AIM: The reservoirs of the medium and low Tietê River are disposed as a "cascade series", in which time, physiographic features and influence from drainage basins present differences that determine variability in their dynamics. These reservoirs are submitted to different impacts from the urban centres which they drain, and also from agricultural activities.Considering these features, the aim of this work was to characterize the Medium and Low Tietê river stretches by limnological analysis (including water and sediments) considering dry and rainy seasons evaluating changes along the reservoirs sequence. METHODS: Based on physical and chemical measurements on water and sediments from 16 points along the reservoirs and in the river mouth of the main tributaries, it was possible to make an environmental characterization of the study area. RESULTS: This was performed focusing on water, sediment, habitat structure and time scale. Generally, the data showed a reduction in the values of the analyzed parameters along the reservoirs sequence. Moreover, it was possible to identify differences between the dry and rainy seasons for water and sediment parameters. Concerning habitat structure, there is a gradient along the river, which shows that the environmental quality improves from the river head to the river mouth. Differences in macrophyte composition, kind of sediments, transparency and habitats were also identified. CONCLUSIONS: Therefore, modifications were identified in the habitats general structure along the medium Tietê stretch: emerging macrophytes and muddy bottom predominate upstream; and submerging macrophytes and sandy bottom predominate downstream. The water characteristics should be taken into consideration, because they reflect the impacts upon the drainage basin of each reservoir.

An evaluation was made of the quality of samples of water and sediment collected from a series of reservoirs in the Tietê River (SP), based on limnological and ecotoxicological analyses. The samples were collected during two periods (Feb and Jul 2000) from 15 sampling stations. Acute toxicity bioassays were performed using the test organism Daphnia similis, while chronic bioassays were carried out with Ceriodaphnia dubia and Danio rerio larvae. The water samples were analyzed for total nutrients, total suspended matter and total cadmium, chromium, copper and zinc concentrations, while the sediment samples were examined for organic matter, granulometry and potentially bioavailable metals (cadmium, chromium, copper and zinc). The results obtained for the limnological variable, revealed differences in the water quality, with high contribution of nutrients and metals for Tietê and Piracicaba rivers, besides the incorporation and sedimentation, consequently causing a reduction of materials in Barra Bonita reservoir, thus promoting the improvement of the water quality in the other reservoirs. The toxicity bioassays revealed acute toxicity for Daphnia similis only in the reservoirs located below Barra Bonita dam. On the other hand, chronic toxicity for Ceriodaphnia dubia and acute for Danio rerio showed a different pattern, decreasing in magnitude from Barra Bonita to Três Irmãos, demonstrating an environmental degradation gradient in the reservoirs.Keywords: reservoirs ecology, limnology, ecotoxicology, Tietê River. RESUMO Estudos limnológicos e ecotoxicológicos na cascata de reservatórios do rio Tietê (São Paulo, Brasil)O presente trabalho visou avaliar a qualidade de amostras de água e sedimento dos reservatórios em cascata do rio Tietê (SP) através de análises limnológicas e ecotoxicológicas. Foram realizadas coletas de água e sedimento em dois períodos (fevereiro e julho de 2000) e em 15 estações de amostragem. Foram realizados bioensaios de toxicidade aguda para Daphnia similis, de toxicidade crônica para Ceriodaphnia dubia e para larvas pós-eclodidas de Danio rerio. Análises de nutrientes totais, material em suspensão total e cádmio, cromo, cobre e zinco foram realizadas para as amostras de água e análises de matéria orgânica, granulometria e metais biodisponíveis (cádmio, cromo, cobre e zinco) para o sedimento. Os resultados obtidos para as variáveis limnológicas, revelaram diferenças na qualidade da água, com elevado aporte 698 RODGHER, S. et al.

With the purpose of analyzing the horizontal distribution of the zooplankton community of the Tucuruí Reservoir in the State of Pará, Brazil, collections were made at 16 stations during August 1988. The stations cover the regions called Caraipé (C), Araçagi (B) and Ararão (A), and represent the different compartments of this ecosystem in regard to the morphometry and the presence or absence of "flooded forest". Our findings showed the existence of three compartments with different limnological characteristics determined as a function of the system's morphometry, a factor that may have influenced the horizontal circulation of the reservoir's water and, consequently, led to physical, chemical and biological differences at each of the sampled stations. All the stations presented physical and chemical stratification and layers of total anoxia or reduced concentrations of oxygen dissolved at depths corresponding to the limit of the euphotic zone. With regard to the zooplankton, a differentiated distribution that was mainly quantitative was found among the compartments (density of organisms and proportion among the species) in the case of Cladocera and Copepoda, and basically qualitative in the case of Rotifera. On the other hand, the greatest densities of organisms for all the groups were recorded at the Caraipé stations. As for the spatial distribution, some species were more restricted or more abundant at given stations. Among the Cladocera, there was a greater abundance of Moina minuta, Bosmina hagmani and Bosminopsis deitersi at the Araçagi stations, while C. cornuta intermedia, C. cornuta rigaudi and C. cornuta cornuta were more plentiful in the Caraipé and Diaphanosoma birgei in the Ararão. Among the Rotifera, Trochosphaera aequatoriales and Lecane proiecta, Ascomorpha ecaudis, besides Polyartha cf vulgaris were restricted, respectively, to the Caraipé, Araçagi and Ararão stations, while the others, such as Keratella americana and Collotheca sp., were more broadly distributed. As regards Copepoda Calanoida, the dominating species was found to be Notodiaptomus maracaibenses, followed by Notodiaptomus henseni and Argyrodiaptomus azevedoi. The most abundant Cyclopoda species was Thermocyclops minutus, although Thermocyclops decipiens, Mesocyclops longisetus, Mesocyclops meridianus e Metacyclops sp. were also found.

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Freshwaters are among the most vulnerable ecosystems regarding biological invasions. Reservoirs built after the last half century in São Paulo State have been continuously subjected to the introduction and establishment of alien species, both exotic and allochtonous. Here we present a short historical and the present status of some recent, still going on, biological invasion processes, for both, invertebrate and vertebrates, with emphasis on Tietê River basin. Among reservoir communities, the zooplankton has the lowest number of established exotic species, with 4 species: the cladocerans Ceriodaphnia dubia and Daphnia lumnholtzi, the cyclopoid copepod Mesocyclops ogunnus and the rotifer Kellikotia bostoniensis. Among the macroinvertebrate communities there is a fast ongoing invasion by three exotic species of mollusks, the Asian clam, Corbicula fluminea, the African-Asian gastropod, Melanoides tuberculata, and the golden mussel, Limnoperna fortunei, this latter a cause of greatest concern. However by far the community most affected by introduction of alien species is the ichthyofauna. In reservoirs of Tietê, Paranapanema and Rio Grande rivers, the main tributaries of the Upper Parana basin, the relative proportion of alien species vary approximately between 10 and 25% of the total number of species. Most introductions were derived from anthropic activities, either from direct local introduction, the so called peixamento or by accidental escape from nearby fish farming installations. Some species have profound effects on the structure of the fish community, as recently happened in Lobo (Broa) reservoir after the introduction of the Cichla kelberi. The African Sarotherodon niloticus and Tilapia rendalli and the allochtonous Plagioscion squamosissimus became the dominant species in many São Paulo state reservoirs. The complex interaction between biological invasions, water pollution and eutrophication, is the possible cause for many native species extinction going on in São Paulo State rivers and cascade of reservoirs. At short time and local scale, management actions are urgently required to eradicate or minimize the impact of exotic species in the reservoirs, especially in the small ones, and at longer time and larger scales, education and law enforcement are essential tools for both prevention of new introductions and management to the already existing problems.

Studies on the longitudinal processes in reservoirs, involving physical, chemical and biological processes have been thoroughly appraised, suggesting the existence of a longitudinal organization controlled by the entrance and circulation of water which inserts modifications in the structuring of the system. To evaluate this effect, the Salto Grande reservoir (Americana, SP) was analyzed in 11 sampling stations in its longitudinal axis, in the rainy and dry seasons of 1997 considering the physical chemical and biological variables. Analyzing the results in agreement with the declining concentration degree of the river-barrage direction, a more significant correlation was verified in the dry period for total phosphorus (r2 = 0.86), dissolved total phosphate (r2 = 0.83), nitrite (r2 = 0.93), inorganic phosphate (r2 = 0.89), ammonium (r2 = 0.84) and suspended material (r2 = 0.85). In the rainy period, only nitrite (r2 = 0.90) and conductivity (r2 = 0.89) presented correlation with the distance of the dam, which demonstrates the effects of precipitation and the operational mechanism of the dam, as well as the distinction among the physical (sedimentation), chemical (oxidation) and biological (decomposition) processes in spatial heterogeneity of the system. These factors were decisive in the organization of these communities, with higher occurrence of rotifers and copepods in relation to cladocerans, the first ones being more abundant in the entrance of the Atibaia river, decreasing towards the dam direction, while copepods presented an inverse pattern. A distribution pattern similar to Copepoda was also verified for the Cladocera, evidencing a tendency to increase the density of organisms in the stations distant to the entrance of the Atibaia river, not being registered, however, a distribution gradient in the longitudinal axis, as observed for rotifers and copepods. In relation to the trophic degree a longitudinal gradient was also verified from eutrophic to oligotrophic depending on the location of the sampling station in relation to the longitudinal axis and period of analysis. The differences obtained, relating to the distribution of the environmental variables, demonstrate a characteristic pattern for reservoirs, with a longitudinal gradient in the sense river-barrage that inserts changes in the physical and chemical composition of the water, contributing to the differentiated establishing of biological communities.

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