Iberian Rivers Research Articles

Northern and Southern expansions of Atlantic brown trout (Salmo trutta) populations during the Pleistocene

The phylogeography of Atlantic brown trout (Salmo trutta) was analysed using mitochondrial DNA control region complete sequences of 774 individuals from 57 locations. Additionally, the available haplotype information from 100 published populations was incorporated in the analysis. Combined information from nested clade analysis, haplotype trees, mismatch distributions, and coalescent simulations was used to characterize population groups in the Atlantic basin. A major clade involved haplotypes assigned to the Atlantic (AT) lineage, but another major clade should be considered as a distinct endemic lineage restricted to the Iberian Peninsula. The phylogeography of the Atlantic populations showed the mixed distribution of several Atlantic clades in glaciated areas of Northern Europe, whereas diverged haplotypes dominated the coastal Iberian rivers. Populations inhabiting the Atlantic rivers of southern France apparently contributed to postglacial colonization of northern basins, but also comprised the source of southern expansions during the Pleistocene. © 2009 The Linnean Society of London, Biological Journal of the Linnean Society, 2009, 97, 904–917.

Population density and age structure of the freshwater pearl mussel, Margaritifera margaritifera, in two Iberian rivers

Summary1. The European freshwater pearl mussel, Margaritifera margaritifera (Bivalvia: Unionoida), is one of the most threatened mussels. The Iberian populations of this species are considered peripheral because their distinct characters such as growth rate and longevity, and require development of effective conservation strategies.2. We assessed population density and age structure of pearl mussels in two Galician rivers (Eo and Masma in north‐west Spain). Four sampling sites were selected in each river to cover stretches of 100 m. The mean density of mussels in each of these sampling transects was estimated using the adaptive sampling technique, given that mussels occur at low densities and are highly aggregated in these rivers.3. Age structure was inferred for each population using length–age keys. The empty shells encountered during sampling were used to determine the length of the specimens at different ages (years), together with length‐at‐age data from shells previously analysed for computing growth rates from the same rivers. Water samples from both rivers were analysed for typical physicochemical parameters.4. Mean densities were very variable, even within the same river (from 0.27 to 6.55 m−2 in the River Eo and from 0.98 to 2.38 m−2 in the River Masma). Individuals in the 0‐ to 5‐year age class were scarce in both rivers.5. Margaritifera margaritifera showed a preference for the strip of river bed within 1.5 m from the river bank and avoided sites at greater distances. The species also showed a preference for sites with more than 80% tree cover and avoided sites with <50% cover.6. Iberian populations exhibit the highest growth rate, together with the lowest maximum age and maximum length known for M. margaritifera. Detailed knowledge about these peripheral Iberian populations will contribute to developing strategies for conservation and management of this endangered species.

The contribution of total suspended solids to the Bay of Biscay by Cantabrian Rivers (northern coast of the Iberian Peninsula)

Information of suspended sediments fluxes of small rivers to the coastal zone is sparse, and this is particularly so for the Iberian Rivers. To help address this shortage of information, the relationship between fluvial discharge and total suspended solids (TSS) for the main 28 Cantabrian Rivers using data from 22 years monitoring by the COCA network has been analysed, and their particulate material fluxes to the Bay of Biscay coasts have been quantified. The Cantabrian Fluvial System (drainage basin area of 20,333 km 2) may be considered as a quasi-homogeneous fluvial system with an average discharge of 561 m − 3 s − 1 and average loads of 35 kg TSS s − 1 with rivers showing similar average yields of 56 t km − 2 a − 1 . The average TSS contribution is 1.2 ± 0.2 10 9 kg a − 1 . This seaward flux of sediment is dispersed along the entire North Iberian coast and is rather modest (25% of the total supply) in comparison with the output from the French Rivers to the Bay of Biscay. The TSS loads of Cantabrian Rivers indicate they are similar to world upland rivers and those of other parts of Northern Europe according to Milliman and Syvistki [Milliman and Syvistki, 1992. Geomorphic/tectonic control of sediment discharge to the ocean: the importance of small mountainous rivers. Journal of Geology, 100: 525–544] and Milliman [Milliman, 2001. Delivery and fate of fluvial water and sediment to the sea: a marine geologist's view of European rivers. Scientia Marina, 65: 121–132]. Although their TSS flux is practically negligible (13,000 times lower) when compared to the world average flux, they provide a good example of the role of small Atlantic temperate rivers.

Monitoring fish passes using infrared beaming: a case study in an Iberian river

Summary An application of a new automated fish counting device - the Riverwatcher System (RW) - was used to monitor upstream fish movements in a pool-and-weir fish pass in the River Zezere, Portugal, for 141 days from June 2002 to May 2003. Fish populations were also collected downstream using mul- timesh gillnets (5 different mesh sizes ranging from 30 mm to 85 mm knot to knot; ratio between mesh sizes of about 1.30) and electrofishing for comparison with fish records produced by the RW. More than 3000 individual Iberian nase Chondro- stoma polylepis ascended the fish pass and moved through the RW during the study period. However, only 18% of the records produced by the RW contained silhouettes similar to fish; no individual smaller than 15 cm TL was recorded by the counter. Most seasonal movements (73.9%) occurred in spring and were associated with reproduction. Displacements seemed to occur independently of time of day. Water temperature (range: 12-22� C) was the only significant environmental variable (P < 0.01) influencing upstream movements of this species. Further development of hardware and software will be necessary to improve performance of the counter, particularly in Mediterranean rivers, where more turbid waters and a greater proportion of small-size species are present.

Eurasian otter (Lutra lutra) diet and prey selection in Mediterranean streams invaded by centrarchid fishes

The diet of the Iberian otter (Lutra lutra) was determined by analysing 547 spraints collected at 28 sites within a wide area invaded by centrarchid fishes (pumpkinseed sunfish, Lepomis gibbosus and largemouth bass, Micropterus salmoides): the middle Guadiana basin (South-west Iberian Peninsula). Fish was the otters’ main prey, representing more than 60% of total individuals and more than 80% of total biomass. Otters preyed on most of the fish species captured in the field; however, the consumption of centrarchids was low compared to their abundance in the streams, and Jacobs’ index of preference showed a clear rejection of both species by the otter. Consumption of native fish genera (Squalius, Barbus and Chondrostoma) by otters increased in relation to their increase in the environment. In contrast, increasing numbers of L. gibbosus in the field was not reflected in otter consumption. The general decline of native freshwater fishes in Iberian rivers, the preferred prey of otters, together with the spread of exotic fish species (centrarchids and others) could put otter populations at risk.

On the spread of the European catfish (Silurus glanis) in the Iberian Peninsula: first record in the Llobregat river basin

On the spread of the European catfish (Silurus glanis) in the Iberian Peninsula: first record in the Llobregat river basin The first record of the European catfish (Silurus glanis L. 1758) introduced to the Llobregat river basin (NE Spain) is reported. We captured one individual of this silurid fish species (of a total of 541 fish) in La Baells reservoir on 30 August 2006. Given the low catchability of this fish species, the popularity among some anglers, and old rumours on this introduction, we hypothesize that this species has been present in the reservoir since a few years ago, despite we did not capture it in two previous surveys. The illegal introduction of this and other exotic species to other Iberian river basins should be prevented by the Spanish administration.

Environmental and geographical constraints on common swift and barn swallow spring arrival patterns throughout the Iberian Peninsula

AbstractAim Still poorly understood, the main migratory pathways for most trans‐Saharan species pass through the Iberian Peninsula, which acts as a gateway to the European–African migratory system. Arrival patterns in this region for the common swift (Apus apus) and barn swallow (Hirundo rustica), of similar morphology and flight capabilities, were described, and the environmental and geographical factors best explaining them were examined, in a search for common ecological constraints on these two migratory species.Location Latitude ranged from 36.02 to 43.68°N, longitude from 9.05°W to 3.17°E, and altitude from 0 to 1595 m a.s.l. for 482 common swift and 812 barn swallow Spanish localities spread widely over the Iberian breeding grounds of the two species.Methods Our data set, covering the years 1960–1990, consisted of 3206 first‐arrival dates for common swifts and 6036 for barn swallows. Forty topographical, climatic, river basin, geographical and spatial variables were used as explanatory variables in general regression models (GRMs). GRMs included polynomial terms up to cubic functions in all variables when they were significant. A backward stepwise selection procedure was applied in all models until only significant terms remained. GRMs were applied in two steps. First, we searched for the best model in each one of the five types of variables (topographical, climatic, river basin, geographical and spatial). To cope with the unavoidable correlation between explanatory variables, the relative importance of each type of variable was assessed by hierarchical variance partitioning. Secondly, we searched for that model able to explain the maximum amount of the observed variability in arrival date. To obtain this model all significant explanatory variables were subjected jointly to a GRM. Spatial variables were then added to this model to take any remaining spatial structure in the data into account. Moran's I autocorrelation coefficient was used to check for spatial autocorrelation.Results Both species arrived earlier in the south‐western Iberian Peninsula, where summers are warmer and drier. From there, both species followed the main southern Iberian river basins towards the north‐east; however, several mountainous regions impede the colonization of eastern Iberia. The best models for each type of variable explained 19–47% of the variability in common swift arrival dates and 14–44% in barn swallow arrival dates. Variance partitioning indicated that climatic and geographical variables best explained variability. The best predictive models built with all variables accounted for 52% of the variability in common swift arrival dates and 50% for the barn swallow. Residuals from both models were not spatially autocorrelated, an indication that all major spatially structured variation had been accounted for.Main conclusions Spring arrival patterns are highly dependent on the geographical configuration of the Iberian Peninsula. This spatial constraint forces both species to converge very closely in their spring migration, because common swifts and barn swallows are subject to a trade‐off between optimum migratory pathways and territories ecologically suitable for breeding.

Comparative ecology of the European eel, Anguilla anguilla (L., 1758), in a large Iberian river

A total of 1,816 eels were sampled in 1988, from seven sampling areas. Four areas were located in brackish water and the remaining three were located in freshwater reaches of the Tagus river basin. Eels were more abundant in the middle estuary and decreased both in the upstream and in the downstream directions, with a predominance of males in higher density areas. Smaller individuals preferred more peripheral areas, such as margins and upper reaches in the brackish water zone, and the tributaries of the freshwater habitats. It was assumed that this distribution pattern resulted from three main factors: (i) the dominance of larger specimens; (ii) the need to avoid predators and; (iii) the search for better trophic conditions. The condition of the individuals generally decreased toward the upper reaches, apparently due to a corresponding decrease in feeding intensity. The presence of the Belver dam in the main river, 158 km upstream from the sea, seemed to impose major alterations to the described patterns. The concentration of specimens below this impassable obstacle yielded a reduction in the proportion of females and a decrease in the condition and survival of the eels, contributing to a reduction in the spawning success of this population. Suggestions to diminish the effects of the dam, and to preserve the fishery are also presented.

Threats to the conservation of biotic integrity in Iberian fluvial ecosystems

Fluvial ecosystems are highly modified by human activity and the species inhabiting them are more imperiled than their aerial or oceanic counterparts. In addition, freshwater biodiversity per surface unit is higher than terrestrial and marine biodiversity. But from a conservation point of view rivers and streams have received less attention than any other natural system. Impacts in water and habitat quality, flow regime and biotic interactions can be considered the main factors responsible for the lack of biotic integrity in Mediterranean rivers. Dams and impoundments alter rastically the physical habitat structure and the ecological unctioning of running waters. But at the same time also modify the flow regime. Biotic interactions are deeply altered by introduced invasive species, which are common and proliferate in eservoirs and other artificial lentic habitats. In this paper we have adapted this classification of the main human impacts on fluvial ecosystems to the Iberian situation summarising them in three groups: water pollution, damming, and introduction of invasive species. We discuss their effects on biotic integrity of running waters through some examples from some of our previous works. Pollution is treated in two alternative ways. Quantifying its importance in the drainage network of a large Mediterranean area in southern Spain, as well as analysing its effect on the structure of macroinvertebrate communities and otter (Lutra lutra) distribution. Invasive species are widespread in Iberian inland waters. We hereby present the spatial relationship between native fish communities and invasive centrarchids that populate the peninsula. Finally, the damming impacts are considered through the analysis of its role in the spread and establishment of the invasive species. All of them are just a few examples about how alterated is the natural composition and dynamics of Iberian rivers and streams, i.e. its biotic integrity. The challenge for the future lies in the efficient protection of the biodiversity of Iberian freshwaters in the face of increasing pressures on the aquatic resources. But the first step in preserving our rivers is the real recognition of the extent of the problem, particularly among scientists.

First record of the common bream (Abramis brama) introduced to the Iberian Peninsula

This is the first record of the common bream, Abramis brama (Linnaeus, 1758), introduced into the Iberian Peninsula. Eight individuals of this cyprinid fish species were captured (of a total of 978 fish) in the Boadella Reservoir (Catalonia, Spain) on August 18, 2004. This reservoir is only 14 km away from France, where the bream is native, and contains several exotic freshwater fish that are still not widespread in Spain. The further introduction of species and the illegal translocation of the bream by anglers to other Iberian river basins should be controlled by the Spanish administration.

Patterns of exotic and native plant species richness and cover along a semi-arid Iberian river and across its floodplain

Patterns of native and exotic species richness and cover were examined together with the composition of species assemblages along a typical Mediterranean river and across its floodplain. Major goals were to characterize species distribution regarding diverse environmental and habitat features, and to explore floodplain landforms relations using plant assemblages and invasion responses as biological indicators of aquatic–terrestrial connectivity. A floristic survey was undertaken during May and early June 1998 in six sites of a southern Iberian basin. Vegetation data were gathered from three transects in three landforms: inundated and frequently flooded banks (FF), infrequently flooded valley floors (IF), and rarely flooded higher floodplains (RF). We used 2×2 m plots to record each species present and visual estimates of percentage cover of each species. Cover of native and exotic species assemblages were also estimated for each plots, for the transects, and for the landforms. The total number of vascular plant species found was 287 of which only 26 were exotic. Despite the high environmental and geomorphological variability, the hierarchical classification showed that species composition varied across the floodplain, rather than along the river. The IF and RF landforms were the most species-rich, both for native and exotic species group, whereas the cover trends of exotic and native plant species were opposed, with exotics being consistently more abundant in FF, and native cover increasing towards RF landforms. We found positive correlations between native and exotic species richness on the floodplain scale, and on a smaller scale in FF and IF landforms, thus supporting the hypothesis that in aquatic and riparian ecosystems of semi-arid landscapes, species-rich communities are preferentially colonized by exotic species. However, this pattern was not consistently coupled with a high cover of exotic species, and it was not sustained for the dry areas of the RF landforms.

Assessing biotic integrity in Iberian rivers: Development of a multimetric plant index

Aquatic and riparian plant species were examined in order to propose a multimetric plant index to assess the biotic integrity in Iberian rivers. An additional analysis with two other plant indices was performed to evaluate the present approach. A floristic survey was undertaken during May and early June 2000 in southern Iberian rivers. After the identification of two vegetation types, a partial canonical correspondence analysis of the floristic data with anthropogenic variables permited the recognition of the least perturbed sites, using the environmental characteristics as co-variables to account for natural site variability. From the 22 candidate-metrics, 8 were selected for inclusion on the final multimetric index: 3 representing composition measures (number of riparian and aquatic species, and number of annual species), 3 reflecting direct human-disturbances (abundance of Arundo donax , number of exotic, ruderal species), 1 trophic measure (number of nitrophyllous species) and 1 related with the riparian integrity (relative riparian width). Quality outputs were correlated with a global disturbance measure in order to evaluate the index performance and compared with the results of other plant indices. The Iberian multimetric plant index (IMPI) displayed a promising ability to relate riparian vegetation features to the current range of global disturbance. The IMPI will need to be further validated, with an enlarged set of metrics and a wider geographical area with clearly unimpaired reference sites in each vegetation type.

A simple method for assessing minimum flows in regulated rivers: the case of sea lamprey reproduction

Abstract The sea lamprey (Petromyzon marinus) is an anadromous species that occurs in some large Iberian rivers, such as the Tagus, and their tributaries. The sea lamprey populations of the River Tagus are threatened by habitat loss and overexploitation, which means that conservation measures, including maintenance of the spawning grounds, are urgent. Habitat loss is mainly due to hydroelectric and flood‐control dams that have affected the migration and spawning habitats of anadromous species. The main objective of this study was to use a simple, cost‐effective method to assess a minimum instream flow for the regulated River Tagus. The method used was based on the wetted perimeter–discharge relationship, but accounted for the habitat requirements of spawning sea lampreys. In 1998, a habitat survey was conducted along a 30 km segment of the River Tagus downstream from Belver Dam (the first impassable obstacle for fish movements), a stretch that includes potential habitats for spawning sea lampreys. Four spawning areas were identified, and in each one a representative reach was selected along which to collect elevation and geo‐positional data. For a range of flows from 0 to 50 m3 s−1, hydraulic modelling and studies of spawning habitat availability were conducted, both for each reach and for the total wetted area of the four reaches. The availability of spawning habitat was limited by flow. Results showed that the present minimum flow downstream from Belver Dam (8 m3 s−1) is insufficient to ensure suitable habitat conditions in a significant part of the segment studied; gains in wetted usable area were more substantial with discharges of up to 20 m3 s−1, and thus a minimum flow of 10–20 m3 s−1 was recommended. Potential spawning habitat could be improved by increasing instream flow to at least 50 m3 s−1. The impacts of the Belver release regime on anadromous species are discussed and a more ‘ecologically oriented’ form of water management is suggested . Copyright © 2004 John Wiley &amp; Sons, Ltd.

Fish fauna in Iberian Mediterranean river basins: biodiversity, introduced species and damming impacts

Abstract A basin approximation was used to analyse distribution patterns of different components of biodiversity (taxonomic richness, endemicity, taxonomic singularity, rarity) and conservation status of freshwater fish fauna in 27 Mediterranean Iberian rivers. Basin area alone explained more than 80% of variation in native species richness. Larger basins featured not only a higher number of native species, but also more endemic and rare species and fewer diversified genera than smaller basins. In contrast, smaller basins scored higher community conservation values, owing to their lower degree of invasion by introduced species. The presence of dams was the most important factor determining the conservation status of fish communities, and it was also positively associated with the number of introduced species. While the most important components of Iberian freshwater fish biodiversity are located in large basins, small unregulated basins feature better conserved fish communities. Copyright © 2004 John Wiley &amp; Sons, Ltd.

North Atlantic oscillation influence on precipitation, river flow and water resources in the Iberian Peninsula

AbstractThe Iberian Peninsula precipitation and river flow regimes are characterized by large values of inter‐annual variability, with large disparities between wet and dry years, especially in southern Iberia. This situation is a major problem for water resources management in general, and for the production of hydroelectricity in particular. Hydroelectric production represents, in an average year of precipitation, 20% of the total Spanish electricity production (and 35% for Portuguese production). Its absolute value, however, can vary by a factor of three between wet and dry years. We have assessed the impact of the North Atlantic oscillation (NAO) on winter precipitation and river flow regimes for the three main international Iberian river basins, namely the Douro (north), the Tejo (centre) and the Guadiana (south). Results show that the large inter‐annual variability in the flows of these three rivers is largely modulated by the NAO phenomenon. Throughout most of the 20th century, the January‐to‐March river flow is better correlated with the December to February (DJF) NAO index than is the simultaneous (DJF) river flow. Correlation values for the period 1973–98 are highly significant (−0.76 for Douro, −0.77 for Tejo and −0.79 for Guadiana), being consistently of higher magnitude than those obtained over previous decades. This impact of the NAO on winter river flow was quantified in terms of total Spanish potential hydroelectricity production. The important control exerted by the NAO and the recent positive trend in the NAO index contribute to a significant decrease in the available flow. This reduction represents an important hazard for the two Iberian economies because of its negative impact on water‐dependent resources, such as intensive agriculture and hydroelectric power production. Copyright © 2004 Royal Meteorological Society

Assessing reference sites and ecological quality of river plant assemblages from an Iberian basin using a multivariate approach

This study attempts to develop and discuss the applicability of a method for using river plant assemblages as indicators of ecological quality in a southern Iberian basin, located in Portugal. Several relevant aspects contribute to make this purpose scientifically challenging. Two thirds of the basin are located in Spanish territory, a large part of the river system is intermittent, the historical reference data on river plants in the basin are non-existent and the distribution patterns of Iberian river plants are scarcely known. A survey of 40 sites was conducted in late spring 1999, covering all the basin. Vegetation data were treated by hierarchical classification and by canonical correspondence analysis, using both environmental (19) and anthropogenic (9) variables. Three plant ecotypes were identified, having different geological and geographical backgrounds. For the selection of reference sites of each ecotype, the environmental variables significantly explaining within-ecotype species distribution were determined by a forward selection procedure and latterly used as co-variables. A partial canonical correspondence analysis was then performed for each ecotype, using a unique variable for the anthropogenic influence, henceforth, leading to a sequence of less human disturbed sites being opposed to the more disturbed ones along the first canonical axis. A cluster analysis for the group of sites of each ecotype contributed to the final distinction of the reference sites, still presenting some intrinsic natural floral variability. The Bray-Curtis dissimilarity within the group of sites from each ecotype ranged between a natural similarity variation and a maximum dissimilarity (a community within the same ecotype but with a plant composition completely different from the floral reference). This range was partitioned into four classes of floristic quality corresponding to an increasing deviation from the pro-natural floristic state. The use of river plants as indicators of river integrity and the method used are discussed, considering the particular features of the river system studied and its plant assemblages.

Role of transient silicon limitation in the development of cyanobacteria blooms in the Guadiana estuary, south-western Iberia

The Guadiana estuary, located between Portugal and Spain, has the fourth largest drainage basin of Iberian river systems. Up to 75% of the catchment area has been regulated by dams since the early 1970s. During the 1980s and 1990s, an increasing occurrence of summer cyanobacte- ria blooms dominated by the potentially toxic Microcystis spp. was reported. In an effort to under- stand the causes of recurrent noxious blooms in the Guadiana estuary, nutrients (phosphorus, nitro- gen (nitrate, nitrite and ammonium), and silicon (silicic acid), chlorophyll a and phytoplankton abundance, specific composition and biomass were evaluated during field surveys, from April 1997 to March 1998. A pattern of successive blooms of different phytoplankton assemblages was observed throughout this period. Diatoms (nano-sized, chain-forming) dominated an early spring bloom (max. 18 × 10 3 cells ml -1 ) in March and April. Following the decrease in diatom abundance, a chlorophyte bloom (max. 11 × 10 3 cells ml -1 ) and then a cyanobacteria bloom (>6 × 10 4 cells ml -1 ) quickly followed during late spring through to early summer. From July to September, a major cyanobacteria bloom dominated by the potentially toxic Microcystis spp. (> 4 × 10 5 cells ml -1 ) developed in the freshwater zone. The results indicate that high winter loads of nitrogen and phosphorus led to the depletion of silicate (down to as low as 0.2 µM) during the early spring diatom bloom, which conditioned the suc- cessive phytoplankton assemblages during the remaining productive period. Low monthly discharge rates during spring and summer further provided an environment with low Si:N and N:P relative availability which, coupled with high water-column temperature (>21°C), seemed to favour the dominance of cyanobacteria over chlorophytes during the summer.

Seasonal and yearly variations of macrophytes in a southern Iberian river

Seasonal and yearly variations of macrophytes in a southern Iberian river

The influence of fish abundance on the otter (Lutra lutra) populations in Iberian Mediterranean habitats

AbstractRecent studies show that the availability of prey influences the ecology of otters by regulating breeding, carrying capacity and mortality. Mediterranean habitats experience extreme seasonal variation in water flow – a stress period can occur in summer when water flows and levels are low with long periods of drought, while torrential floods usually occur in autumn and spring. Reservoirs can affect this situation by further influencing water flow regimes and acting as a barrier. Our data show the close relationship between the amount of food (fish) available to otters and their use of a stretch of water and the otter density there. The adjustment of the number of individuals (and carrying capacity) in an area can be seen from their mortality, a decrease in their reproductive success, or migration. Our results are consistent with the hypothesis that otters are food‐limited. Torrential rains, summer droughts and dams for water management have repercussions for otter populations in as much as they affect their prey‐species. Barbels Barbus spp. and, secondarily, nases Chondrostoma spp. are shown to be important to otter ecology and breeding in Mediterranean Iberian rivers. The conservation and management of the otter in these habitats must not ignore the management of food sources. In order not to affect otter populations, the effect of the reservoirs and their dams also must be considered. The release and cessation of floodwater from reservoirs must be progressive and mimic more closely the flow regime of natural rivers.

Exotic and native vegetation establishment following channelization of a western Iberian river

Channelization is often a major cause of human impacts on river systems. It affects both hydrogeomorphic features and habitat characteristics and potentially impacts riverine flora and fauna. Human-disturbed fluvial ecosystems also appear to be particularly vulnerable to exotic plant establishment. Following a 12-year recovery period, the distribution, composition and cover of both exotic and native plant species were studied along a Portuguese lowland river segment, which had been subjected to resectioning, straightening and two-stage bank reinforcement, and were compared with those of a nearby, less impacted segment. The species distribution was also related to environmental data. Species richness and floristic composition in the channelized river segment were found to be similar to those at the more ‘natural’ river sites. Floral differences were primarily consistent with the dominance of cover by certain species. However, there were significant differences in exotic and native species richness and cover between the ‘natural’ corridor and the channelized segment, which was more susceptible to invasion by exotic perennial taxa, such as Eryngium pandanifolium, Paspalum paspalodes, Tradescantia fluminensis and Acacia dealbata. Factorial and canonical correspondence analyses revealed considerable patchiness in the distribution of species assemblages. The latter were associated with small differences in substrate composition and their own relative position across the banks and along the river segments in question. Data was also subjected to an unweighted pair-group arithmetic average clustering, and the Indicator Value methodology was applied to selected cluster noda in order to obtain significant indicator species. Copyright © 2001 John Wiley & Sons, Ltd.