Upstream-downstream Gradient Research Articles

How Does an Invasive Cyprinid Benefit from the Hydrological Disturbance of Mediterranean Temporary Streams?

The Iberian Peninsula has been subjected to numerous fish introductions and the colonization of new areas by non-native species is constantly reported. However, there is a lack of knowledge about many aspects of the bio-ecology of these species and their invasive success within the environmental context of Mediterranean temporary rivers. This study aimed to address the following questions: (i) what are the main regional and local environmental drivers influencing fish assemblages and differentiating native from non-native species, particularly A. alburnus?; (ii) what are the environmental and anthropogenic disturbance factors responsible for the occurrence and abundance of A. alburnus?; (iii) is there a pattern in the spatiotemporal distribution of A. alburnus size classes, along the tributaries of reservoirs where the species occurs? Data on fish species, environmental variables, and anthropogenic disturbance were collected in 77 sites of the Guadiana and Sado river basins in the south of Portugal. Additionally, a seasonal sampling was performed along an upstream-downstream gradient of several tributaries from three reservoirs in these river basins. A multivariate analysis and a multi-model approach were used in data analyses. Native and non-native fish assemblages showed different environmental drivers and responses to anthropogenic disturbance levels, though A. alburnus has revealed similarities with native species. The occurrence of A. alburnus was mainly determined by hydrological and morphological disturbances driven by anthropogenic activities. Furthermore, this species apparently performed seasonal movements along the tributaries of several reservoirs, profiting from these lentic habitats as a stepping-stone for further invasions. This study highlighted the wide ecological plasticity of A. alburnus, as it benefits from the anthropogenic hydrological disturbance (induced by reservoirs), and is also able to cope with the natural hydrological disturbance (resulting from the intermittency of these streams), to guarantee and enhance its invasive success in Mediterranean intermittent streams. It also gives a sound contribution to understand the spread of A. alburnus in these vulnerable freshwater ecosystems, and to delineate management measures, namely by identifying critical points in the river network along with prioritizing river restoration measures that benefit native species.

Ecomorphological diversity of freshwater fishes as a tool for conservation priority setting: a case study from a Balkan hotspot

Biodiversity studies commonly focus on taxonomic diversity measures such as species richness and abundance. However, alternative measures based on ecomorphological traits are also critical for unveiling the processes shaping biodiversity and community assembly along environmental gradients. Our study presents the first analysis of habitat-trait-community structure in a Balkan biodiversity hotspot (Louros river, NW Greece), through the investigation of the relationships among freshwater fish assemblages’ composition, morphological traits and habitat features. In order to provide a hierarchical classification of species’ priority to protection measures, we highlight the most ecomorphologically distinct species using originality analysis. Our results suggest that the longitudinal changes of habitat variables (water temperature, depth, substrate, altitude) drive the local fish assemblages’ structure highlighting the upstream-downstream gradient. We also present evidence for environmental filtering, establishing fish assemblages according to their ecomorphological traits. The calculation of the seven available indices of ecomorphological originality indicates that Valencia letourneuxi and Cobitis hellenica, which are endemic to Louros and threatened with extinction, exhibited the highest distinctiveness; thus their protection is of great importance. The methodological approach followed and the patterns described herein can contribute further to the application of community ecology theory to conservation, highlighting the need to use ecomorphological traits as a useful ‘tool’.

Headwater–river gradient: trait-based approaches show functional dissimilarities among tropical fish assemblages

Traditionally, species richness and composition of freshwater communities are expected to change according to the structural variation from headwaters to river mouth. In the present study, we hypothesised that fish functional traits will respond to this gradient, leading to functional dissimilarities between fish assemblages of headwaters and those of rivers. We addressed the following questions: (1) which functional traits are present in fish assemblages of each habitat type; (2) which environmental factors determine the occurrence of fish functional traits; and (3) how functional traits are related to each environmental variable? Our findings showed dissimilarities between fish assemblages of headwaters and those of rivers, mainly driven by channel depth, channel width, water temperature, water turbidity and concentration of dissolved oxygen, whereas altitude, water temperature and pH were mainly correlated with variation within each habitat type. These results suggested that not only do the physical variables proposed in classical theory of the river-continuum concept follow the upstream–downstream gradient in structuring the functional organisation of fish assemblages, but that also physicochemical variables, such as turbidity and concentration of dissolved oxygen, display this trend in tropical river systems. Moreover, our results have given a first perspective on what kind of fish species and fish functional traits to expect in each habitat type and, consequently, along the headwater–river mouth gradient.

The relative contribution of river network structure and anthropogenic stressors to spatial patterns of genetic diversity in two freshwater fishes: A multiple‐stressors approach

Abstract Recent findings highlighted the central role of the structure of the river network in shaping spatial patterns of genetic diversity in riverscapes. However, the influence of multiple anthropogenic stressors on these patterns may be just as important and the relative impacts of these two types of predictors have rarely been quantified simultaneously in river networks. Here, we contributed to filling this gap by investigating the relative contribution of both network structure and multiple anthropogenic stressors in shaping spatial patterns of genetic diversity in two freshwater fishes (Gobio occitaniae and Phoxinus phoxinus). We focused on two rivers in which the two fish species were sampled along the upstream–downstream gradient. Microsatellite markers were used to quantify genetic diversity from three indices: allelic richness, private allelic richness and genetic uniqueness. Each sampling site was physically characterised according to its position in the network, and was described for multiple anthropogenic stressors including habitat degradation, fragmentation and stocking. This multiple‐stressors approach was conducted using a fully explicit and generalisable analytical framework designed to cope with strong collinearity among environmental variables. Overall, the contribution of network structure to the variance in genetic diversity was 1.8 times higher than the contribution of anthropogenic stressors. Both the position of sites along the upstream–downstream gradient and stocking were strong and consistent drivers of genetic variability. Conversely, the local influences of habitat degradation and fragmentation were species‐ and river‐specific, sometimes even varying along the river channel, thus preventing any generalisations. We concluded that the natural structure of networks and stocking strongly influence spatial patterns of genetic diversity in a predictable way, whereas the influence of other human activities may be much more difficult to predict over species and contexts.

Disentangling spatial and environmental determinants of fish species richness and assemblage structure in Neotropical rainforest streams

Abstract Freshwater ecology templates were developed in temperate streams, but whether they also apply to tropical streams that harbour a higher biological diversity than their temperate counterparts remains uncertain. This is particularly true for tropical fish assemblages inhabiting small streams that have been less studied than larger, higher‐order lowland streams. Here, we disentangled the strength of spatial (longitudinal and environmental) drivers, and scale‐specific (drainage basin, reach and local scale) determinants of species richness and composition of freshwater fish assemblages inhabiting small streams in French Guiana. We found that species richness increased from upstream to downstream but also with increasing local habitat structural diversity independently of stream position in the upstream–downstream gradient. This pattern was shared by the two most speciose fish orders (Characiformes and Siluriformes), demonstrating that species addition rather than species replacement shaped species richness in these assemblages. Species composition of fish assemblages was determined equally by their spatial structure within drainage and by the environment, and assemblages differed both with distance and along an upstream–downstream gradient. The environmental effect on species assemblages indicated by the fact that almost all environmental descriptors had slight but nonetheless significant effects on assemblage composition, probably reflecting species‐specific responses to the local environment. In contrast, despite a strong micro‐endemism between drainages for some taxa, assemblages were only slightly affected by river drainage identity, since widespread species were a common constituent of assemblages in all rivers. We identified five species assemblages characterising different local habitat features from torrential areas to lowland muddy areas. We also distinguished fish assemblages from confluence areas with larger rivers, which differed from the other five assemblages. The fish zonation patterns we report can constitute a benchmark for future studies measuring the impact of anthropogenic disturbances on Neotropical forest streams.

High salmon density and low discharge create periodic hypoxia in coastal rivers

AbstractDissolved oxygen (DO) is essential to the survival of almost all aquatic organisms. Here, we examine the possibility that abundant Pacific salmon (Oncorhynchus spp.) and low streamflow combine to create hypoxic events in coastal rivers. Using high‐frequency DO time series from two similar watersheds in southeastern Alaska, we summarize DO regimes and the frequency of hypoxia in relationship to salmon density and stream discharge. We also employ a simulation model that links salmon oxygen respiration to DO dynamics and predicts combinations of salmon abundance, discharge, and water temperature that may result in hypoxia. In the Indian River, where DO was monitored hourly during the ice‐free season from 2010 to 2015, DO levels decreased when salmon were present. In 2013, a year with extremely high spawning salmon densities, DO dropped to 1.7 mg/L and 16% saturation, well below lethal limits. In Sawmill Creek, where DO was monitored every six minutes across an upstream–downstream gradient during the 2015 spawning season, DO remained fully saturated upstream of spawning reaches, but declined markedly downstream to 2.9 mg/L and 26% saturation during spawning. Modeled DO dynamics in the Indian River closely tracked field observations. Model sensitivity analysis illustrates that low summertime river discharge is a precursor to salmon‐induced oxygen depletion in our study systems. Our results provide compelling evidence that dense salmon populations and low discharge can trigger hypoxia, even in rivers with relatively cold thermal regimes. Although climate change modeling for southeastern Alaska predicts an increase in annual precipitation, snowfall in the winter and rainfall in the summer are likely to decrease, which would in turn decrease summertime discharge in rain‐ and snow‐fed streams and potentially increase the frequency of hypoxia. Our model template can be adapted by resource managers and watershed stakeholders to create real‐time predictive models of DO trends for individual streams. While preserving thermally suitable stream habitat for cold‐water taxa facing climate change has become a land management priority, managers should also consider that some protected watersheds may still be at risk of increasingly frequent hypoxia due to human impacts such as water diversion and artificially abundant salmon populations caused by hatchery straying.

Effects of anthropogenic disturbances on α and β diversity of fish assemblages and their longitudinal patterns in subtropical streams, China

AbstractIdentifying the spatial patterns of α and β diversity of biotics is an important yet little‐understood area of basic and applied ecological research. Although the upstream–downstream patterns of α diversity of stream fishes are numerously reported, β diversity has received less attention. In this study, we surveyed fishes along the upstream–downstream gradients in three headwater streams of the Qingyi River, China, which were affected by different extents of human activities. We aimed to assess how anthropogenic disturbances affect α and β diversity of stream fishes and their upstream–downstream patterns. We found that, compared with that in the Shuxi Stream disturbed less, endemic species decreased and cosmopolitan species increased in the Maxi and Puxi Stream disturbed heavily. The streams disturbed heavily showed lower α diversity and higher β diversity than that in the stream disturbed less. This among‐stream variations in fish diversity only occurred at the mid‐downstream, not upstream segments. α diversity increased downstream and β diversity decreased downstream from headwaters to mouth in the stream disturbed less, whereas this upstream–downstream pattern in fish diversity shifted in the streams disturbed heavily, in which both α and β diversity showed the quadratic distributions with the hump‐shape for α diversity and the U‐shape for β diversity respectively. Our results suggest that anthropogenic disturbances cause some cosmopolitan fishes replacing many endemic fishes in upland streams. This replacement processes result in α diversity decreasing and β diversity increasing, and shift the spatial patterns in fish diversity along the upstream–downstream gradient.

Modern palynological record in the Bay of Brest (NW France): Signal calibration for palaeo-reconstructions

Modern palynological investigations (pollen and non-pollen palynomorphs) have been carried out for the first time in the entire Bay of Brest (BB; NW France), a semi-enclosed oceanic basin flooded during the Holocene that today is exposed to both marine (North Atlantic surface waters) and fluvial (Aulne and Elorn rivers) influences. Palynological analyses were conducted on 41 surface sediments collected in several mudded bays where low-energy conditions prevail, allowing for fine particle decantation, and also on 12 moss samples considered to be pollen rain collectors taken on the periphery of the BB. Furthermore, the BB allows a direct comparison between landscapes and fossilized pollen assemblages in sediments thanks to relatively small watersheds and well-studied vegetation cover on land. Our data display a well-known distortion between pollen taxa percentages calculated from palynological slides and current vegetation cover on land, with a particular overestimation of arboreal taxa as well as an underestimation of herbaceous ones, especially of cereal crops that represent a major component of the regional continental vegetation. Various factors may be involved, including pollen transport issues, taphonomic processes, and different pollination rates according to trees/herbaceous taxa. Under close examination, we find that while all surface sediments show the same major species, palynological maps established in the BB regarding some specific pollen taxa exhibit differences and upstream-downstream gradients. Also, the comparison between the pollen content in BB surface sediments and in mosses supports the hydrodynamic influence acting on the pollen distribution in sediments. More specifically, we suggest that some arboreal taxa reflect a watershed effect, especially Alnus, which seems to be highly correlated to river influences and could therefore be considered as a potential proxy for fluvial Holocene palaeo-discharges within the BB.

Water abstraction affects abundance, size-structure and growth of two threatened cyprinid fishes.

Hydrologic alteration is a major threat to freshwater biota, and particularly fish, in many river courses around the world. We analyzed and compared the effects of water abstraction on two threatened cyprinid fishes of contrasting ecology (the Mediterranean barbel Barbus meridionalis and the Catalan chub Squalius laietanus) in a Mediterranean stream. We compared abundance, size-structure, growth, and condition of both species across perennial and artificially intermittent reaches affected by water abstraction. Both species were less abundant, had scarce large individuals, and displayed slower growth rates (length-at-age) in intermittent reaches, showing clear detrimental effects of water diversion. Mixed-effect models of scale increments showed variation among individuals and among sites, years and age classes for both species. The larger-sized, water-column species (chub) disappeared or was rare in many intermittent reaches. The barbel present in intermittent reaches showed better somatic condition than in sites with permanent flow, perhaps due to reduced competition after rewetting or colonization by better fitted individuals. This benthic, rheophilic species seems more resilient to moderate water abstraction than chub. Many effects of water flow intermittency were only detected on fish life-history traits when accounting for natural, often non-linear, variation, along upstream-downstream gradients. Our results suggest that abundance was the strongest indicator of effects of water abstraction on fish populations, whereas condition was a more labile trait, rapidly recovering from anthropogenic disturbance.

Biological impacts of local vs. regional land use on a small tributary of the Seine River (France): insights from a food web approach based on stable isotopes.

As part of the landscape, streams are influenced by land use. Here, we contributed to the understanding of the biological impacts of land use on streams, investigating how landscape effects vary with spatial scales (local vs. regional). We adopted a food web approach integrating both biological structure and functioning, to focus on the overall effect of land use on stream biocœnosis. We selected 17 sites of a small tributary of the Seine River (France) for their contrasted land use, and conducted a natural experiment by sampling three organic matter sources, three macroinvertebrate taxa, and most of the fish community. Using stable isotope analysis, we calculated three food web metrics evaluating two major dimensions of the trophic diversity displayed by the fish community: (i) the diversity of exploited resources and (ii) the trophic level richness. The idea was to examine whether (1) land-use effects varied according to spatial scales, (2) land use affected food webs through an effect on community structure and (3) land use affected food webs through an effect on available resources. Beside an increase in trophic diversity from upstream to downstream, our empirical data showed that food webs were influenced by land use in the riparian corridors (local scale). The effect was complex, and depended on site's position along the upstream-downstream gradient. By contrast, land use in the catchment (regional scale) did not influence stream biocœnosis. At the local scale, community structure was weakly influenced by land use, and thus played a minor role in explaining food web modifications. Our results suggested that the amount of available resources at the base of the food web was partly responsible for food web modifications. In addition, changes in biological functioning (i.e. feeding interactions) can also explain another part of the land-use effect. These results highlight the role played by the riparian corridors as a buffer zone, and advocate that riparian corridor should be at the centre of water management attention.

Lateral and longitudinal patterns of water physico-chemistry and trace metal distribution and partitioning in a large river floodplain

Floodplain water bodies provide habitat to diversified ecological communities. Floodplains are also among the most impacted aquatic ecosystems. While the link between the lateral connectivity of floodplain sites to the main channel and their plant, fish and invertebrate communities has been well established, detailed information on chemical characteristics and particularly on trace metal spatial distribution and partitioning is scarce. The aim of this study was to examine the link between the lateral connectivity and physico-chemical variables, their trace metal concentrations and partitioning as well as the upstream-downstream gradient of these parameters. In connected and disconnected water bodies of the Rhône River upstream and downstream of the city of Lyon, we measured major ions, dissolved organic carbon, trace metal concentrations (Al, Cr, Ni, Cu, Zn, Cd, Pb and U) in water, suspended particulate matter (SPM) and sediment. The results revealed a clear difference between connected and disconnected water bodies. pH, SPM, Na+, and NO3− concentrations were lower in disconnected sites while conductivity, DOC, Ca2+ and Mg2+ were higher. Conductivity and a part of the major ion concentrations increased in the downstream sections. Trace metal concentrations and partitioning varied between connected and disconnected sites. In the dissolved fraction, trace metal concentrations were higher in connected sites. In the surface sediment, concentrations were higher in disconnected sites for the majority of metals. The upstream-downstream gradient was less important than the connected-disconnected gradient. Only three metals in the dissolved fraction (Cu, Cd and Pb) showed a clear increase in downstream sections. Overall, the study shows that the functioning of floodplains produces strong spatial patterns concerning the concentrations and partitioning of trace metals. These findings improve our understanding of trace metal biogeochemistry in floodplains and have important applications for floodplain restoration projects.

Spatial processes structuring riparian plant communities in agroecosystems: implications for restoration

The disruption of hydrological connectivity by human activities such as flood regulation or land-use changes strongly impacts riparian plant communities. However, landscape-scale processes have generally been neglected in riparian restoration projects as opposed to local conditions, even though such processes might largely influence community recovery. We surveyed plant composition of field edges and riverbanks in 51 riparian zones restored by tree planting (565 1-m2 plots) within two agricultural watersheds in southeastern Québec, Canada. Once the effects of environmental variables (hydrology, soil, agriculture, landscape, restoration) were partialled out, three models of spatial autocorrelation based on Moran's eigenvector maps and asymmetric eigenvector maps were compared to quantify the pathways and direction of the spatial processes structuring riparian communities. The ecological mechanisms underlying predominant spatial processes were then assessed by regression trees linking species response to spatial gradients to seed and morphological traits. The structure of riparian communities was predominantly related to unidirectional spatial gradients from upstream to downstream along watercourses, which contributed more to species composition than bidirectional gradients along watercourses or overland. Plant traits selected by regression trees explained 22% of species response to unidirectional upstream-downstream gradients in field edges and 24% in riverbanks, and predominantly corresponded to seed traits rather than morphological traits of the adult plants. Our study showed that even in agriculturally open landscapes, water flow remains a major force structuring spatially riparian plant communities by filtering species according to their seed traits, thereby suggesting long-distance dispersal as a predominant process. Preserving hydrological connectivity at the watershed-scale and restoring riparian plant communities from upstream to downstream should be encouraged to improve the ecological integrity of rivers running through agricultural landscapes.

Environmental factors structuring benthic primary producers at different spatial scales in the St. Lawrence River (Canada)

The influence of environmental factors controlling the biomass of submerged aquatic macrophytes, cyanobacterial mats, and epiphyton was examined at three nested spatial scales within the St. Lawrence River: (1) along a 250-km-long upstream–downstream river stretch, (2) among three fluvial lakes located within that river stretch and (3) within each fluvial lake at sites located upstream, at the mouth, and downstream of the St. Lawrence River tributaries. Over its 250-km-long course, large increases of water colour (fivefold), suspended matter (tenfold), dissolved organic carbon (DOC) (twofold) and dissolved N and P concentrations (2.5-fold) were observed in the St. Lawrence River, showing the cumulative effects of human activities on water quality. In contrast, biomass of submerged vascular macrophytes dropped tenfold along the sampled reach whereas biomass of epiphytes and cyanobacterial mats rose significantly. Biomass of the three benthic primary producers (PP) was explained (59 %) by the combined effects of conductivity, TP and spatial structure. Macrophyte biomass was related to changes in conductivity (+), biomass of epiphyton responded to DIN:TDP ratio (+) and light extinction coefficient (+) and cyanobacterial mats coincided with differences in DOC (+) and NH4+ (−). Within-lake structure was the most important spatial component for all benthic PP, suggesting that local effects, such as enrichment by the inflow of tributaries, rather than upstream–downstream gradients, determined the biomass of benthic PP. Our study shows that the sum of local tributary inflows exerts major overall pressures on benthic PP in the St. Lawrence River and that conversely, small-scale management of individual watersheds, can markedly improve local ecological condition of the river ecosystems.

Stable isotopes reveal food web modifications along the upstream–downstream gradient of a temperate stream

The upstream–downstream gradient (UDG) is a key feature of streams. For instance food webs are assumed to change from upstream to downstream. We tested this hypothesis in a small European river catchment (937 km2), and examined whether food web modifications are related to structural (i.e. food web composition) or functional changes (i.e. alteration of linkages within the web). We adopted a double approach at two levels of organisation (assemblage and species levels) using two isotopic metrics (isotopic space area and isotopic niche overlap), and proposed a new hypothesis-testing framework for exploring the dominant feeding strategy within a food web. We confirmed that the UDG influenced stream food webs, and found that food web modifications were related to both structural and functional changes. The structural change was mainly related to an increase in species richness, and induced functional modifications of the web (indirect effect). In addition, the UDG also modified the functional features of the web directly, without changing the web composition. The proposed framework allowed relating the direct effect of the UDG to a diet specialisation of the species, and the indirect effect via the structural changes to a generalist feeding strategy. The framework highlights the benefits of conducting the double approach, and provides a foundation for future studies investigating the dominant feeding strategy that underlies food web modifications.

Dendritic connectivity shapes spatial patterns of genetic diversity: a simulation-based study.

Landscape features notoriously affect spatial patterns of biodiversity. For instance, in dendritic ecological networks (such as river basins), dendritic connectivity has been proposed to create unique spatial patterns of biodiversity. Here, we compared genetic datasets simulated under a lattice-like, a dendritic and a circular landscape to test the influence of dendritic connectivity on neutral genetic diversity. The circular landscape had a level of connectivity similar to that of the dendritic landscape, so as to isolate the influence of dendricity on genetic diversity. We found that genetic diversity and differentiation varied strikingly among the three landscapes. For instance, the dendritic landscape generated higher total number of alleles and higher global Fst than the lattice-like landscape, and these indices also varied between the dendritic and the circular landscapes, suggesting an effect of dendricity. Furthermore, in the dendritic landscape, allelic richness was higher in highly connected demes (e.g. confluences in rivers) than in low-connected demes (e.g. upstream and downstream populations), which was not the case in the circular landscape, hence confirming the major role of dendricity. This led to bell-shaped distributions of allelic richness along an upstream-downstream gradient. Conversely, genetic differentiation (Fst ) was lower in highly than in low-connected demes (which was not observed in circular landscape), and significant patterns of isolation by distance (IBD) were also observed in the dendritic landscape. We conclude that in dendritic networks, the combined influence of dendricity and connectivity generates unique spatial patterns of neutral genetic diversity, which has implications for population geneticists and conservationists.

Distribution patterns of fish assemblages in an Eastern Mediterranean intermittent river

The distribution patterns of fish assemblages within streams can provide insights for river type classifications and may warrant specific conservation actions. However, there is limited knowledge of how fish assemblages assort along a longitudinal axis in Mediterranean intermittent streams. Patterns in spatial and temporal distribution of fish communities were analysed in a Mediterranean intermittent river (Evrotas River) located in Southern Greece, hosting three endemic range restricted species of high conservation concern, during the period 2007−2009, with 80% of the river’s total length desiccating in the 2007 and 2008 droughts. The general trend was an increase in fish density and species richness along an upstream-downstream gradient. Fish assemblages from upstream to downstream were characterized by a decrease of the most rheophilic species (Squalius keadicus ) and an increase of the most stagnophilic species (Tropidophoxinellus spartiaticus ). Three river segments, characterized by a high degree of homogeneity were delineated. Habitat and environmental preferences for the studied fish species were identified, with elevation and low flowing habitats being the most important environmental factors affecting fish distribution patterns. The current study provides evidence that even in an intermittent river an assemblage pattern following a longitudinal gradient can be identified, mainly due to the lack of instream barriers that allows recolonization after flow resumption.

River fish assemblages in relation to environmental factors in the eastern extremity of Europe (Tatarstan Republic, Russia)

Fish assemblages and environmental variables were studied in 245 sites in small and mid-sized rivers of the Tatarstan Republic. The basic aim of this investigation was to model the habitat requirements of the 26 most common freshwater fish species at the eastern edge of Europe since the spatial variation of the assemblages of these fish species is little understood there. The impact of environmental factors on species distribution using both presence-absence and abundance data was analyzed with regression methods and ordination methods, which are widely used by European ichthyologists elsewhere. Ordination analysis demonstrated an upstream-downstream gradient of the fish assemblages. Regression analysis indicated that basic predictors of species preference were elevation and “size” of river. For the first time in Eastern Europe, optimum points and niche breadth for fish species along basic environmental gradients were estimated. Niche breadth (tolerance) was highly variable between species. Generally, estimated relationships between environmental factors and the probability of fish presence and abundance were consistent with known fish ecology.

Water quality dynamics in the Boro-Thamalakane-Boteti river system, northern Botswana

The quality of water in aquatic systems is subject to temporal and spatial variations due to varying effects of natural and anthropogenic factors. This study assessed the dynamics of water quality in the Boro-Thamalakane-Boteti river system along an upstream–downstream gradient above and below Maun during February, May, September and December 2012. Temperature, conductivity, pH, dissolved oxygen, turbidity, Escherichia coli and faecal streptococci were monitored in the settlements of Boro, Maun, Xobe, Samedupi, Chanoga and Motopi along the river system. Comparisons of water quality among settlements using ANOVA and Tukey's honest significant difference test highlighted a significant decline in water quality from upstream to downstream, indicated by increases in microbial numbers and turbidity. The quality of water improved as the water level rose during the peak flooding season in September and declined thereafter. This study highlights the possible influence of human settlements and associated developments on waterbodies, and lends strong support to management efforts to maintain river water quality to ensure the suitability of the water for various ecosystem uses.

Response of macroinvertebrate communities to anthropogenic pressures in Tajan River (Iran)

AbstractThe Tajan River was investigated for one year in seven stations, analyzing the relationships between physical properties, water chemistry and aquatic macroinvertebrates. Biotic and diversity indices were compared with canonical unconstrained (CA) and constrained (CCA) ordination to test different methods able to estimate river ecology status. An upstream-downstream gradient was emphasized, in presence of anthropogenic stressors, coming from trout farms, paper factory, agriculture, urbanization, river regulation; the first CCA axis emphasized a natural gradient, bound to altitude, source distance, water temperature, the second CCA axis a pollution gradient. Biotic and diversity indices detected three polluted stations: S3, downstream the Korcha tributary, S6 downstream a paper factory and S7 situated after the Sari town. S4 showed high macroinvertebrates densities, which were attributed to the presence of a dam. Both multimetric and multivariate methods emphasized the need to separate the influence of natural variables from anthropogenic stressors in the Tajan River. To separate the influence of longitudinal gradient from the influence of pollution, it was suggested to evaluate the anthropogenic impact as deviation from a regression line, considering a multimetric index as dependent variable and source distance as predictor variable. The definition of reference sites was problematic in this poorly investigated area and a progress in taxonomic resolution is in any case recommended to better define the ecological status of these waters.

Health status of Largescale Sucker (Catostomus macrocheilus) collected along an organic contaminant gradient in the lower Columbia River, Oregon and Washington, USA

The health of Largescale Sucker (Catostomus macrocheilus) in the lower Columbia River (USA) was evaluated using morphometric and histopathological approaches, and its association with organic contaminants accumulated in liver was evaluated in males. Fish were sampled from three sites along a contaminant gradient. In 2009, body length and mass, condition factor, gonadosomatic index, and hematocrit were measured in males and females; liver and gonad tissue were collected from males for histological analyses; and organ composites were analyzed for contaminant content in males. In 2010, additional data were collected for males and females, including external fish condition assessment, histopathologies of spleen, kidney and gill and, for males, liver contaminant content. Multivariate analysis of variance indicated that biological traits in males, but not females, differed among sites in 2009 and 2010. Discriminant function analysis indicated that site-related differences among male populations were relatively small in 2009, but in 2010, when more variables were analyzed, males differed among sites in regards to kidney, spleen, and liver histopathologies and gill parasites. Kidney tubular hyperplasia, liver and spleen macrophage aggregations, and gill parasites were generally more severe in the downstream sites compared to the reference location. The contaminant content of male livers was also generally higher downstream, and the legacy pesticide hexachlorobenzene and flame retardants BDE-47 and BDE-154 were the primary drivers for site discrimination. However, bivariate correlations between biological variables and liver contaminants retained in the discriminant models failed to reveal associations between the two variable sets. In conclusion, whereas certain non-reproductive biological traits and liver contaminant contents of male Largescale Sucker differed according to an upstream–downstream gradient in the lower Columbia River, results from this study did not reveal the specific environmental factors responsible for the differences in health status among fish populations.