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Habitat structure and predation are major factors that influence the distribution of zooplankton species and functional traits. Here, we analyzed how the structure provided by macrophytes (richness, Shannon’s diversity, and biomass) and potential predation by small-bodied fishes (richness and abundance) determine the richness of species and functional traits of zooplankton, as well as the spatial dissimilarity (beta-diversity) of species and traits. Zooplankton, fish, and macrophytes were simultaneously sampled across a gradient of 30 multi-species macrophyte beds. We assessed spatial patterns of zooplankton under taxonomic and functional approaches, using linear regression models, Generalized Dissimilarity Models, a Structural Equation Model, and a Fourth-Corner Analysis. Zooplankton taxonomic beta-diversity was most represented by the turnover component and zooplankton functional beta-diversity by nestedness. Zooplankton taxonomic richness and taxonomic beta-diversity were positively related to macrophyte biomass, richness, and Shannon’s diversity, whereas zooplankton functional richness and functional beta-diversity were positively related to fish richness and abundance. Macrophyte biomass and diversity oppositely influenced fish structure, which had influence on zooplankton structure. Macrophytes also negatively influenced the zooplankton traits such as body size, reproduction type, habitat, lifespan, and predatory escape response and positively influenced the trait feeding type. Fish were negatively related to the trait body size. The spatial structure generated by macrophyte beds and fish community determined the distribution of zooplankton species and functional traits.

The diversity of aquatic macrophytes can offer different local conditions required to support an increased number of microhabitats, therefore resulting in diverse biotic communities. This study assessed the influence of macrophyte diversity on the ecological attributes of the associated invertebrates (diversity, richness and abundance), as well as the diet composition of a small fish, Moenkhausia forestii, that inhabits the Upper Parana River floodplain in abundance, and present great contributions of invertebrates in its diet. The richness and diversity of invertebrates increased with increasing macrophyte diversity, while the abundance of invertebrates didn’t show a significant relationship. The diet of M. forestii differed among stands and the consumption of invertebrates increased with increasing macrophyte diversity, while the consumption of aquatic plants decreased. Moreover, the trophic niche breadth of M. forestii expanded, a probable result of the increase in the ecological opportunity promoted by higher macrophyte diversity. Our study emphasizes the importance of the diversity of these plants as they structure the environment and the associated communities, and changes in the attributes of these plants could be reflected on other trophic levels and even on a local scale.

Habitat complexity is recognized to mediate predator–prey relationships by offering refuge or not. We investigated the availability of planktonic microcrustaceans and the diet of a planktivorous fish (Hyphessobrycon eques) at different levels (low, intermediate and high) of aquatic macrophyte biomass. Sampling was carried out in a river with low flow speed, located in a Neotropical floodplain. We collected fish and microcrustaceans in macrophyte stands with variations in biomass. There were no differences in microcrustacean density in the water among the levels of macrophyte biomass, but microcrustacean richness and diet composition of H. eques differed. Microcrustacean richness and trophic niche breadth of the planktivorous fish were higher in high biomass stands. There was high consumption of a small cladoceran species in low macrophyte biomass, which was replaced by larger species, such as copepods, in intermediate and high biomass. Thus, the selection of some species was different among the biomass levels. These results suggest that plant biomass plays an important role in the interaction between fish and microcrustaceans, and prey characteristics such as size, escape ability and energy value make them more or less subject to predation by fish according to habitat structuring.

Populations of generalist fish can be composed of individuals specialised to a small subset of food resources, a process known as individual specialisation. In this study, we hypothesised that variation in the diet and morphology of individuals is related to habitat complexity and that greater levels of individual specialisation are found at intermediate levels of macrophyte biomass. Fish and aquatic plants were sampled in 15 multispecific macrophyte stands with different biomasses in the Baia River (Upper Parana River floodplain). We assessed habitat complexity using the biomass of aquatic macrophytes (three levels of classification: low, intermediate and high). The diet of Moenkhausia forestii was evaluated with the volumetric method. To assess individual diet specialisation, we used the IS (specialisation index) and NODF (nestedness based on overlap and decreasing fill) index and for individual morphological specialisation we used ecomorphological metrics. We tested for a correlation between diet and morphology with a Mantel test. Contrary to our hypothesis, IS and NODF values indicated greater individual specialisation in the diet of M. forestii from low macrophyte biomass. However, the greatest individual morphological specialisation was found in macrophyte stands with intermediate biomasses. The Mantel test showed that there was no significant correlation between diet and morphology. Thus, individual specialisation of M. forestii diet may be related to an increase in competition (likely intraspecific) in stands with low macrophyte biomass, while morphological specialisation is probably related to greater resources availability and accessibility within stands of intermediate macrophyte biomass.

Owning to dissimilarity with native macrophytes, invasive macrophytes may negatively affect the diet and foraging efficiency of fishes. We evaluated the invertebrate availability, diet composition, and foraging efficiency of four fish species that use macrophytes as habitat in a native and a highly invasive macrophyte. The samples were taken in a reservoir in Brazil (22° 33′ S, 52°54′ W). We collected invertebrates and fishes simultaneously in macrophyte stands dominated by the invasive Poaceae Urochloa arrecta and the native Eichhornia azurea. We evaluated the foraging efficiency by the total number of prey consumed and by the degree of stomach fullness. We used an approach based on resampling procedures to compare differences in composition of invertebrates, composition of fish diet, and the similarity between invertebrates and fish diet. The abundance of invertebrates and the diet composition differed significantly between plants. On the other hand, fish foraging efficiency did not differ between macrophytes. Despite this lack of difference of foraging efficiency, the lower abundance of food items in the invasive macrophyte and the significant changes in fish diet suggest that U. arrecta is not a favorable feeding habitat for fishes, compared to the native E. azurea.

Although it is widely known that dams can have large impacts on the environmental and biological characteristics of downstream rivers, there is a substantial lack of studies focusing on which ecological processes cause longitudinal changes in biological communities downstream of reservoirs. We investigated longitudinal patterns in the total beta diversity and its replacement and richness difference components for actively (fish) and passively (phytoplankton) dispersing biological groups. Our results, obtained from a 230 km sampling stretch, demonstrated the key role played by tributaries in the downstream direction from main river impoundment, which influenced local environmental conditions and beta diversity patterns of each biological group. Both replacement and richness difference contributed to high values of total beta diversity for fish (average = 0.77) and phytoplankton (average = 0.79), but their relative importance was more associated with the replacement component for both biological groups (average = 0.45 and 0.52, respectively). Moreover, we observed clear differences between fish and phytoplankton in beta diversity patterns operating at small and broad scales, as well as in the mechanisms driving each beta diversity component. Directional dispersal-related processes and environmental filtering played a major role in shaping total beta diversity and its components for fish, while temporal factors explained considerable parts of phytoplankton beta diversity. Our findings contributed to understanding of tributary-induced heterogeneity and highlight the importance of dam-free stretches of rivers for preserving the integrity of dammed river basins.

Invasive macrophytes usually increase the homogenisation of habitats and can affect local communities with negative effects on the food chain. We evaluated experimentally the composition, density, richness and diversity of the invertebrates in the invasive macrophyte Hydrilla verticillata and assessed the potential effects of this macrophyte on the diet and foraging efficiency of small native fishes. The native macrophyte Egeria najas was used as control, since it has very similar physical structure and physical complexity to H. verticillata. Macrophytes of both species were maintained separately in aquaria with the fishes, and the invertebrates richness and abundance associated with plants and within fish stomach were evaluated. No significant difference was found in the richness and diversity of invertebrates attached to macrophytes; however, the two macrophytes differed in regard to invertebrate composition, and the density was significantly higher in H. verticillata. The fish foraging efficiency was high in both macrophytes, and the difference between them was not significant. The fish diet did not differ significantly between the native and the invasive macrophyte in respect to invertebrate composition, abundance and richness, and trophic niche breadth. We found no evidence that H. verticillata negatively affects the density of associated invertebrates, and as a consequence, fish diet and foraging efficiency. Our results indicate that the replacement of E. najas by a similar biomass of H. verticillata does not affect the feeding behaviour of associated fish species.

Plants that colonize wet habitats may become temporarily submerged forming a new habitat for fish in the flood period. In this study, the amphibian macrophyte Polygonum punctatum was evaluated as a temporary habitat and feeding ground for fish (≤ 108 mm) during the flood period in a river–floodplain ecosystem. The fish were sampled in January and February of 2015 in a lake of the Upper Parana River floodplain, inside patches dominated by P. punctatum (flood-formed, temporary habitat) and Eichhornia azurea (permanent aquatic habitat, used as control). The abiotic data did not differ between the two plants, unlike their biomass, which was higher for E. azurea. Fish abundance did not differ between macrophytes but the composition and richness did, with more species registered in E. azurea. The foraging efficiency of fish was significantly lower in individuals using the amphibian macrophyte for most species. Differences in the diet composition were verified for half of the species, but the main items consumed were the same in both macrophytes, changing only the proportion. The niche breadth was similar in the patches of both macrophytes. The results suggest that during flood periods, the new habitat provided by the macrophyte P. punctatum was colonized by small fish that probably used this new habitat as a refuge and feeding ground. However, the lower foraging efficiency in this macrophyte suggests that it does not provide an amount of food resources comparable to the permanent habitat represented by E. azurea.

Most studies that address microplastic (MP) ingestion by fish are conducted in marine environments; however, freshwater ecosystems such as rivers and streams are also important sources of these particles in coastal areas. Considering that increasing urbanization surrounding watersheds increases the sources of plastic pollution and that fish feeding behavior may influence the probability of ingestion of these particles, the aim of this study was to evaluate the ingestion of MP by fish of different feeding habits in urbanized and non-urbanized streams. The fish were captured in ten streams in Southern Brazil and the stomach contents of 294 individuals belonging to 13 species were analyzed. Individuals of ten species ingested MP of fiber type. From a generalized linear mixed model, we observed that the urbanized streams and the omnivorous habit showed a positive correlation with MP intake. Our results suggest that both types of streams present MP, but this pollutant is probably more prominent in heavily urbanized sites, which may represent important sources of MP for larger systems along the river basin. This evidences the importance of preserving riparian areas of small order streams as a means to reduce MP inputs into these ecosystems.

This study tested the hypothesis that the flood pulse affects the diet composition and the niche breadth of Moenkhausia forestii, a small characid fish inhabiting the littoral zone of lakes. To this end, we compared the diet composition (at the population and individual levels) between hydrological periods (high and low water phases) in a floodplain lake of the Upper Parana River. PERMANOVA revealed differences in the diet between periods (pseudoF1,38 = 8.5; p