We compared the species richness and abundance of fish, zooplankton and phytoplankton in nine mesotrophic coastal shallow lakes (Northeastern Brazil) with and without the exotic predator cichlid tucunaré or 'peacock bass' (Cichla cf. ocellaris). We hypothesized that the introduction of tucunaré would lead to decreased abundance and species diversity of native fish assemblages and cause indirect effects on the abundance and species diversity of the existing communities of zooplankton and phytoplankton and on water transparency. Our hypotheses were only partly confirmed. Although fish richness and diversity were, in fact, drastically lower in the lakes hosting tucunaré, no significant differences were traced in total fish catch per unit of effort, zooplankton and phytoplankton biomass, plankton diversity or the zooplankton:phytoplankton biomass (TZOO:TPHYTO) ratio. However, zooplankton biomass and TZOO:TPHYTO tended to be higher and the phytoplankton biomass lower in lakes with tucunaré. Our analyses therefore suggest that the introduction of tucunaré had marked effect on the fish community structure and diversity in these shallow lakes, but only modest cascading effects on zooplankton and phytoplankton.

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ABSTRACT. Food fl exibility and omnivory are important features pronounced in Neotropical freshwater fi sh species, particularly for Astyanax species. Traditionally most fi sh diet resources are known to be originated in the aquatic environment, however recent studies have pointed to the importance of allochthonous sources. Besides, the colonization of macrophytes, common at several tropical reservoirs, may enhance insectivory in fi sh diet expanding or concentrating the area of available resources for feeding. Here we employed stomach contents analysis of two sympatric Astyanax species to access the importance of habitat diff erentiations as spatial complexity in two tropical reservoirs with diff erent environmental features. The NMDS analysis indicated separation in the diet of these species between reservoirs (Stress= 8.28%). Additionally, analysis of variance revealed a signifi cative diff erence in the volume of food itens ingested between the reservoirs (Anova one-way F(1, 132)=4.4446; p= 0.037). This points out the importance of environmental conditions on the composition of the diet of fi shes. This study highlighted the insectivorous feeding habit of Astyanax species and revealed diff erent feeding strategies between sympatric fi shes despite high niche overlap in both environments. Habitat heterogeneity increasing food resources availability plays an important role in the diet strategy of these Astyanax species and on their constant maintenance in the two diff erent reservoirs.KEYWORDS. Feeding resources, land-water ecotone, trophic ecology, Astyanax aff . bimaculatus, Astyanax parahybae. RESUMO. Heterogeneidade de habitat no hábito alimentar de duas espécies simpátricas e congenéricas de peixes caracídeos em dois reservatórios tropicais.A fl exibilidade alimentar e onivoria são características das espécies de peixes de água doce neotropicais, especialmente em espécies de Astyanax. Tradicionalmente a maioria dos recursos na dieta de peixes é reconhecidamente originária do ambiente aquático, porém estudos recentes têm apontado para a importância das fontes alóctones. Além disso, a colonização de macrófi tas, comuns em vários reservatórios tropicais, pode melhorar os aspectos de insetivoria na dieta dos peixes expandindo ou concentrando a área disponível para a alimentação. Aqui analisamos o conteúdo estomacal de duas espécies de Astyanax simpátricas para avaliar a importância das diferenças de habitats, como a complexidade espacial, em dois reservatórios tropicais com diferentes características ambientais. A análise NMDS indicou separação na dieta destas espécies entre reservatórios (stress = 8.28%). Além disso, a análise de variância revelou uma diferença signifi cativa no volume de itens alimentares consumidos entre os reservatórios (Anova one-way F (1. 132) = 4,4446; p = 0.037), salientando a importância das condições ambientais na composição da dieta de peixes. O estudo destacou o hábito alimentar insetívoro das espécies de Astyanax e revelou diferentes estratégias de alimentação entre esses peixes ...

Beta diversity, the spatial variation in species composition, has been related to different explanatory variables, including environmental heterogeneity, productivity and connectivity. Using a long-term time series of zooplankton data collected over 62 months in a tropical reservoir (Ribeirão das Lajes Reservoir, Rio de Janeiro State, Brazil), we tested whether beta diversity (as measured across six sites distributed along the main axis of the reservoir) was correlated with environmental heterogeneity (spatial environmental variation in a given month), chlorophyll-a concentration (a surrogate for productivity) and water level. We did not found evidence for the role of these predictors, suggesting the need to reevaluate predictions or at least to search for better surrogates of the processes that hypothetically control beta diversity variation. However, beta diversity declined over time, which is consistent with the process of biotic homogenization, a worldwide cause of concern.

Zooplankton may be regarded as a sensitive tool for monitoring environmental variations in coastal lagoons due to their ability to immediately react to changes in the water column trophic features and salinity levels. As a coastal lagoon with a broad history of anthropic influence, Rodrigo de Freitas Lagoon (Rio de Janeiro, Brazil) is widely used for water sports and artisanal fishing. The present study aimed to expand the knowledge base about zooplankton in the Rodrigo de Freitas Lagoon by assessing the composition and time-spatial distribution of the major zooplankton groups. Samples were collected fortnightly from at four distinct sampling points August 2001 to July 2002. At each point, salinity, temperature, dissolved oxygen and water transparency were measured. During the study period, the lagoon behaved as an spatially homogeneous environment in what regards the abiotic variables. However, all these variables showed significant differences along the time, mainly related to seasonality (air temperature and rainy and dry periods). The zooplankton community showed low taxonomic richness, with the predominance of species commonly found in coastal lagoons, especially with mesohaline conditions, as well as those found in estuaries. An interesting fact was the rise in zooplankton abundance at all sampling points right after a fish kill event. Such increase was caused mainly by the Brachionus plicatilis O.F. Müller 1786 species. Thus, the zooplankton community was affected by physical and chemical factors, mainly by the dissolved oxygen decline event and variations in the influx of seawater into the lagoon. In addition, phytoplankton availability and fish predation pressure were suggested as important regulating factors of the zooplankton community

Rotifers have often been used as indicators of sudden changes in physical and chemical features of the aquatic environment. Such features vary greatly during flood pulse events in small lakes connected to major floodplains. However, few are the studies that investigate the consequences of the flood pulse in rotifer species composition, abundance, richness and diversity, especially in Amazonian lakes. We analyzed samples from a small blackwater lake of an "igarapé" connected permanently to the Negro river, in Central Amazonia. Samples were taken twice a year for two years, comprising flooding and receding periods of the flood pulse. Rotifer abundance increased significantly after draught events, and electrical conductivity and turbidity were intrinsically related to such variation. Species composition also changed from flooding to receding periods. Some taxa, such as Brachionus zahniseri reductus and Lecane remanei were restricted to receding periods, while Brachionus zahniseri, Brachionus gillardi and Lecane proiecta were only present during flooding. A shift in the composition of rotifer families was observed from one period to another, showing the effect of renewing waters of the flood pulse. These results suggest that the flood pulse acts as a driving force and stressing condition, considerably altering rotifer community dynamics, either changing species composition or decreasing abundance.

Several studies have shown that spatial synchrony, which is defined as the strength of the correlation between time series, is a common pattern that occurs in different regions, types of ecosystems and groups of organisms. In addition to applied research (optimisation of monitoring networks and population persistence), spatial synchrony has important implications for unravelling the processes underlying environmental and biological dynamics. We quantified the levels of synchrony among six sites in a tropical reservoir (Lajes Reservoir, Rio de Janeiro, Brazil) over 85 months (from August 2001 to December 2009) for twelve environmental variables, four broad zooplankton groups (testate amoebae, rotifers, cladocerans and copepods) and 23 abundant taxa. We hypothesised that (a) environmental synchrony would be higher than biological synchrony; (b) high synchrony would be found; and (c) biological synchrony matrices would be positively correlated with environmental synchrony and negatively correlated with both environmental and geographic distances. A strong relationship between biological synchrony and environmental matrices, when compared to geographic distance, would be consistent with the hypothesis that similar environmental variability synchronises the dynamics of local populations (Moran effect). In comparison to previous studies, we found high levels of synchrony. Environmental synchrony was often higher than biological synchrony, which was in line with our expectation and previous findings. The high levels of synchrony found in this study suggest that reservoir zonation was not enough to desynchronise the dynamics at the sites for both environmental and biological variables. Spatial synchrony in zooplankton populations declined more consistently with environmental distance than with geographic distance. Additionally, environmental distances were unrelated to geographic distances. Taken together, these results indicate a preponderant role of the Moran effect. Thus, as synchrony increases extinction rates, we speculate that novel ecosystems may be highly susceptible to regional‐wide changes in environmental factors.

The efficiency of grass carp (Ctenopharyngodon idella Val.) for biological control of floating aquatic macrophytes in net cages was assessed in two eutrophic tropical hydroelectric reservoirs through experiments using three macrophyte species (Eichhornia crassipes, Pistia stratiotes and Salvinia auriculata). A total of twenty experiments were performed in these reservoirs with a duration period between 21 and 30 days, during distinct seasons of the year. Sets of experiments were conducted using the three macrophyte species simultaneously in both reservoirs, and additional experiments were performed in one reservoir involving the separate use of each species. All macrophytes demonstrated significant growth in the absence of fish with total per day biomass increase rate ranging from 0.06 to 17.00%. The presence of grass carp significantly reduced macrophyte biomass in both reservoirs independent of fish size, stocking rate and available cage space. In all seasons, grass carp grazing pressure was higher for S. auriculata and lower for P. stratiotes. The results show that grass carp in net cages was able to use aquatic macrophytes as the only food source, controlling massive plant growth (especially S. auriculata and E. crassipes). Fish cages, already implemented in several reservoirs in Brazil, are easily manageable units at relatively low cost. Thus, biocontrol of macrophytes using caged grass carp is proposed as an effective low‐budget ecotechnological tool to control consumable plants while avoiding the removal of desirable aquatic vegetation and the resultant impact on local fauna.