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Metacommunity structure depends on environmental and spatial factors. Stream fishes are constrained to disperse within dendritic networks and waterfalls and other barriers add resistance to species dispersal. We evaluated the importance of environmental and spatial effects on fish metacommunity composition, adopting different models of spatial distance: overland, watercourse, resistance (physical barriers), cost distance (watercourse resistance), and asymmetrical dispersal (considering flow direction). Fishes were sampled in 31 riffle sites of a small subtropical basin of southern Brazil (~ 250 km2 effective study area). Using redundancy analysis (RDA) and variance partitioning, we found the best model was the one including flow direction weighted by watercourse distance plus environmental variables (overall adj-R2 = 0.42). However, the explanation of fish composition was greatly improved by weighting spatial distance by stream channel sinuosity or height of barriers. Reach slope and width were the two significant environmental variables. Our study supports that dispersal, in addition to environmental factors, affects stream fish metacommunity even in a watershed of small spatial extent. Detecting spatial effects in a small watershed depends on considering realistic watershed features that affect fish dispersal in drainage networks such as waterfalls and other physical barriers, and distance along the watercourse and flow direction.

This study explored the interactions of phytoplankton species during the invasion of Ceratium furcoides and the environmental variables that contributed to its establishment and ecological success in a shallow eutrophic reservoir (Garças Reservoir, southeast Brazil), which has been monitored monthly for 20 years (1997–2017). The Ceratium furcoides invasion in September 2014 was preceded by disturbance events (macrophyte removal and a historical drought period), which disrupted the dominance of cyanobacteria by modifying resource availability (high water transparency and soluble reactive phosphorus concentrations) and recruiting other species. Ceratium blooms at the water surface were preceded by high abundance near the bottom, suggesting the importance of the propagule bank. However, the pattern of Ceratium-Microcystis coexistence that is usually recorded in temperate lakes was not observed. Instead, Ceratium replaced Cylindrospermopsis raciborskii in mixing periods with high light and nitrogen availabilities, significantly influencing the abundance of Trachelomonas spp. Flagellated forms became dominant in the Garças Reservoir, due to the higher water transparency and relatively lower water-column stability, and alternative states between Ceratium-Trachelomonas in mixing periods and Microcystis-Cryptomonas in stratified periods have been repeated. Since then, cyanobacterial dominance ceased, and the “skillful” Ceratium apparently has come to stay, influencing interactions among phytoplankton species.

The stream slope influences the diversity of stream fish, as it is an important riverscape characteristic related to flow energy, substrate size, and channel morphology. These conditions limit the movement and local persistence of species across the network, affecting local and regional species composition. Therefore, we can predict that streams with steep channels and harsh flow conditions would select a restricted set of species and ecomorphological traits, whereas those with gentle slopes would allow more species to coexist. This would cause a nested pattern in which the species and trait composition of streams with steep slopes (e.g., high swimming capacity) are a subset of those with gentle slopes (e.g., varied swimming capacities). We evaluated the influence of stream slope on a fish metacommunity, investigating the response of trait-based diversity to the stream slope for 21 fish assemblages sampled in two subtropical watersheds of Brazil. We found that trait-based alpha diversity was negatively related to slope. Moreover, stream sites with steep channels showed a subset of species and traits present in streams with gentle slopes. Finally, the spatial scale at which slope was measured had different effects on fish diversity, and the strongest effect was observed when the slope was estimated using the entire channel. Thus, we suggest caution in choosing the spatial scale, as the local slope may not be a good representation of the riverscape characteristics that drive the taxonomic and trait diversity of local fish. The stream slope is an important driver of fish diversity in streams by filtering species traits and determining species occurrence, while nestedness is a relevant pattern emerging from differences in stream gradient among sites.

Loss of native vegetation cover in the watershed is known to affect stream fish communities. However, community response may not occur immediately after land cover change, and may become apparent only years later. In this study, we investigated how a c.a. 30-year (1985–2017) historical trajectory of native vegetation loss affected fish communities in 47 streams of southern Brazilian grasslands. We tested whether current species richness and composition would be more related to past or to recent natural vegetation cover. We then defined trajectories of vegetation loss and tested whether different attributes (frequency, magnitude and duration of disturbance) affected particular species and functional groups. We found that current species richness and composition were more related to past than to current vegetation cover. However, we also observed that responses of particular species and functional groups were dependent on specific trajectory attributes. We concluded that present fish community characteristics may respond not only to past watershed conditions but also to how watershed conditions have changed over the time. Exploring land use trajectories seems a promising approach for improving our understanding of landscape effects on stream ecosystems, as well as the predictability of present landscape management and planning on freshwater communities.

Channel slope is an important variable in lotic systems because it mediates flow. However, stream slope can be measured at various spatial scales and the relation with fish assemblage characteristics may be scale dependent. In this study we investigated the influence of stream slope measured at different spatial scales on fish species richness. We tested three hypotheses: 1) the relation between slope and species richness will be dependent on the spatial scale at which slope is measured; 2) species richness will be inversely related to mean stream slope; 3) species richness will be positively related to stream slope variability. We sampled riffle fish assemblages in 21 streams in southern Brazil. For each stream we measured slope at five different scales and calculated mean slope and slope variation. The influence of slope on local species richness was tested by simple regression analysis using rarefied richness as response variable. We found that the relation between slope and species richness was scale-dependent, because only slope at the whole stream channel was significantly related to species richness. We also observed that steeper streams tended to have less slope variability and lower species richness. These results suggest that processes occurring at the whole stream scale are determinant for local fish species richness. More importantly, they reinforce the need to measure slope at different scales when investigating fish-habitat relations in streams, because the influence of slope may not be detected if assessed at only one scale. We suggest that assessing mean slope and slope variability at different scales should be considered in sampling design of research and conservation aimed at stream fishes.

Eutrophication is a key threat to aquatic biodiversity around the world, but especially in the Cerrado biome that has undergone intensive land use conversion and fertilizer use. In this study, we investigated how water conditions and different taxonomic and functional indicators of phytoplankton communities responded to eutrophication over time and also identified which of these variables can act as early indicators of eutrophication. We used a mesocosm experiment to simulate shallow lakes and added nutrients to produce eutrophic and oligo-mesotrophic environments. We found that conductivity and turbidity increased with the nutrient enrichment process. Eutrophication did not change species richness; however, it increased the total phytoplankton density and chlorophyll-a concentration and decreased the diversity indices (Shannon–Wiener diversity and Pielou’s equitability) over time, for both taxonomic and functional facets. Our results indicate that eutrophication may not have marked effects on the composition of the initially dominant organisms in the short term but does increase the density of certain organisms and exclude rare species and functional groups in eutrophic environments along the time. Furthermore, the diversity indices are more sensitive indicators of eutrophication compared to abiotic indicators and composition of species and functional groups. Therefore, they may be considered reliable early warnings of ecosystem changes.

The reproductive tactics of Deuterodon stigmaturus were investigated in this study, testing whether the species presents a seasonal or opportunistic life-history strategy in a high latitude Atlantic Forest stream in Brazil. The relation between the development of anal-fin spinelets (sexually dimorphic structures in characids) and the reproductive period of D. stigmaturus was also investigated. The species displayed a seasonal life-history strategy, with the reproductive period occurring between austral spring and summer. The reproductive period was related to increase in temperature and photoperiod, corroborating several studies that suggest positive relation between seasonal reproduction and abiotic predictability. The data indicated total spawning occurring in January, coinciding with the month of greatest rainfall (historical and in the study year). However, the observation of postovulatory follicles in previous months suggests small spawning before January. The development of spinelets on the anal-fin was the only sexual dimorphic trait between males and females. Contrasting with several other studies, seasonal development of anal-fin spinelets was observed in adult males, following the reproductive period and regressing afterwards. Thus, some characid species probably retain this structure throughout the year and others do not, which may be related to life-history strategy differences.

In communities or regions where non-native fish species still do not predominate, changes in the assemblage composition are driven by loss, gain or substitution by native species only. We investigated the native invasion hypothesis in small streams, in which human modifications may influence fish assemblage composition by boosting the expansion and establishment of widespread species, as well as of species commonly found in large streams and rivers. Fish community data from 54 lowland streams from South Brazil were used to investigate this hypothesis. We found a positive relationship between cropland cover at the catchment scale and the dominance of fish species that commonly inhabit large streams or rivers (inferred on museum records). We also observed a weak and negative relationship between site elevation and the percentage of widely distributed species in fish assemblages. Our results partly support the hypothesis of native invasion in lowland streams, but the low explanatory power of the models suggests that it is less pronounced compared to highland streams. Our results contribute to understand inconsistencies among studies on the effects of land use on stream fish assemblages using traditional metrics (alpha and beta diversities). For instance, land use can initially increase fish species richness in small streams by favouring the occurrence and establishment of fish species common to rivers. In this sense, alternative metrics that consider specific changes in native species distribution, such as proliferation of common species, should be used to better assess the mechanisms that drive changes in communities of aquatic ecosystems.

Although longitudinal position is recognized as a major predictor of fish assemblage structure in stream ecosystems, how mesohabitat type interacts with longitudinal position to drive different spatial and temporal patterns in fish assemblages is still unknown. We investigated adjacent pool and riffle mesohabitats in a paired sampling design along the longitudinal gradient of a coastal subtropical drainage in Brazil. We tested whether the structure of fish assemblages of contrasting mesohabitat types respond similarly to position along the longitudinal riverine axis. Mesohabitat type by itself was by far the most important predictor of fish assemblage composition in multivariate models. Nektonic characid species were predominant in pools while benthic loricariids were most abundant in riffles. However, the interaction between longitudinal position and mesohabitat explained an additional significant proportion of the variation in fish assemblage composition. Changes in species composition with longitudinal position differed between riffles and pools. Models for predicting descriptors of assemblage structure revealed that fish abundance and spatial beta diversity responded strongly to the interaction between mesohabitat type and longitudinal position. Specifically, downstream fish abundance increased more for pools than for riffles. Spatial beta diversity showed opposite trends between pools and riffles along the longitudinal gradient, indicating that the combination and strength of niche and dispersal processes are different between these mesohabitat types. In summary, pool and riffle mesohabitats maintain fish assemblages that are taxonomically and functionally different and that respond differently to longitudinal position. Considering pool and riffle assemblages separately in modeling fish assemblage patterns can contribute greatly to our understanding of human modifications and other mechanisms driving spatial and temporal variation of fish assemblages within stream networks.

Neste trabalho são apresentados dados referentes a um inventário da fauna de peixes da Floresta Nacional de Canela, uma Unidade de Conservação Federal localizada no trecho superior da bacia hidrográfica do Rio Caí, em elevadas altitudes da Serra Geral do Rio Grande do Sul. Foram amostrados nove pontos de coleta em diversos ambientes aquáticos, incluindo um banhado, açudes e arroios, situados tanto no interior quanto no entorno da unidade. Para a captura dos exemplares utilizou-se puçá, picaré, linhas de pesca, redes de emalhar e armadilhas do tipo covo. Ao total, foram encontradas 20 espécies pertencentes a 10 famílias e 6 ordens. Hyphessobrycon luetkenii teve distribuição mais ampla, ocorrendo em quase todos os pontos amostrados, enquanto Callichthys callichthys foi registrado somente em um ponto. Nos açudes, a ordem Perciformes foi a mais representativa, seguida por Cypriniformes e Characiformes. Esses ambientes caracterizaram-se pela predominância da família Cichlidae (Perciformes), cuja maioria das espécies Neotropicais tem preferência por ambientes de água parada. Com distribuição restrita aos açudes, destaca-se a ocorrência de 4 espécies exóticas introduzidas (Ctenopharingodon idella, Cyprinus carpio, Hypophthalmichthys molitrix e Micropterus salmoides). Para os arroios, foi observado maior riqueza específica de Characiformes e Siluriformes de pequeno porte, padrão já encontrado em muitos estudos nos riachos sul-americanos. Destaca-se a presença de espécies com \"status\" taxonômico indefinido, tais como Trichomycterus sp. e Astyanax aff. scabripinnis, reforçando a importância dos levantamentos em riachos nas cabeceiras de rios do Rio Grande do Sul, para a obtenção de informações que subsidiem medidas de conservação desses ambientes. Embora as espécies exóticas tenham sido registradas apenas em açudes, recomenda-se a erradicação destas da unidade de conservação, em função das ameaças que podem representar à ictiofauna nativa e aos demais ecossistemas aquáticos da região.