Canonical Correspondence Analysis Axis Research Articles

Detecting the spatial component of variation in the weed community at the farm scale with variation partitioning by canonical correspondence analysis

SummaryVariation partitioning by canonical correspondence analysis (CCA) was applied to analyse spatial variation in the species composition of a weed community for an area of farmland in southern Finland. The farmland, covering 450 ha, was sampled with a 60 m × 60 m grid. Data on weed species were collected along with the following groups of explanatory variables: spatial variables (the terms of second‐order polynomial trend surface regression equation generated on the x and y co‐ordinates of sample quadrats: x, y, x2, xy, y2, x2y and xy2), farmer variables (nine farms), soil variables (four soil types and pH value), crop variables (barley, oats, sugarbeet, potato and turnip rape) and physical variables (area of field, altitude, slope and aspect in four directions). The main variation in species composition along the first and second CCA axes was caused by interplay between farmer and crop variables. Farmer and crop variables explained more of the variation than did soil or physical variables. All the variables were to some extent spatially structured. The spatial variables contributed 54.5% of the total variation, of which pure spatial variation accounted for 12.2%. The highest covariation with spatial variables was detected with farmer (33.7%) and crop variables (25.7%). Variation partitioning by CCA is recommended for studying the relationship between the spatial variation in weed communities and explanatory variables.

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� 47¢ N, 103� 32¢ W), 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 (k ¼ 0.44) was primarily controlled by conductivity-related variables, while CCA axis 2 (k ¼ 0.21) was related to particulate concentrations. Diatom-based inference models were generated for the reconstruction of conductivity (RMSEPjack ¼ 0.32, r 2 jack ¼ 0.76) and pH (RMSEPjack ¼ 0.40, r 2 jack ¼ 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.

Planktonic foraminiferal assemblages preserved in surface sediments correspond to multiple environment variables

Here we investigate the relationships between modern planktonic foraminiferal species assemblages from Atlantic and Pacific core-top sediment samples and 35 water-column and preservation properties using Canonical Correspondence Analysis (CCA). CCA finds two faunal dimensions (axes) that are most highly correlated to the environmental variables, and describes each axis in terms of the best linear combination of environmental variables. CCA Axis 1 (30.4% of the faunal variance) is related primarily to mean annual sea-surface temperature (SST, r=−0.96). CCA Axis 2 (7.9% of the faunal variance) is related to environmental variability associated with an inverse relationship between SST and surface salinity, as well as pycnocline phosphate concentrations, the seasonal range in nitrate concentrations, water depth, and chlorophyll concentrations at the sea surface. Based on this clustering of nutrient and chlorophyll on Axis 2, we infer an ecological response to oceanic fertility. No evidence is found for a unique dissolution influence, suggesting that sea-floor carbonate ion concentration cannot be estimated reliably from planktonic foraminiferal assemblages. Our results support the use of foraminiferal assemblages to estimate mean annual SST.

Occurrence of epiphytic bryophytes in a 'tetrad' transect across southern Britain. 2. Analysis and modelling of epiphyte–environment relationships

Frequency data for epiphytic bryophytes in systematically sampled 2 x 2 km grid squares within a belt transect aligned S. W. to N. E. across southern Britain were analysed by canonical correspondence analysis (CCA). The environmental data used in the analysis included variables representing climate, atmospheric pollutants, forest cover, geology, altitude and presence of water courses. The two most important canonical gradients obtained expressed the effects of moisture availability and atmospheric pollution/geology on the epiphytic flora. Frullania tamarisci, Neckera pumila, Metzgeria temperata, Microlejeunea ulicina and Hypnum andoi were restricted to tetrads with high moisture availability, whereas Syntrichia ruralis, Grimmia pulvinata, Tortula muralis and Aulacomnium androgynum only occurred as epiphytes in tetrads with low moisture status. At one end of the second CCA axis were species characteristic of acid woodland, including Dicranum tauricum and D. montanum, which may be indicative of elevated deposition of sulphur and nitrogen from atmospheric pollution. Calcicole bryophytes and taxa that avoid acid substrata (e.g. Syntrichia laevipila, S. papillosa and Ulota phyllantha) were positioned on the other end of this axis. The CCA axes were used to generate log-linear regression models for individual epiphyte species. For 12 species, fitted distributions for the transect tetrads were compared with the observed distributions and used to predict epiphyte distributions for a wide area of southern Britain based on an independent set of environmental data. A separate analysis was made of the associations between particular epiphytes and their phorophytes by detrended correspondence analysis (DCA). The primary DCA axis represented a sequence of epiphytes from species more commonly associated with calcareous masonry (e.g. Syntrichia ruralis and Grimmia pulvinata) to calcifuges such as Tetraphis pellucida, Hypnum jutlandicum, Dicranum montanum and Orthodontium lineare. The corresponding ranking of host trees was from Sambucus nigra, Malus sp., Ulmus spp., Salix spp. and Acer campestre, all species characterized by nearly neutral or nutrient-rich bark, to those with strongly acidic bark including Carpinus betulus, Betula spp. and Castanea sativa.

Identification of the spectral characteristics of British semi-natural upland vegetation using direct ordination: a case study from Dartmoor, UK

The direct ordination technique Canonical Correspondence Analysis (CCA) is applied to the examination of the relationships between the floristic composition of semi-natural vegetation and its spectral reflectance. Paired measurements of floristic and spectral characteristics, the latter being measured using a portable field spectrometer, were collected at ground level for a range of upland semi-natural vegetation within two study sites on Dartmoor, south-west England. The spectral data were converted to simulate a Compact Airborne Spectrographic Imager (CASI) bandset. Both floristic and spectral data sets were then jointly ordinated using CCA. A sequence of individual species changes along the first ordination axis was identified that showed a strong correspondence with variation in the simulated CASI wavebands covering the 736 nm to 870 nm wavelengths. A significant relationship between first axis CCA scores for quadrats and their estimated total percentage vegetation cover was also identified. Although UK National Vegetation Classification (NVC) categories corresponded to a general sequence of plant community types along the first CCA axis, with the possible exception of the U20 Pteridium aquilinum-dominated community, it proved impossible to demonstrate any close link between any specific plant community type and a distinct set of spectral characteristics because of the continuum nature of the vegetation. The second axis of the CCA showed no interpretable relationship with variation in floristic/spectral data and this was confirmed further by use of Detrended Canonical Correspondence Analysis (DCCA). Possibilities for further research into floristic/spectral data using CCA/DCCA are discussed.

Distribution and habitat niches of 37 peatland Cyperaceae species across a broad geographic range in Canada

The presence and absence of 37 Cyperaceae species found on 498 peatlands across Canada were examined along surface water chemistry, shade, height above the water table, and climatic gradients. A canonical correspondence analysis of the species distribution along the environmental gradients indicated that surface water chemistry and climate were highly correlated with the first and second axes and were the two most important gradients distinguishing among groups. The climatic gradient was further subdivided into western oceanic continental and eastern oceanic continental gradients. Height above the water table and shade were correlated to the third canonical correspondence analysis axis and were of secondary importance in explaining species distributions. A TWINSPAN analysis of the species separated them into eight groups: (1) widespread, obligate rich fen, wet, shade-tolerant species; (2) widespread rich fen preferential species; (3) continental and eastern oceanic, rich fen preferential, wet, shade-intolerant species; (4) widespread fen, wet, shade-intolerant species; (5) widespread bog or fen, shade-tolerant species; (6) widespread subcontinental and oceanic, bog or fen, shade-intolerant species; (7) eastern subcontinental and oceanic, poor fen preferential species; (8) western oceanic, bog and poor fen preferential, shade-intolerant species. Response surfaces were generated by quantifying the frequency of occurrence of representative species in each group along pH and height above the water table, shade and mean annual temperature, and mean annual total precipitation gradients. Frequency of occurrence values for several of the rarer species indicated that they were not limited by the number of suitable habitats analyzed in this study but by other factors such as competition, failure to establish, or dispersal. This study emphasizes the importance of habitat and climate in determining the local and regional diversity and distribution of the most common Cyperaceae on peatlands in Canada.Key words: sedges, Carex, peat lands, response surfaces, climate, water chemistry.

Effects of scale‐dependent factors on herbaceous vegetation patterns in a wetland, northern Japan

Herbaceous vegetation was examined in an Otanoshike wetland in northern Japan to clarify the relationships between vegetation patterns and environmental factors with different scales. Alders (Alnus japonica) have recently invaded and might modify the herbaceous vegetation. In total, 150 50 × 50 cm plots were established on the transitional areas between alder thickets and grassy marshland. Cover was measured for the vascular plant taxa, and canopy area, number of stumps, number of mounds, water depth, elevation difference, litter thickness, soil organic matter, and soil pH were measured in each plot. TWINSPAN cluster analysis classified four vegetation groups: (i), grasslands represented by Phragmites australis, Trientalis europaea, Lythrum salicaria, and Hosta rectifolia; (ii), Calamagrostis langsdorfii, and Polygonum thunbergii grasslands with Spiraea salicifolia; (iii), reed swamp dominated by Phragmites australis, and (iv), marshland dominated by Carex lyngbyei. Canonical correspondence analysis indicated that water depth primarily divided vegetation groups 1–2 and 3–4. Alder established in drier sites mostly by re‐sprouting, and the canopy affected light and soil conditions on the ground surface. The second axis of canonical correspondence analysis was related to the canopy area and soil pH, and explained the vegetation differentiation between groups 1 and 2, and groups 3 and 4. In conclusion therefore, scale‐dependent or hierarchical variables affected the vegetation patterns in different ways, that is, the herbaceous vegetation was first differentiated by water depth that was corresponding to alder establishment on a large scale, and subsequent light and soil conditions were second determinants on a small scale.

Correspondence analysis of functional groups in a riparian landscape

We used multivariate analysis and ordinations to characterize thecomposition and distribution of woody vegetation within the Ozark NationalScenic Riverways (ONSR), Missouri, USA. The objectives of the study were to: 1)evaluate patterns of woody species distributions along existing environmentalgradients; 2) determine if different classes of woody plants (i.e., dominantoverstory trees, all trees, understory trees, and shrubs) responded similarlytothe same suite of environmental variables; and 3) determine if discreteecotonaland/or ecoclinal vegetation patterns were present across the landscape. Woodyvegetation was sampled from 94 plots along 35 transects positioned at rightangles to the river channel. Sample plots were analyzed with DetrendedCorrespondence Analysis (DCA), Canonical Correspondence Analysis (CCA), andTWINSPAN. Overall, woody vegetation was correlated with several environmentalgradients, including elevation of the plot above the river, soil pH, soilmoisture, and soil particle size. Responses to secondary gradients differedamong the four classes of plants analyzed, however. CCA biplots of understorytrees indicated that patterns of those species were strongly correlated withslope through the plot and sand content of soil. CCA biplots of shrubs showedthat CCA axes were most strongly correlated with soil organic matter content,soil moisture, and silt content. Further, there was limited evidence fordiscrete assemblages of woody species, with the exception of streamsidevegetation. Instead, mixing of woody species was observed across a broadtransition zone. Because there is little correspondence between vegetationlayers, our results demonstrate including plant classes other than a subset ofcanopy dominant trees can provide additional resolution in characterizingvegetation responses along complex environmental gradients.

MULTIVARIATE ANALYSIS OF PERIPHYTON ASSEMBLAGES IN RELATION TO ENVIRONMENTAL GRADIENTS IN COLORADO ROCKY MOUNTAIN STREAMS 1

We assessed relationships between chemical and physical characteristics and periphyton assemblages in stream reaches of the mineral belt in the Southern Rockies ecoregion of Colorado, United States. Using canonical ordination analyses, we contrasted results wherein assemblage structure was assessed using community metrics or species abundances. Our objectives were to identify community metrics or individual species diagnostic of the primary environmental stressors in these streams, to compare the sensitivity of these two approaches to determining the primary stressors, and to determine how these approaches may be used to differentiate the effects of these environmental stressors from other gradients. For periphyton metrics, the first axis extracted by redundancy analysis correlated with total phosphorus, substrate coarseness and embeddedness, and riparian vegetation condition, whereas the second axis correlated with dissolved metals. For species abundances, the three axes extracted by canonical correspondence analysis (CCA) were correlated with (1) stream size and types of in‐stream habitats; (2) total phosphorus, dissolved ions, and riparian disturbance by agriculture; and (3) sediment coarseness and embeddedness and riparian vegetation condition. Concentrations of dissolved metals were not correlated with the CCA axes. Analyses of species abundances were sensitive to effects associated with nutrients, substrates, and riparian vegetation, whereas analyses of periphyton metrics were sensitive not only to these nutrient and physical habitat effects but also to toxicological effects associated with metals. As a result, the analyses of periphyton metrics may be used to identify which metrics would be useful for a periphyton index of biotic integrity and would also be individually diagnostic of the larger scale stressors in these streams, nutrient, substrate, and riparian vegetation effects of livestock grazing and increased metal concentrations associated with metal mining.

Use of Diatoms and Macroinvertebrates as Bioindicators of Water Quality in Southern Ontario Rivers

The effectiveness of diatoms and macroinvertebrates as indicators of environmental conditions in lotic systems was compared in a regional assessment. Benthic samples were collected during summer 2000 from 35 Provincial Water Quality Monitoring river stations in the Grand, Credit, Maitland and Upper Thames watersheds in southern Ontario. Patterns of diatom and macroinvertebrate taxon distributions in relation to environmental variables were determined using canonical correspondence analysis (CCA). Total nitrate, phosphate, conductivity and alkalinity were significant in explaining diatom data, while alkalinity, total nitrate, ammonium and total Kjeldahl nitrogen were significant in explaining invertebrate data. The eigenvalues of the first two CCA axes were significant (p < 0.05) for both diatoms and invertebrates, while the invertebrate analysis explained more taxonomic variation (19% vs. 12%). Regression and calibration models were developed for total nitrate. The correlation between taxon-inferred and observed values was higher for diatoms than invertebrates in the analysis (0.70 vs. 0.59), however, cross-validation with bootstrapping indicated that apparent coefficients may be inflated. Biotic indices were also calculated. The composite invertebrate metric scores gave a slightly closer representation of water quality conditions than the diatom trophic index, but, biotic indices were not as effective as CCA in describing sites. Overall, the invertebrate and diatom communities were similar in their abilities to predict water quality.

Exploration of spatial and temporal patterns of fish diversity and composition in a tropical estuarine system of Mexico

There are many studies of fish assemblages intemperate systems, but little informationexists about tropical estuarine systems,particularly in the Western Atlantic Regionincluding Mexico. We investigated the fishcommunity structure in the Pueblo Viejo lagoon,Veracruz. Biological samples were collectedmonthly for one year at six sites: three withdense stands of Ruppia maritima and threesites without submerged vegetation. For eachsample, temperature, salinity, dissolvedoxygen, turbidity and depth were analyzed. Theunbiased Simpson diversity index, CanonicalCorrespondence Analysis (CCA) and DiscriminantAnalysis (DA) were used to analyze the fishcommunity. Environmental abiotic variablestended to show significant temporal, but notspatial differences. Fish diversity showedonly a weak significant correlation with watertemperature, and relatively strong diversitypeaks, from June to August and April, wererelated to production peaks in the system. Thetwo first axes of CCA accounted for 65% of thespecies-environment biplot variance, whichsuggested that monthly changes of salinity,turbidity and precipitation, and presence orabsence of submerged vegetation, were the mostimportant environmental variables indetermining the observed variability in fishcommunity composition. Further, fish diversitywas significantly different between habitatswith and those without the presence ofsubmerged vegetation (P < 0.02). A DA showedsignificant differences (P < 0.03) in fishcommunity composition between both kinds ofhabitats, with Lagodon rhomboides,Mugil curema and Menidia beryllina(substantially more abundant in habitat withsubmerged vegetation) as the most importantspecies in the discrimination of spatial fishcomposition. Considering both habitatscombined, fewer differences were observed infish diversity and community compositionbetween rainy and dry seasons. Fish communitycomposition showed greater similarity betweenseasons than between habitats, despite the factthat environmental abiotic variables showed aninverse pattern, suggesting that site factors,such as the presence of submerged vegetation,play a more important role in the maintenanceof fish community patterns than those relatedto temporal influence.

Development of Species Dominance along an Elevational Gradient: Population Dynamics ofPinus edulisandJuniperus monosperma

We evaluated species‐environment relationships within piñon‐juniper woodlands in northern New Mexico (United States) using canonical correspondence analysis (CCA). The first CCA axis was associated primarily with elevation. Our results showed separation between piñon and juniper along the elevation gradient, as expected: piñon is relatively more dominant at higher sites, whereas juniper is relatively more dominant at lower sites. To examine how this pattern of dominance might emerge with time, we plotted the position of centroids for three piñon and juniper size classes along the first CCA axis. We found that small piñons and junipers were distributed relatively uniformly across the gradient, whereas large piñons and junipers were strongly segregated along the gradient, with intermediate‐sized piñons and junipers intermediate on the CCA axis between small and large. This produced a pattern of increased divergence between the two species that increased with size. We suggest that this pattern emerges as a result of differential mortality between the species rather than as a result of differences in seedling establishment along the gradient. These differences between the species could result from differences in resource use (i.e., physiology) and resource acquisition (i.e., rooting patterns relative to plant available water). We present a conceptual model of how differences between the species in resource acquisition increase with size (age). We suggest that differences in resource acquisition between species, which increase as individuals mature, may play a greater role in determining species dominance along resource gradients than has been previously appreciated.

A CANONICAL CORRESPONDENCE ANALYSIS OF THE EFFECTS OF THEEXXON VALDEZOIL SPILL ON MARINE BIRDS

To assess how the Exxon Valdez oil spill affected habitat occupancy by communities of marine-oriented birds in Prince William Sound, Alaska, we conducted a canonical correspondence analysis (CCA) using data collected between 1989 and 1996 in 10 bays that had been exposed to different levels of oiling. CCA creates a multivariate space defined by a combination of environmental variables and measures of the abundances of the bird species present in the bays. The locations of individual sites (site scores) in this multivariate space indicate how bird-community composition at the sites varies in relation to the combination of environmental variables, while the locations of individual bird species (species scores) indicate the mean values of the response curves (abundance variations) of species on the CCA axes. Relationships among the site scores of sites exposed to different levels of oiling may therefore be used to evaluate the effects of the spill on the bird communities occupying these sites, and the relationship of a species score to the site scores indicates the conditions under which the species is, on average, most abundant. In surveys conducted the summer after the oil spill, the site scores for unoiled or lightly oiled (“unoiled”) bays occupied a region of the CCA space that was largely separate from that occupied by the moderately or heavily oiled (“oiled”) bays, indicating consistent differences in bird-community composition. Although the structure of the CCA ordination and site locations in that space varied among seasons and years, the separation of the unoiled and oiled site scores in the CCA space continued. Rather than indicating continuing spill effects, however, the separation of site scores was probably related to environmental differences among the bays and chance differences among bays in the reassembly of bird communities following the spill. Consideration of the locations of species scores in the CCA space clarified the patterns of spill impacts and subsequent recovery. The increase in the number of bird species present in early summer surveys between 1989 and 1990 was associated with the occupancy of previously oiled bays by species that were absent in 1989 or that previous analyses had shown to be initially impacted by the spill. Species scores for birds for which the previous analyses showed continuing spill impacts in early summer 1990 were associated most closely with the site scores of the unoiled bays. The species scores of several species that occurred too infrequently to be evaluated in the previous studies were associated with the site scores of the unoiled bays in 1989, suggesting initial spill impacts. In subsequent years, these species scores were associated with the site scores of the previously oiled bays, suggesting reoccupancy of those bays. Patterns for surveys conducted in midsummer generally were similar to those in early summer. Surveys conducted in midsummer 1996 suggested continuing recovery in the use of initially oiled habitats by birds, including some species that did not show clear evidence of recovery in 1991. Overall, these analyses showed that the Exxon Valdez oil spill had clear initial impacts on bird species and communities in the study bays in Prince William Sound, but they also provided clear evidence of increasing occupancy of previously oiled sites. Multivariate analyses such as CCA can provide valuable insights into the complex responses of environments and biological communities to large-scale contamination events when precontamination data are lacking, especially when the analyses are repeated over time and the changing relationships of both species and site scores in the multivariate space are considered.

Analysis of Macroinvertebrate Assemblages in Relation to Environmental Gradients in Rocky Mountain Streams

Using redundancy analysis (RDA) and canonical correspondence analysis (CCA), we assessed relationships among chemical and physical characteristics and macroinvertebrate assemblages at stream sites sampled by the Regional Environmental Monitoring and Assessment Program (R-EMAP) in the mineral belt of the Southern Rockies Ecoregion in Colorado. We contrasted results of analyses where community structure was summarized as community metrics and analyses based on genera abundances. Our objective was to identify metrics or taxa diagnostic of major environmental stressors in these streams. When RDA was used to analyze the community metrics data, three axes were significant, accounting for 96% of the metric–environment relation. The first RDA axis was correlated with dissolved cadmium, sediment zinc, and total suspended solids, variables that indicate it was related to mining effects. The second and third RDA axes were correlated with water temperature, mean substrate embeddedness, mean canopy density at the banks, and a riparian human disturbance index for agriculture, variables associated with riparian and substrate alterations associated with grazing by livestock. When CCA was used to analyze the genera abundance data, four axes were significant, accounting for 45% of the species–environment relation. The CCA axes were correlated with total and dissolved iron, water temperature, dissolved and total organic carbon, mean bank height, and mean water surface gradient, variables associated with riparian disturbance from livestock grazing, but not with mining effects. Because CCA measures variation in community structure in terms of changes in the absolute abundances of different genera relative to one another, that analysis of genera abundances was sensitive to the effects of riparian disturbance and stream size, but not to the general toxicological effects associated with mining that reduced the abundances of all genera. Community metrics measure various aspects of community structure, including taxa richness, taxa relative abundances, and taxa relative dominance, and these metrics were sensitive to the effects of mining, riparian disturbance, and stream size. Some community metrics, such as the percentage abundance of the most dominant taxon, the total number of individuals, the total number of taxa, and the number of chironomid taxa, may be used to diagnose the environmental stressors in these streams, while the results of the CCA for genera abundances may be used to design new metrics for this purpose.

ANALYSIS OF MACROINVERTEBRATE ASSEMBLAGES IN RELATION TO ENVIRONMENTAL GRADIENTS IN ROCKY MOUNTAIN STREAMS

Using redundancy analysis (RDA) and canonical correspondence analysis (CCA), we assessed relationships among chemical and physical characteristics and macroinvertebrate assemblages at stream sites sampled by the Regional Environmental Monitoring and Assessment Program (R-EMAP) in the mineral belt of the Southern Rockies Ecoregion in Colorado. We contrasted results of analyses where community structure was summarized as community metrics and analyses based on genera abundances. Our objective was to identify metrics or taxa diagnostic of major environmental stressors in these streams. When RDA was used to analyze the community metrics data, three axes were significant, accounting for 96% of the metric–environment relation. The first RDA axis was correlated with dissolved cadmium, sediment zinc, and total suspended solids, variables that indicate it was related to mining effects. The second and third RDA axes were correlated with water temperature, mean substrate embeddedness, mean canopy density at the banks, and a riparian human disturbance index for agriculture, variables associated with riparian and substrate alterations associated with grazing by livestock. When CCA was used to analyze the genera abundance data, four axes were significant, accounting for 45% of the species–environment relation. The CCA axes were correlated with total and dissolved iron, water temperature, dissolved and total organic carbon, mean bank height, and mean water surface gradient, variables associated with riparian disturbance from livestock grazing, but not with mining effects. Because CCA measures variation in community structure in terms of changes in the absolute abundances of different genera relative to one another, that analysis of genera abundances was sensitive to the effects of riparian disturbance and stream size, but not to the general toxicological effects associated with mining that reduced the abundances of all genera. Community metrics measure various aspects of community structure, including taxa richness, taxa relative abundances, and taxa relative dominance, and these metrics were sensitive to the effects of mining, riparian disturbance, and stream size. Some community metrics, such as the percentage abundance of the most dominant taxon, the total number of individuals, the total number of taxa, and the number of chironomid taxa, may be used to diagnose the environmental stressors in these streams, while the results of the CCA for genera abundances may be used to design new metrics for this purpose.

Ostracoda (Crustacea) as ecological indicators: a case study from Iberian Mediterranean brooks

Although ostracods are common inhabitants of most types of continental water bodies, their ecologies are generally poorly known. Despite this, there is a genuine need for data on species-habitat relationships, particularly because of the recent expansion of their use in Quaternary palaeoecology. We studied the responses of ostracod assemblages to environmental gradients in a variety of Mediterranean brooks. These differed in their geographical and geological settings, water chemistries and degrees of anthropogenic disturbance. The use of Canonical Correspondence Analysis (CCA) showed that the main sources of assemblage variation were related to the physical and chemical properties of the host environment. CCA separated species that inhabited chloride-rich, shallow streams on evaporitic rocks at medium altitudes, from deeper, lowland streams with higher carbonate than chloride contents. The second axis of CCA reflected ostracod species segregations derived from nutrient (nitrate) pollution of aquifers. It also showed a gradient from high altitude sites with high BMWP' values, to disturbed lowland sites that were nitrate-enriched and had high oxygen concentrations. Multivariate logistic regression showed that Sarscypridopsis lanzarotensis and Heterocypris salina both relate to waters with low alkalinities with respect to calcium and, in the former case, also to high relative chloride contents. Both species are good indicators of aquatic ionic composition, which is related to local geology. Our results also suggest that Herpetocypris brevicaudata, a common ostracod in the area, may be valid as indicator of clean waters. Despite these findings, further data are required before Ostracoda can be used effectively to monitor water quality. However, the present survey provides evidence that supports their potential in the assessment of the present and past conditions of flowing waters.

Invertebrate communities and hydrological variation in Cairngorm mountain streams

Macroinvertebrates, discharge and 16 chemical variables were monitored over a 14-year period in four small streams (catchment area <15 km2) in the Cairngorm mountains, Scotland. Canonical Correspondence Analysis (CCA) was used to assess relationships between invertebrates and environmental conditions on the day of sampling, average conditions over the preceding 1, 2 and 3-month periods and indices of hydrological and hydrochemical variation over preceding monthly intervals. CCA detected subtle inter-catchment differences in invertebrate community structure, with catchments separated along axes representing streamwater calcium, alkalinity and total organic carbon concentrations. Invertebrate communities varied seasonally, with spring, summer and autumn samples separated along CCA axes representing temperature, orthosilicate and discharge. Hydrochemically, spring was the most variable season, characterised by increased frequency of both high and low flow events and acid, snowmelt episodes. In two of the streams, invertebrate community structure varied more in spring than in other seasons. CCA ordinations using indices of hydrological and hydrochemical variation over preceding time periods were more successful (increased eigenvalues) at explaining temporal variation in invertebrate community structure than those using conditions on the day of sampling or average conditions over preceding time periods. For one of the catchments, 40% of the seasonal and between-year variation over the 14-year period could be explained by the frequency of high and low flow events, maximum and minimum water temperatures and acid episodes in the two months prior to the invertebrate samples being collected. The single most important flow parameter (longest CCA arrow) was the frequency of high flow events greater than three times the median discharge. No significant trends in invertebrate community composition were found in any of the streams over the 14-year period so, despite the apparent importance of hydrological and hydrochemical variation, communities appeared stable over the long-term.

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.

Gradient analysis of diatom assemblages in a Puget Sound salt marsh: can such assemblages be used for quantitative paleoecological reconstructions?

Taphonomy is important to coastal paleoecologists because processes acting on diatom thanocoenoses tend to work towards obscuring original ecological relationships between diatom assemblages and the environment. The purpose of this paper is to briefly describe diatom taphonomy and present a method for quantitative reconstruction of environmental parameters from salt marsh diatom assemblages. The main hypothesis for this study is that major environmental and taphonomic processes (e.g., tides) act in predictable ways to distribute living and dead diatoms along environmental gradients. To test this hypothesis, a modern transect was established across a large salt marsh in southwestern Puget Sound for the purpose of determining modern species/environment gradients and calibrating species assemblages to environmental variables. Canonical correspondence analysis (CCA) relates modern species assemblages to environmental gradients, and weighted averaging calibration is used to develop transfer functions for predicting environmental information. CCA showed that the effect of salinity and elevation on the species distributions is significant, indicating that environmental processes control the distribution of sedimentary diatoms across the salt marsh surface in predictable ways. Salinity was strongly correlated with CCA Axis 1 and elevation with Axis 2. The calibration results indicate that, although mixing of allochthonous and autochthonous diatoms does occur, salt marsh diatom assemblages reflect major environmental gradients in Puget Sound salt marshes and can be effectively used for quantitative reconstructions of former environmental conditions.

Hydraulic and Geomorphic Influence on Macroinvertebrate Distribution in the Headwaters of a Small Watershed

ABSTRACT Spatial variability of aquatic macroinvertebrates was examined in riffles of second to fourth order streams in Onondaga Creek, in central New York, USA. Even with only small differences in stream sizes, aquatic macroinvertebrates were distributed primarily by a headwater-to-valley gradient as defined by mean stream width and water surface slope. Secondary and tertiary gradients were based on hydraulic character. Direct gradient analysis using canonical correspondence analysis (CCA) examined the common and rare (respectively, < 5—0.5% and 0.5—0.001% of total) macroinvertebrate assemblage among sites. The variables used in the analysis were mean wetted width, water surface slope, mean Froude number and Froude number variance. The first CCA axis explained 43.1% of the variability. Froude number variance and Froude number affected the second and third CCA axes most strongly. The gradients revealed by the second and third CCA axes did not influence lower gradient valley sites, but did separate the headwater sites based on hydraulic character. Taxa were also distributed by functional feeding groups (i.e., collector-gatherer). The headwater-to-valley gradient did separate taxa slightly by functional feeding groups; however, the hydraulic gradient clearly separated scrapers and shredders from collector filterers.