference document providing basic information, and must be improved by updating the plant list with new data from other sites. 相似文献