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Data of Thesis

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Author: Andreia Maria da Anunciação Gomes.

Title: Impact of intensive fish farming activity and the addition of inorganic nutrients (N and P) in water quality in the reservoir of Ribeirão das Lages-RJ.

Year: 2005.    Full text (in Portuguese)

Abstract

This study aims to determine the influence of inorganic nutrient addition (N and P) and fish culture on physical, chemical and biologic features of the water of Ribeirão das Lajes Reservoir, a oligo-mesotrophic system. This research was conducted through in situ experiments performed during february 2004. For this purpose we used 9 polyethylene bags (2900 liters), placed in the reservoir at the same time. The enclosures were divided in three treatments: one of them containing fishes (10 tilapia per enclosure); the second of them with nutrient addition (nitrate, ammonium and phosphorus addition with amounts of 70; 1,3 and 1,2 μg/L/day, respectively) and the third of them containing no fishes and no nutrient addition (control group). Water samples were collected from each enclosure and from the reservoir in alternated days until the 15th day and the last ones were collected 25 days after the beginning of the experiments. The water temperature, pH, conductivity and turbidity were measured in situ with specific electrodes. Secchi depth also was determined. Chlorophyll a and nutrient concentrations were measured spectrophotometrically. Algal numbers were determined with an inverted microscope from lugol-fixed samples. During experimental periods, oligotrophic conditions characterized the reservoir, like: low concentration of nitrogen and phosphorus, high secchi depth, low turbidity and low phytoplankton density. Phytoplankton community was dominated by cyanobacteria in Ribeirão das Lajes reservoir. The control group remained these features. Nutrient addition caused an increase of nitrate concentration, but the P addition was not sufficient to increase SRP concentration. Although Chlorophyta had replaced Cyanobacteria, the phytoplankton density was not increased in these enclosures. However, changes related to the trophic conditions had taken place in the enclosures containing fishes. The tilapia presence increased water temperature, pH, conductivity, turbidity values and secchi depth values. The chlorophyll increased from 1,5μg/L, at the beginning of the experiment, to 542,28 μg/L at the end of experiment. The growth of algae leads to decreases of the transparency level. Phytoplankton community structure was changed in enclosures containing fishes. Cyanobacteria were excluded from these enclosures and Chlorophyta were dominant at the end of experiment. The present study results suggest that nutrient addition was not sufficient to increase phytoplankton density and the major limiting factors for the phytoplankton growth could be phosphorus deficiency. However, the high nitrogen concentrations observed in all treatment enclosures were sufficient to change the phytoplankton community structure, leading chlorophyta increase. Chorophyta dominance in spite of cyanobacteria has been observed in hypereutrophic system.

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