ABSTRACT

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.

The dominance patterns of the main zooplankton groups depend on whether they are analysed by numerical density or dry weight. This fact is discussed in relation to the trophic state of the system.Biomass of planktonic crustaceans from Broa reservoir was analysed by dry weight. For Argyrodiaptomusfurcatus organic carbon was also determined. Values of 4.42 pg C ind. -' and 6.83 pg C ind. -r were obtained for males and females, respectively.Seasonal fluctuation of the species showed two peaks of abundance: the rainy season (summer), and the windy period (winter), these factors being thus the principal controlling forces for the growth of the populations.

The population dynamics, life cycle and feeding of Argyrodiaptomus furcatus and Notodiaptomus iheringi, were studied in Broa reservoir from August 1988 to August 1989, period when a replacement of A. furcatus by N. iheringi was observed. Some abiotic factors such as temperature, dissolved oxygen, pH and conductivity were measured to characterize the limnological conditions of the reservoir. Also, phytoplankton composition was analyzed and related to the feeding of the two species. Experimental data on developmental time and reproduction of A. furcatus and N. iheringi under different temperatures showed that lower temperatures were responsible for density decreasing of both populations in the reservoir during the dry season. Chlorophyta and Chrysophyta smaller than 20 microns were the most abundant phytoplankton groups in the reservoir as well as in the gut content of A. furcatus and N. iheringi, representing an important food source for both species. The temporary disappearance of Argyrodiaptomus furcatus, observed between 1988 and 1989 and its replacement by Notodiaptomus iheringi was related to mining activities upstream, modifying the water turbidity, pH and conductivity. However, the reappearance and maintenance of A. furcatus for another ten years and a recent replacement re-incidence indicates that these two calanoids do not coexist in this environment. Adaptive strategies of both species, related to changes in environmental conditions, are discussed. Probably, Argyrodiaptomus furcatus is an indicator of less eutrophic environments, while Notodiaptomus iheringi of more eutrophic systems.

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.