This study examined the effects of the damming of the Mogi-Guaçu River (São Paulo State, Brazil) on the surface current velocity, water temperature, Secchi disc transparency, turbidity, colour, conductivity, pH and concentrations of nutrients and pigments. Surface-water samples were taken before, during and after the reservoir was filled. Three sampling sites were established, one in the upper reach of the reservoir, one in the central area of the reservoir and one downstream from the dam. An additional sampling site was established on the Peixe River, the major tributary of the Mogi-Guaçu in the study area. After filling of the reservoir, the surface current velocity tended to decrease, excepting downstream of the dam. The pH, and the Kjeldahl nitrogen, ammonia and chlorophyll-a concentrations, tended to increase. The nitrite concentrations increased mainly in the upper reach and central area of the reservoir. The Secchi disc transparency and colour tended to decrease. A decreasing trend in dissolved oxygen concentration was observed mainly at the central area of reservoir. The conductivity tended to decrease, later returning to levels observed prior to reservoir filling. The nitrate, total phosphorus and orthophosphate concentrations exhibited an increasing trend after reservoir filling, followed by a decreasing concentration, reaching lower levels than those found prior to reser voir filling. High phaeophytin concentrations were measured for the filling phase. The observed water quality changes for Mogi-Guaçu Reservoir generally were not as extreme as those observed for other tropical reservoirs. This trend was related to the operation of the reservoir. As MogiGuaçu Reservoir is a run-of-the-river reservoir with a short water retention time, the flooded area is not extensive and the retention of material and sedimentation upstream from the dam is not remarkable. These facts explain the small water quality changes observed for most of the variables after reservoir filling. The water quality decreased at the in-lake site in the central part of the reservoir, attaining a hypereutrophic condition. This fact was related to the ageing of the reservoir and to cultural eutrophication.
We examined the effects of the Mogi-Guaçu river damming (São Paulo State, Brazil) on the Chironomidae fauna. Pre, during, and post-filling sampling was carried out in the main channel and margins of one site in the upper zone of the reservoir, using a modified Petersen grab (325 cm 2 ). We evaluated the total, subfamily, and tribe densities and also their relative abundance. Analysis of genera included densities, relative abundance, richness, and dominance. The Rosso's ecological value index (EVI) determined the ecological importance of each genus. There was a tendency of decrease of the total Chironomidae density, increase in the percentage of Chironomini, and decrease in densities and percentages of Orthocladiinae and Tanytarsini. These changes in percentage were respectively related to Polypedilum, Lopescladius, and Rheotanytarsus, the genera with the highest EVI values. After-filling richness was lower in the margins and dominance of genera did not change significantly. Chironomidae in the margins was more sensitive to damming than in the main channel. This difference in sensibility sustains the use of Chironomidae as bioindicators. Damming impact was indicated by the reduction of both genera richness in the margins and relative abundance of groups typical of faster waters. The results have highlighted the need for multi-habitat analysis combined with a before-after sampling approach in the environmental impact studies concerning the damming impact on the benthic fauna.Keywords: bioindicators, taxocenose composition and structure, before after sampling design. Impacto de um represamento sobre Chironomidae do remanso de um reservatório fio d'água tropical ResumoEste trabalho teve como objetivo avaliar o impacto do represamento do Rio Mogi-Guaçu, em um ponto situado na zona de remanso do reservatório, através da utilização da taxocenose Chironomidae. A amostragem foi realizada antes, durante e após o represamento, no canal e nas margens, com o auxílio de um pegador Petersen modificado (área de 325 cm 2 ). Calculou-se a densidade total, de subfamílias, tribos e gêneros de Chironomidae, bem como sua abundância relativa. Estimou-se a riqueza e a dominância de gêneros. O índice de valor ecológico de Rosso (IVE) foi utilizado para avaliar a importância ecológica dos diferentes gêneros. Houve tendência de redução da densidade total de Chironomidae, aumento da abundância relativa de Chironomini e diminuição das abundâncias relativas de Orthocladiinae e Tanytarsini. Estas alterações de abundância relativa foram relacionadas respectivamente a Polypedilum, Lopescladius e Rheotanytarsus, gêneros que apresentaram os maiores valores de IVE. Após o represamento, houve decréscimo da riqueza de gêneros na margem, enquanto a dominância não apresentou alteração significativa. A taxocenose de Chironomidae das margens mostrou-se mais sensível ao represamento que a do canal. Esta diferença de sensibilidade confirma o potencial dos Chironomidae como bioindicadores. A ocorrência de impacto foi observada através da diminuição...
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