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The construction of large dams has changed natural hydrology of many rivers in South America. Considering that little is known about the reorganisation of nonmigratory fish assemblages following an impoundment, we investigated changes caused by the construction of Lajeado Dam on the spatial distribution of nonmigratory fish along the Tocantins River, a large Amazonian river. In particular, we investigated changes in distance decay of community similarity (initial similarity and halving distance) in periods that preceded and followed the construction of the dam. For this study, we analysed data collected over a four‐year period, including seven sites distributed along the Tocantins River (~270 km). Initial similarity showed high values and little variation over the years. Halving distance, on the other hand, changed considerably after impoundment, showing progressive smaller values. In the first 2 years after impoundment, halving distance decreased by more than half of the initial value, indicating changes in distance decay relationships. An ordination analysis nonmetric multidimensional scaling (NMDS) indicated substantial changes in assemblage structure between pre‐ and postimpoundment periods, especially in sites close to the dam. In addition, after river regulation, we recorded shifts in species abundance across sites, while numerical dominance increased and local richness decreased in sites near the dam. Our results indicate that the dam changed diversity gradients along the river corridor, increasing the distance decay of assemblage similarity. All these findings evidence that nonmigratory fish assemblages are particularly vulnerable to river regulation.

In this work, spatial and temporal variations in the diversity (species richness and Simpson's Diversity Index) and abundance (indexed by the capture per unit effort -CPUE; total and for reproductive groups) of fish from three rivers (Baía, Ivinheima and Paraná) located in a floodplain of the Upper Paraná River basin were analyzed over a period of 20 years . In addition, we evaluated the relationships of these ecological attributes with variations in the hydrologic regime, considering the possible effects of natural (climatic events) and artificial (discharge control by dams) disturbances. Annual variations in hydrometric attributes were calculated using PULSO software and daily water level data. We applied analysis of covariance to determine the relationships between ecological and hydrometric attributes, the latter summarized in axes of a principal component analysis. Lower values of the fish assemblage attributes (diversity and abundance) were registered in the Paraná River. Species richness, total CPUE and CPUE of long-distance migratory species were positively related to the duration of the floods and the connectivity of the area. Variations in the annual hydrological cycle and their effects on fish assemblage appear to be affected by extreme natural (ENSO) and artificial (discharge control by dams) events.Keywords: floods, recruitment, abundance, fish diversity, fish assemblage. Efeitos do regime hidrológico sobre a ictiofauna de ambientes fluviais da planície de inundação do Alto Rio Paraná ResumoNesse trabalho foi avaliada a variação espaço-temporal na diversidade (riqueza de espécies e Índice de Diversidade de Simpson) e abundância (indexada pela captura por unidade de esforço -CPUE; total e por grupo reprodutivo) de espécies de peixes de três rios (Baía, Ivinheima e Paraná), localizados na planície de inundação do Alto Rio Paraná, ao longo de 20 anos . Ainda, avaliamos as relações desses atributos ecológicos (diversidade e abundân-cia) à variação anual do regime hidrológico, considerando a influência de distúrbios naturais (eventos climáticos) e artificiais (regulação hidrológica por barragens). Para representar a variação anual do regime hidrológico de cada rio, foram utilizados os atributos hidrométricos calculados pelo programa PULSO, a partir de medidas do nível hidromé-trico diário. Análise de covariância foi aplicada para avaliar a relação entre os atributos ecológicos das assembléias de peixes e a variação no regime hidrológico, esta sumarizada em eixos de uma análise de componentes principais. A riqueza de espécies, a CPUE total e a de migradores de longa distância foram relacionadas positivamente à duração das cheias e conectividade, sendo que o Rio Paraná apresentou os menores valores médios desses atributos. Dessa forma, as variações constatadas no ciclo hidrológico anual e seus reflexos sobre a estrutura das assembléias de peixes do rio Paraná parecem decorrer do efeito combinado de eventos naturais (El Niño -ENSO) e artificiais (controle da vazão pelas barragens).Palavras-chave: ch...

Summary Riverine metacommunities have been studied mainly in streams and headwaters, and little is known about how dendritic networks affect metacommunity structure in large river systems. We, therefore, examined fish assemblages in the Upper Tocantins River, Amazon Basin, to investigate the hypothesis that riverine networks constrain metacommunity structure. In particular, we investigated (i) taxonomic and functional β‐diversity along the river channel (main stem) and major tributaries (branches); (ii) the relative importance of the turnover and nestedness components; and (iii) the relationship between β‐diversity and spatial structure (the distance decay of similarity). Fish assemblages were sampled over a year (2007–2008) at 17 sites distributed along the main stem (n = 8) and branches (n = 9). We recorded 170 fish species in the 100‐km river section studied. Local species richness ranged from 45 to 79, with higher mean values in the main stem; local assemblages, however, showed high functional richness, with no significant difference between the main stem and branches. We observed that the turnover component contributed 87.7% of the variation in taxonomic β‐diversity (Jaccard), while the nestedness‐resultant contributed 12.3%. Taxonomic β‐diversity was on average 2.5 times higher than functional diversity, and taxonomic turnover was on average four times greater than functional turnover. In addition, a null model showed that observed values of functional β‐diversity were not different from random expectations, given the observed level of taxonomic β‐diversity. Branches showed higher species turnover, while significant distance decay of similarity was observed only along the main stem. Functional β‐diversity was not different between the main stem and branches and did not correlate with watercourse distance. Our study revealed that fish metacommunity in this large river shows species turnover, but with high functional similarity. In addition, different β‐diversity patterns between the main stem and branches indicate that the dendritic network constrained metacommunity structure. Mass effects and dispersal limitations probably explain β‐diversity patterns in the main stem, while a combination of mechanisms (i.e., disturbance, specific biological functions and dispersal constraints) affect diversity in the branches.

Abstract. This paper presents the structure and application of a regional scale operational modelling tool for the West Iberian coast, and discusses its potential for products and services for both scientific and coastal management activities. The forecasting suite includes nested hydrodynamic models forced with up-to-date meteorological forecast data and large-scale model results. The present status of the system and its recent upgrades are reviewed, offering a general description of the main components of the system: the forcing data, the circulation model, the model outputs and the validation methodology of model results. Seasonal differences in temperature, salinity and current velocity fields are illustrated and show satisfactory reproduction of the top and deep layer thermodynamics. The system provides boundary forcing for a number of local-scale model applications via downscaling of the solution and enables potential products and services from which civil society will benefit.