We report the occurrence of cyanobacterial blooms and the presence of cyanotoxins in water samples from the Armando Ribeiro Gonçalves reservoir (06° 08' S and 37° 07' W), located in the state of Rio Grande do Norte, in the semiarid region of northeastern Brazil. The cyanobacterial species were identified and quantified during the rainy and dry seasons in the year 2000. Cyanotoxins such as microcystins, saxitoxins and cylindrospermopsins were analyzed and quantified using hPLC and ELISA methods. The mixed toxic blooms of Cylindrospermopsis raciborskii, Microcystis spp (M. panniformis, M. protocystis, M. novacekii) and Aphanizomenon spp (Aphanizomenon gracile, A. cf. manguinii, A. cf. issastschenkoi) were persistent and represented 90-100% of the total phytoplankton species. Toxic cyanobacterial blooms from the Armando Ribeiro Gonçalves reservoir were analyzed and found to have three phases in relation to the annual cycle. During the rainy season, an intense toxic bloom of Cylindrospermopsis raciborskii was recorded along with saxitoxins (3.14 µg.L -1 ). During the transition period, between the rainy and dry seasons, different species of Microscytis occurred and microcystin as high as 8.8 µg.L -1 was recorded. In the dry season, co-dominance of Cylindrospermopsis raciborskii, Microcystis spp and Aphanizomenon spp occurred and the concentrations of saxitoxin remained very low. Our results indicate the presence of microcystins (8.8 µg.L -1 ) and saxitoxins (3.14 µg.L -1 ) into the crude water, with increasing concentrations from the second fortnight of April to late May 2000. The occurrence of toxic blooms in this reservoir points to a permanent risk of cyanotoxins in supply waters, indicating the need for the implementation of bloom control measures to improve the water quality. Exposure of the local population to cyanotoxins through their potential accumulation in fish muscle must also be considered.Keywords: reservoir, semiarid, cyanobacteria, microcystin, saxitoxin. RESUMO Ocorrência de florações tóxicas de cianobactérias em um reservatório no semi-árido brasileiroNós relatamos a ocorrência de florescimentos de cianobactérias e a presença de cianotoxinas em amostras de água do reservatório Armando Ribeiro Gonçalves (06° 08' S; 37° 07' W) situado no Estado do Rio Grande do Norte, na região semi-árida do Brasil. Cianobactérias foram identificadas e quantificadas nos períodos seco e chuvoso do ano 2000. Cianotoxinas tais como, microcistinas, saxitoxinas e cilindrospermopsinas foram quantificadas por hPLC e ELISA. Florescimentos tóxicos mistos de Cylindrospermopsis raciborskii, Microcystis spp (M. panniformis, M. protocystis, M. novacekii)

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a b s t r a c tThe native fish fauna commonly found in the drainage basins of rivers and reservoirs of Latin America, including those of the semi-arid Northeastern Brazil, are representatives of the Neotropical region. This work reports on the reproductive ecology of five commercially important and consumable native fish species, in relation to rainfall and hydrological variables of the semi-arid reservoirs in Brazil. Pluviometric precipitation, temperature, pH, dissolved oxygen and electrical conductivity of the water were registered, and maturation of fish gonads was assessed on a monthly basis. This region is characterized with short spells of rain interspersed with long dry season and rainfall seems to be the main environmental factor which modulates the timing of the spawning period of fish. Construction of reservoirs without adequate facilities for fish migration has an adverse impact on the migratory fish species which are of commercial and ecological importance to semi-arid Northeastern Brazil.

The reproductive ecology of the freshwater fish Cichla monoculus Spix, 1831 (Osteichthyes: Cichlidae) was investigated in the Campo Grande Reservoir, Northeast Brazil. Rainfall, temperature, pH, dissolved oxygen and electrical conductivity of the water were recorded monthly. Fish collected on a monthly basis were measured, weighted, dissected, sexed and the stage of maturation of the gonads were assessed by macro and microscopic means. The semi-arid study region has short spells of rain of 2-3 months duration interspersed with dry seasons. A positive correlation was observed between rainfall and air and water temperatures and conductivity of the water. The study population had an extended spawning season, with peak reproductive activity coinciding with low water temperatures. Males were longer and heavier than females on average and were larger at onset of sexual maturity. The size frequency distributions of the oocytes indicate that C. monoculus is a multiple spawner with an estimated batch fecundity of 3100. Condition factor showed an inverse relationship in relation to gonad size during maturation in both sexes and spent fish were in poor condition. In mature males, lipid stores in the post-occipital cephalic protuberance, a secondary sexual characteristic developed during the reproductive phase, which depleted in spent individuals. The success of this fish is attributed to its reproductive capacity and to the phenotypic plasticity that allows it to adapt to the harsh ecological conditions that prevail in the semi-arid region of Northeast Brazil.

a b s t r a c tThe ecology of phytoplankton reservoir communities plays a pivotal role in their management and in the development of inland fisheries in the water scarce semi-arid regions of Rio Grande do Norte State, Brazil. The hydrological conditions characterizing these reservoirs include infrequent rainfall and high evaporative loses. The phytoplankton assemblages in tropical climate are structured primarily on dry and wet cycles, not the annual temperature and light regime. The purpose of the current study was to investigate the ecological aspects of phytoplankton communities from three reservoirs during the period of July 2003-June 2004, which include atypical summer rainfall and reservoir overflow. The environmental data broadly divided into two categories, stress factors and disturbance factors. The stress factors deals with the impact of high particulate organic matter and low dissolved oxygen concentrations and disturbance factors, linked to water level fluctuation through flushing and reservoir drawdown, and these events are associated with the phytoplankton assemblages. Results indicated that the three distinct hydroperiods determined the structure of phytoplankton and chlorophyll levels and limited the presence and relative abundance of cyanobacterial species. Phytoplankton succession accompanied changes in clear and turbid water phase represented by the alternate dominance between diatoms and chlorophytes. Inundation and complete filling of reservoir stimulated ''s'' and ''r'' strategists species (o20 um) of unicellular and some colonial members of chlorophytes and cryptomonads in highly turbid water environment, and ''c'' strategist species in rest of the wet period following disturbance gradients. Statistical analysis elicited a significant relationship between particulate organic matter and relative abundance of smaller chlorophytes, which was deemed as stress factor. The flushing and reservoir drawdown indicate disturbance gradient and as a consequence high diversity. Furthermore, a substantial reduction in chlorophyll levels was registered in turbid water phase is related to the reduction of light penetration and re-suspension of sediments. Considering these results, it can be suggested that the well-managed reservoir drawdown can possibly maintain an environment free of eutrophication and cyanobacterial dominance.