Relationships Between Environmental Variables and Benthic Diatom Assemblages in California Central Valley Streams (USA)

This study examines distributional patterns of benthic diatom assemblages in relation to environmental characteristics in streams and rivers in the California Central Valley ecoregion. Benthic diatoms, water quality, and physical habitat conditions were characterized from 53 randomly selected sites. The stream sites were characterized by low mid-channel canopy cover and high channel substrate embeddedness. The waters at these sites were enriched with minerals and turbidity varied from 1.3 to 185.0 NTU with an average of 13.5 NTU. A total of 249 diatom taxa were identified. Average taxa richness was 41 with a range of 7–76. The assemblages were dominated by Staurosira construens (11%), Epithemia sorex (8%), Cocconeis placentula (7%), and Nitzschia amphibia (6%). Multivariate analyses (cluster analysis, classification tree analysis, and canonical correspondence analysis) all showed that benthic diatom assemblages were mainly affected by channel morphology, in-stream habitat, and riparian conditions. The 1st CCA axis negatively correlated with mean wetted channel width (r = −0.66) and thalweg depth (r = −0.65) (Table 4). The 2nd axis correlated with % coarse substrates (r=0.60). Our results suggest that benthic diatoms can be used for assessing physical habitat alterations in streams.     

SURVEY OF BENTHIC DIATOM COMMUNITIES FROM LOTIC SYSTEMS WITHIN THE WESTERN ALLEGHENY PLATEAU

Over the course of 3 years (1997–1999), 72 stream sites were sampled for epilithic diatom communities. The analysis of these samples has led to the identification of over 325 species of diatoms. In addition to sampling the diatom community, selected physical and chemical parameters were recorded from each stream reach. These parameters included pH, specific conductance, current velocity, SRP, nitrate, silica, and total alkalinity. Canonical Correspondence Analysis (CCA) was used to identify influential environmental parameters and to assess the response of the diatom community to prominent anthropogenic inputs in the region (i.e. coal mine drainage, eutrophication). The initial analyses indicate that pH was the most influential environmental parameter along the first CCA axis. This shift was not unexpected, as acid mine drainage (AMD) in the region leads to a wide range of pH values (2.8–7.93). The highly acidic sites were characterized by species of the genus Eunotia (specifically E. exigua and E. steineckei), Frustulia rhomboides, and Pinnularia subcapitata. Furthermore, Achnanthidium minutissimum was the most widely distributed of the diatom species encountered, being found at 94% of the sites sampled. Streams that fluctuated between acidic and circumneutral pH (termed “teeter‐totter”) had greater abundances of Brachysira vitrea than other streams in this survey. Further implications for the use of these diatom communities as biomonitoring tools and the distribution of assemblages within the Western Allegheny Plateau will be discussed.     

Diatom species–environment relationships and inference models from Isachsen, Ellef Ringnes Island, Canadian High Arctic

Surface sediment diatom assemblages were examined from 26 freshwater sites near Isachsen (78°47N, 103°32W), Ellef Ringnes Island, a region of diverse and atypical water chemistry for high arctic sites. One hundred and sixty eight diatom taxa were identified from these samples, over 50% of which had not previously been recorded in the Canadian High Arctic. Variations in diatom assemblages were related to changes in measured environmental variables using multivariate techniques. Canonical correspondence analysis (CCA) indicated that five variables contributed significantly to explaining patterns of diatom variation (i.e., COND, DIC, Mn, TPF, TPU). The first CCA axis (=0.44) was primarily controlled by conductivity-related variables, while CCA axis 2 (=0.21) was related to particulate concentrations. Diatom-based inference models were generated for the reconstruction of conductivity (RMSEP_jack=0.32, r²_jack=0.76) and pH (RMSEP_jack=0.40, r²_jack=0.69). The strengths of these models indicate that it will be possible to reliably infer past trends in conductivity and pH from diatom assemblages preserved in dated sediment cores from the Isachsen region.     

SPATIAL PATTERNS AND ECOLOGICAL DETERMINANTS OF BENTHIC ALGAL ASSEMBLAGES IN MID-ATLANTIC STREAMS, USA

We attempted to identify spatial patterns and determinants for benthic algal assemblages in Mid-Atlantic streams. Periphyton, water chemistry, stream physical habitat, riparian conditions, and land cover/use in watersheds were characterized at 89 randomly selected stream sites in the Mid-Atlantic region. Cluster analysis (TWINSPAN) partitioned all sites into six groups on the basis of diatom species composition. Stepwise discriminant function analysis indicated that these diatom groups can be best separated by watershed land cover/use (percentage forest cover), water temperature, and riparian conditions (riparian agricultural activities). However, the diatom-based stream classification did not correspond to Omernik's ecoregional classification. Algal biomass measured as chl a can be related to nutrients in habitats where other factors do not constrain accumulation. A regression tree model indicated that chl a concentrations in the Mid-Atlantic streams can be best predicted by conductivity, stream slope, total phosphorus, total nitrogen, and riparian canopy coverage. Our data suggest that broad spatial patterns of benthic diatom assemblages can be predicted both by coarse-scale factors, such as land cover/use in watersheds, and by site-specific factors, such as riparian conditions. However, algal biomass measured as chl a was less predictable using a simple regression approach. The regression tree model was effective for showing that ecological determinants of chl a were hierarchical in the Mid-Atlantic streams.     

BIOGEOGRAPHY OF BATRACHOSPERMUM HELMENTOSUM (BATRACHOSPERMALES, RHODOPHYTA) IN NORTH AMERICA USING MOLECULAR AND MORPHOLOGICAL DATA

Over the course of 3 years (1997–1999), 72 stream sites were sampled for epilithic diatom communities. The analysis of these samples has led to the identification of over 325 species of diatoms. In addition to sampling the diatom community, selected physical and chemical parameters were recorded from each stream reach. These parameters included pH, specific conductance, current velocity, SRP, nitrate, silica, and total alkalinity. Canonical Correspondence Analysis (CCA) was used to identify influential environmental parameters and to assess the response of the diatom community to prominent anthropogenic inputs in the region (i.e. coal mine drainage, eutrophication). The initial analyses indicate that pH was the most influential environmental parameter along the first CCA axis. This shift was not unexpected, as acid mine drainage (AMD) in the region leads to a wide range of pH values (2.8–7.93). The highly acidic sites were characterized by species of the genus Eunotia (specifically E. exigua and E. steineckei ), Frustulia rhomboides , and Pinnularia subcapitata. Furthermore, Achnanthidium minutissimum was the most widely distributed of the diatom species encountered, being found at 94% of the sites sampled. Streams that fluctuated between acidic and circumneutral pH (termed "teeter-totter") had greater abundances of Brachysira vitrea than other streams in this survey. Further implications for the use of these diatom communities as biomonitoring tools and the distribution of assemblages within the Western Allegheny Plateau will be discussed.     

Changes in algal assemblages along observed and experimental phosphorus gradients in a subtropical wetland, U.S.A.

• 1 We wanted to determine if changes in algae in the Everglades were due to increased phosphorus (P) loading. Epiphytic algae, water chemistry, and surface sediment chemistry were characterized from 32 sloughs along a P gradient in the Everglades and changes in the algal assemblages along the P gradient were compared with those along an experimental P gradient of in situ mesocosms. The sloughs are the wettest open water habitats characterized by floating and submerged aquatic plants in the Everglades.
• 2 Algal species composition was much more sensitive to P concentration than algal biomass. The diatom species variance among sloughs, captured by 1st ordination axis, was more highly correlated with total P (TP) in surface sediments (r = ‐ 0.79), than soluble reactive P (SRP) (r = ‐ 0.08) and TP (r = ‐ 0.48) in the water column. Algal biomass (µg chl a cm^‐2) was not significantly correlated with P (SRP: r = 0.22, TP: r = 0.19, sediment TP: r = 0.07) along the P gradient in the Everglades. Cluster analysis classified diatom species assemblages in 32 sloughs into three groups (TWIN I, II, III), which corresponded to three zones along the P gradient. Dominant diatom species shifted from Mastogloia smithii (40.3%), Cymbella scotica (22.3%), and Fragilaria synegrotesca (21.8%) in TWIN I to Nitzschia amphibia (22.4%) and C. microcephala (12.4%) in TWIN III. TP in surface sediments and TP in epiphyton assemblages increased 4‐ and 5‐fold from TWIN I to TWIN III, respectively.
• 3 Patterns in epiphytic assemblages along the experimental P gradient in the mesocosms were very similar to those along the Everglades P gradient. Shannon diversity indices and species richness significantly increased along both P gradients. TN : TP ratio in epiphyton assemblages significantly decreased as sediment TP increased along both P gradient. Ordination analysis showed that diatom assemblages in the impacted zone (TWIN III) were ordinated closely to the assemblages from the highest P treatments in the mesocosms. The assemblages from the less impacted zone (TWIN I) were ordinated closely to the assemblages from controls in the mesocosms.
• 4 Concurrence between results of our survey and experiments suggest that changes in epiphytic assemblages along the P gradient in the Everglades are caused by increases in P concentrations.

     

Diatoms are better indicators of urban stream conditions: A case study in Beijing,China

Urbanization dramatically affects hydrology, water quality and aquatic ecosystem composition. Here we characterized changes in diatom assemblages along an urban-to-rural gradient to assess impacts of urbanization on stream conditions in Beijing, China. Diatoms, water chemistry, and physical variables were measured at 22 urban (6 in upstream and 16 in downstream) and 7 rural reference stream sites during July and August of 2013. One-way ANOVA showed that water physical and chemical variables were significantly different (p < 0.05) between urban downstream and both reference and urban upstream sites, but not between reference and urban upstream sites (p > 0.05). Similarly, structural metrics, including species richness (S), Shannon diversity (H′), species evenness (J′) and Simpson diversity (D′), were significantly different (p < 0.05) between urban downstream and both reference and urban upstream sites, but not (p > 0.05) between reference and urban upstream sites. However, diatom assemblages were very different among all sites. Achnanthidium minutissima was a consistent dominant species in reference sites; Staurosira construens var. venter and Pseudostaurosira brevistriata were the dominant species in urban upstream sites; and Nitzschia palea was the dominant species in urban downstream sites. Clustering analyses based on the relative abundance of diatom species, showed all the samples fit into three groups: reference sites, urban upstream sites, and urban downstream sites. Canonical correspondence analysis (CCA) and Monte Carlo permutation tests showed that concentration of K⁺, EC, TN, Cl⁻ and pH were positively correlated with relative abundance of dominant diatom species in urban downstream samples; WT and F⁻ were correlated with reference and urban stream diatom composition. Our results demonstrate that the composition of diatom species was more sensitive to urbanization than the water physical and chemical parameters, and that diatom assemblage structure metrics more accurately assessed water quality. Some species, such as Amphora pediculus and Cocconeis placentula were among the dominant species in low nutrients stream sites; however, they were considered to be high nutrient indicators in some streams in USA. We suggest using caution in applying indicator indices based on species composition from other regions. It is necessary to build a complete set of diatom species data and their co-ordinate environment data for specific regions.     

Linkages among land-use, water quality, physical habitat conditions and lotic diatom assemblages: A multi-spatial scale assessment

We assessed the importance of spatial scales (catchment, stream network, and sample reach) on the effects of agricultural land-use on lotic diatom assemblages along a land-use gradient in the agricultural Willamette Valley Ecoregion of Oregon. Periphyton, water chemistry, and physical habitat conditions were characterized for 25 wadeable streams during a dry season (July to September, 1997). Additional water chemistry samples were collected in the following wet season (February 1998) to assess seasonal effects of land-use on stream water chemistry. Percent agricultural land-use in the study catchments ranged from 10% to 89% with an average of 52%. Partial canonical correspondence analysis (CCA) with the first axis constrained by % agricultural land-use showed that % agricultural land-use at 3 spatial scales explained between 3.7%–6.3% of variability in the diatom species dataset. Monte Carlo Permutation tests indicated that the variance explained by % agricultural land-use was only significant at the spatial scale of the stream network with 10- and 30-m band width (p<0.05, 999 permutations). In addition to the effects of % agricultural land-use, partial CCAs with a forward selection option showed that water chemistry (e.g., SiO₂), reach-scale stream channel dimensions (e.g., width, depth, and slope), reach-scale in-stream habitats (substrates and filamentous algal cover in stream beds), and riparian vegetative buffer were all important with relation to diatom species assemblages. Percent of obligately nitrogen-heterotrophic taxa was the only diatom autecological metric that showed a significant but weak correlation with % agricultural land-use along the stream network (r=0.50), but not at catchment or sample reach scale. Correlation between % agricultural land-use and water chemistry variables varied among the spatial scales and between seasons. Physical habitat variables (log₁₀ erodible substrate diameters and stream reach slope) were significantly correlated with % agricultural land-use along the stream network but not at catchment or sample reach scale. Our data suggest that spatial scales are important in assessing effects of land-use on stream conditions but the spatial scale effects may vary between seasons. Direct linkages between agricultural land-use and lotic diatom assemblages were weak during summer base-flow time regardless of the spatial scales. Summer sampling may underestimate the effects of catchment land-use on stream conditions in areas where seasonal patterns are so distinctive as in the Willamette Valley.     

Longitudinal structure of fish assemblages in a minimally disrupted stream

The longitudinal distribution patterns of fish species are affected by both natural and anthropogenic variables. The role of these factors on the formation of species assemblages is well documented in North America and Western Europe, but detailed information is lacking from Central and Eastern Europe, and the Carpathian region especially. Therefore, we examined the structure of fish assemblages in response to six key environmental parameters in a natural stream system (Udava stream basin, Slovakia). We used the indirect ordination method of gradient analysis (Detrended Correspondence Analysis, DCA) to analyse the species groups and their connections to the sampled sites and to recognize the strongest gradient of assemblage composition. Subsequently, we used the direct ordination method (Canonical Correspondence Analysis, CCA) to identify the strongest gradients in relation to selected variables. Two major gradients were identified that follow the upstream-downstream pattern of fish communities and three variables (distance from source, depth and site slope) are correlated with the first CCA axis (P < 0.05) and two variables (depth and vegetation cover) are correlated with the second CCA axis (P < 0.05). We assume that these factors influence the temperature and the amount of dissolved oxygen that can cause oxygen and temperature stress to intolerant species (e.g., salmonids). Based on these results, we assume that the economically important species, brown trout and grayling, are not native to the stream basin and this status is only the consequence of natural factors. Furthermore, the results suggest that the Udava stream offers favourable conditions for fish species distribution — a view supported by the high variability of particular variables within the proposed model.     

Stream macroinvertebrate response to catchment urbanisation (Georgia, U.S.A.)

SUMMARY 1. The effects of catchment urbanisation on water quality were examined for 30 streams (stratified into 15, 50 and 100 km² ± 25% catchments) in the Etowah River basin, Georgia, U.S.A. We examined relationships between land cover (implying cover and use) in these catchments (e.g. urban, forest and agriculture) and macroinvertebrate assemblage attributes using several previously published indices to summarise macroinvertebrate response. Based on a priori predictions as to mechanisms of biotic impairment under changing land cover, additional measurements were made to assess geomorphology, hydrology and chemistry in each stream. 2. We found strong relationships between catchment land cover and stream biota. Taxon richness and other biotic indices that reflected good water quality were negatively related to urban land cover and positively related to forest land cover. Urban land cover alone explained 29–38% of the variation in some macroinvertebrate indices. Reduced water quality was detectable at c. >15% urban land cover. 3. Urban land cover correlated with a number of geomorphic variables such as stream bed sediment size (–) and total suspended solids (+) as well as a number of water chemistry variables including nitrogen and phosphorus concentrations (+), specific conductance (+) and turbidity (+). Biotic indices were better predicted by these reach scale variables than single, catchment scale land cover variables. Multiple regression models explained 69% of variation in total taxon richness and 78% of the variation in the Invertebrate Community Index (ICI) using phi variability, specific conductance and depth, and riffle phi, specific conductance and phi variability, respectively. 4. Indirect ordination analysis was used to describe assemblage and functional group changes among sites and corroborate which environmental variables were most important in driving differences in macroinvertebrate assemblages. The first axis in a non‐metric multidimensional scaling ordination was highly related to environmental variables (slope, specific conductance, phi variability; adj. R²=0.83) that were also important in our multiple regression models. 5. Catchment urbanisation resulted in less diverse and more tolerant stream macroinvertebrate assemblages via increased sediment transport, reduced stream bed sediment size and increased solutes. The biotic indices that were most sensitive to environmental variation were taxon richness, EPT richness and the ICI. Our results were largely consistent over the range in basin size we tested.     

Comparison of Freshwater Diatom Assemblages from a High Arctic Oasis to Nearby Polar Desert Sites and Their Application to Environmental Inference Models

Arctic oases are regions of atypical warmth and relatively high biological production and diversity. They are small in area (<5 km²) and uncommon in occurrence, yet they are relatively well studied due to the abundance of plant and animal life contained within them. A notable exception is the lack of research on freshwater ecosystems within polar oases. Here, we aim to increase our understanding of freshwater diatom ecology in polar oases. Diatoms were identified and enumerated from modern sediments collected in 23 lakes and ponds contained within the Lake Hazen oasis on Ellesmere Island, and compared with diatom assemblages from 29 sites located outside of the oasis across the northern portion of the island. There were significant differences in water chemistry variables between oasis and northern sites, with oasis sites having higher conductivity and greater concentrations of nutrients and related variables such as dissolved organic carbon (DOC). Taxa across all sites were typical of those recorded in Arctic freshwaters, with species from the genera Achnanthes sensu lato, Fragilaria sensu lato, and Nitzschia dominating the assemblages. A correspondence analysis (CA) ordination showed that oasis sites generally plotted separately from the northern sites, although the sites also appear to plot separately based on whether they were lakes or ponds. Canonical correspondence analysis (CCA) identified specific conductivity, DOC, and SiO₂ as explaining significant (P < 0.05) and additional amounts of variation in the diatom data set. The most robust diatom‐based inference model was generated for DOC, which will provide useful reconstructions on long‐term changes in paleo‐optics of high Arctic lakes.     

Diatom Assemblages and their Associations with Environmental Variables in Oregon Coast Range Streams, USA

The Oregon Coast Range, rich in natural resources, is under increasing pressure from rapid development. The purpose of this study was to examine diatom species patterns in relation to environmental variables in streams of this region. Diatoms, water quality, physical habitat and watershed characteristics were assessed for 33 randomly selected stream sites. Watershed size, elevation, geology, vegetation and stream morphology varied substantially among sites. Streams were characterized by dilute water chemistry and a low percent of fine substrate. A total of 80 diatom taxa were identified. Taxa richness was low throughout the region (median 15, range 10–26). Assemblages were dominated by two adnate species, Achnanthidium minutissimum and Achnanthes pyrenaicum. Diatoms sensitive to organic pollution dominated the assemblages at all sites (median 85%). Non-metric multidimensional scaling (NMDS) and correlational analysis showed quantitative relationships between diatom assemblages and environmental variables. NMDS axes were significantly correlated with watershed area, watershed geology, conductivity, total nitrogen, total solids and stream width. Diatom-based site classification (Two-way Indicators Species Analysis, (TWINSPAN)) yielded 4 discrete groups that displayed weak correlations with environmental variables. When stream sites were classified by dominant watershed geology, overall diatom assemblages between groups were significantly different (Analysis of Similarity (ANOSIM) global R = 0.19, p < 0.05). Our results suggest that streams in the coastal region are in relatively good condition. High natural variability in stream conditions in the Oregon Coast Range ecoregion may obscure quantitative relationships between environmental variables and diatom assemblages. A bioassessment protocol that classifies sites by major landscape variables and selects streams along the major human disturbance gradient might allow for detection of early signs of human disturbance in environmentally heterogeneous regions, such as the Pacific Northwest.     

桂江流域附生硅藻群落特征及影响因素

研究了桂江流域水质、土地利用、地理因素对河流附生硅藻群落的影响。结果显示,桂江流域电导率(Conductivity,Cond.)由下游至源头呈降低趋势,其它水质参数变化趋势不明显。主成分分析(Principle Component Analysis,PCA)显示前两个主成分共解释了56.2%的水质特征,第一轴反映了氨氮(NH4-N)、硝氮(NO3-N)、总氮(Total Nitrogen,TN)的变化梯度,第二轴反映了水温(WT)、pH、Cond.、溶解氧(Dissolve Oxygen,DO)的变化梯度。桂江流域硅藻特定污染敏感指数(Specific PolluoSensitivity Index,IPS)和硅藻生物指数(Biological Diatom Index,IBD)下游低于源头,差异不显著,与多项水质、土地利用以及地理因子呈线性显著相关。24个样地共发现112种硅藻,丰富度大于5%的37种,丰富度最大的几个种类依次为Achnanthidium minutissimum,A.pusilla,A.tropica,Cymbella laevis。对应分析(Corresponding Analysis,CA)显示桂江流域存在3个差异较大的硅藻群落,流域下游以Nitzschia recta为优势种,A.lanceolata、Amphora montan、Planothidium frequentissimum在中下游丰度较高,上游区域种类较多。典型相关分析(Canonical Correspondence Analysis,CCA)排序前两轴解释了硅藻群落变异程度的28.60%,CCA排序轴1与水质(Cond.、WT、NH4-N、NO3-N、TN)和土地利用(城市面积、农田面积、植被覆盖)显著负相关,与地理因素(流域面积、海拔、坡度)显著正相关,第二轴与浊度(NTU)显著正相关(P<0.05)。偏典型相关分析(Partial CCAAnalyses)显示,土地利用、地理因子、水质分别解释了桂江流域硅藻群落变异的7.20%,17.50%,48.50%。结果表明,桂江流域附生硅藻群落结构是水质、地理因子和土地利用共同作用的结果,水质起决定性作用,电导和不同形态的氮是影响附生硅藻群落结构的主要水质因素。     

Stream fish assemblages in relation to environmental factors on a montane plateau (Nyika Plateau,Malawi)

Few studies have examined river fishes of Malawi. This study is one of the first to examine the stream fish assemblages on the Nyika Plateau in northern Malawi. Twenty four sites were sampled over three different periods in two river systems of the plateau. Eighteen species were collected and among these was Hippopotamyrus ansorgii, the first collection of this species in the Lake Malawi catchment. Three species, including a non-native trout, were common in the two systems studied. Correspondence analysis (CA) suggested gradients in species composition related to altitude and river type. Species succession, from a trout dominated upstream to a downstream dominated by indigenous species, was shown on the first CA axis. The second CA axis showed the assemblage of the plateau separated by river type. A direct gradient analysis method, canonical correspondence analysis (CCA), showed the importance of two stream position metrics (stream order and c-link), depth, water temperature and substrate type in determining species composition. According to variation partitioning in CCA, the spatial and temporal components respectively explained 46% and 3.6% variation in assemblage composition based on the all species data matrix, and 48.7% and 2.6% variation in assemblage composition based on the native species data matrix. The species collected were also discussed in relation to the morphological adaptations in their body forms to the environmental conditions of the streams studied.     

Benthic diatom communities in boreal streams: community structure in relation to environmental and spatial gradients

An important goal for community ecology is the characterization and prediction of changes in community patterns along environmental gradients. We aimed to identify the major environmental correlates of diatom distribution patterns in boreal running waters. We classified 197 stream sites based on their diatom flora. Direct ordination methods were then used to identify the key environmental determinants of this diatom-based stream typology. Finally, we tested whether a regional classification scheme based on terrestrial landscapes (ecoregions) provides a reasonable framework for a regional grouping of streams based on their diatom flora. Two-way indicator species analysis produced 13 site groups, which were primarily separated by chemical variables, mainly conductivity, total P and water colour. In partial CCA, the environmental and spatial factors accounted for 38% and 24%, respectively, of explained variation in community composition. A high proportion (almost 40%) of variation explained by the combined effect (spatially-structured environmental) indicated that diatom communities of boreal streams incorporate a strong spatial component. At the level of subecoregions, classification strength was almost equally strong for all sites as for near-pristine reference sites only. Procrustes analysis indicated that spatial factors and patterns in diatom community structure were strongly concordant. Our data support the argument that diatom communities are strongly spatially structured, with distinctly different communities in different parts of the country. Because of the strong spatial patterns of community composition, bioassessment programs utilising lotic diatoms would clearly benefit from regional stratification. A combination of regional stratification and the prediction of assemblage structure from local environmental features might provide the most robust framework for diatom-based assessment of the biological integrity of boreal streams.     

Response of epilithic diatom assemblages to water pollution in rivers in the Tokyo Metropolitan area, Japan

1. During the spring of 1992, fifty-two quantitative diatom samples were collected from twenty-eight rivers located in the Tokyo Metropolitan area, Japan, to study the response of the diatom assemblages to water pollution (assessed using physical and chemical data determined monthly from April 1987 to March 1992). 2. Species composition was analysed by means of biotic indices (Pantle and Buck's saprobic index) and multivariate analyses [two-way indicator species analysis (TWINSPAN) for classification and canonical correspondence analysis (CCA) for ordination]. Species-abundance relationships were analysed using diversity indices (species richness, Shannon's diversity index and Pielou's evenness index) and rank-abundance patterns (rank-abundance curves). 3. CCA revealed two major gradients. The first corresponded to organic pollution and eutrophication. The second corresponded to variables related to geographical location. Four main station groups were determined by TWINSPAN. The location of the indicator species of groups 1–3 along the CCA axis 1 is consistent with their known pollution tolerance characteristics. Indicator species for group 4 had larger scores on CCA axis 2, and are representative of brackish water environments. 4. Species richness tended to be higher in the intermediate range of water pollution. Pielou's evenness index and Shannon's diversity index followed the same tendency but only weakly. 5. The rank-abundance patterns of diatom assemblages were more or less constant in all stations. The curves were very similar in shape, differing only in length and gradient (directly related to species richness and evenness, respectively). 6. The results of this study indicate that the response of diatom assemblages to environmental change can be observed in species compositional variation. Multivariate analyses and pollution indices revealed this response and are to be preferred to species diversity measures.     

Different roles of environmental variables and spatial factors in structuring stream benthic diatom and macroinvertebrate in Yangtze River Delta,China

Exploring the relative contribution of spatial factors and environmental variables in shaping communities is of widespread interest in biodiversity conservation and environmental management. Stream communities are hierarchically regulated by environmental variables over multiple spatial scales, and the reaction of different organisms to stressors are still equivocal. We sampled both macroinvertebrates and diatom at 80 sites and additional 10 sites for macroinvertebrates, field measured and laboratory analyzed environmental variables, from the tributaries of Qiantang River, Yangtze River Delta China in 2011. We used PCNM (principal coordinates of neighbor matrices) to generate spatial predictors. We applied redundancy analysis and variation partitioning procedures to identify key spatial and environmental factors, and to quantify their relative roles in shaping diatom and macroinvertebrate assemblages. Our results demonstrated the role of spatial and environmental variables differed in shaping benthic diatom and macroinvertebrate. Diatom assemblage variations were better explained by spatial factors, however macroinvertebrate assemblage variations were better explained by environmental variables. In terms of environmental variables, catchment scale variables (e.g., land use estimators, land use diversity) played the primary role in determining the patterns of both diatom and macroinvertebrate assemblages, whereas the influence of reach-scale variables (e.g., pH, substrates, and nutrients) appeared less. However, nutrients were the stronger factors influencing benthic diatom, whereas physical habitat (e.g., substrates) played more important role than water chemistry in structuring macroinvertebrates. Our results provided more evidence to the incorporation of spatial factors interpreting spatial patterns of stream organisms, and highlighted the useful of multiple organisms and environmental variables at different spatial scales in diagnosing mechanism of stream degradation and in building a sound stream conditions monitoring program for Yangtze River Delta.     

Effects of land cover on chemical characteristics of streams in the Cerrado region of Brazil

The Cerrado is the second largest Brazilian biome and contains the headwaters of three major hydrological basins in Brazil. In spite of the biological and ecological relevance of this biome, there is little information about how land use changes affect the chemistry of low-order streams in the Cerrado. To evaluate these effects streams that drain areas under natural, rural, and urban land cover were sampled near Brasília, Brazil. Water samples were collected between September 2004 and December 2006. Chemical concentrations generally followed the pattern of Urban > Rural > Natural. Median conductivity of stream water of 21.6 (interquartile: 22.7) ??S/cm in urban streams was three and five-fold greater relative to rural and natural areas, respectively. In the wet season, despite of increasing discharge, concentration of many solutes were higher, particularly in rural and natural streams. Streams also presented higher total dissolved N (TDN) loads from natural to rural and urban although DIN:DON ratios did not differ significantly. In natural and urban streams TDN was 80 and 77% dissolved organic N, respectively. These results indicate that alterations in land cover from natural to rural and urban are changing stream water chemistry in the Cerrado with increasing solute concentrations, in addition to increased TDN output in areas under urban cover, with potential effects on ecosystem function.     

Developing diatom-based transfer functions for Central Mexican lakes

This paper is the first attempt to produce diatom-based transfer functions for the northern tropical Americas. A dataset of 53 modern diatom samples and associated hydrochemical variables from 31 sites in the volcanic highlands of central Mexico is presented. The relationship between diatom species distribution and water chemistry is explored using canonical correspondence analysis (CCA) and partial CCA. Variance partitioning indicates that ionic strength and ion type both account for significant and independent portions of this variation. Transfer functions are developed for electrical conductivity (r ² = 0.91) and alkalinity (as a percentage of total anions) (r ² = 0.90), reflecting ionic strength and ionic composition respectively. Prediction errors, estimated using jack-knifing, are low for the conductivity model, but the carbonate transfer function performs less well. This study highlights the potential for diatom-based quantitative palaeoenvironmental reconstructions in central Mexico. However, a number of key diatom species found in fossil material are not represented in the modern flora. Sampling of additional sites may resolve this, but it is thought that the lack of modern analogues may reflect the high degree of anthropogenic disturbance in many of the catchments. This highlights the problem of trying to reconstruct pre-disturbance environmental changes in highly modified ecosystems. One possible solution is to merge the central Mexican data with the African dataset, which includes sites of similar chemical composition, but which have not suffered the same degree of disturbance.     

Local and regional factors determining aquatic and semi-aquatic bug (Heteroptera) assemblages in rivers and streams of Greece

Heteroptera species were collected from 48 sites distributed throughout the mainland and island complexes of Greece during 1999–2004. The aims of this study were to investigate Heteroptera distribution and abundance in Greek streams, identify the environmental factors that are linked to variation in their assemblages and to partition the influence of environmental and spatial components, alone and in combination, on Heteroptera community composition. Canonical ordination techniques (CCA) were used to determine the relationship between environmental variables and species abundance, while variation partitioning was performed using partial CCA to understand the importance of different explanatory variables in Heteroptera variation. Heteroptera variation was decomposed into independent and joint effects of local (physicochemical variables, microhabitat composition, stream width and depth), regional (land use/cover) and geographic variables (longitude, latitude, altitude and distance to source). Land use/cover, aquatic and riparian vegetation, stream size and water chemistry were the most important factors structuring Heteroptera assemblages. At regional scale, bug assemblages were mainly divided into those found in forested and agricultural landscapes, following water quality and microhabitat composition at local scale. Local variables accounted for 48% of the total explained variation, regional variables for 20% whereas geographical position appeared to be the least influencing factor (8.5%). The results of partial constraint analyses suggested that local variables play a major role in Heteroptera variation followed by regional variables. Electronic supplementary material Electronic supplementary material is available for this article at and accessible for authorised users.