Limnological Characteristics of an Old Tropical Reservoir (Ribeirão das Lajes Reservoir, RJ, Brazil)

GUARINO¹ ,A.W.S.; BRANCO¹, C.W.C.; DINIZ¹, G.P. & ROCHA², R.

¹ Departamento de Ciências Naturais, ECB, Universidade Federal do Estado do Rio de Janeiro (UNIRIO), Av. Pasteur 458, CEP 22290-040, Rio de Janeiro, Brazil, guarino@unirio.br; cbranco@unirio.br

² Convênio Light/Unesp, Ger. de Meio Ambiente (GUM), Piraí-RJ, rinaldo.rocha@light.com.br

ABSTRACT: Ribeirão das Lajes Reservoir was filled in 1908 for electric power generation and domestic supply to districts of Rio de Janeiro City. In order to determine spatial and temporal patterns of limnological variables of Ribeirão das Lajes Reservoir, samples were taken monthly, from July 2001 to December 2002, at eight sampling stations. The waters were characterized by a neutral pH (7.19) and low average content of nutrients, 19.61 mg.L^-1 of total P and 397.66 mg.L^-1 of total N. Temporal fluctuation pattern was associated with column stability in most part of the year, and to the water mixing in July. The main horizontal spatial heterogeneity corresponded to differences between the riverine region, with influence of Tócos Tunnel discharge with higher nutrient concentration, and the limnetic area of the reservoir, with comparatively higher water temperature, transparency, conductivity and pH. According to average values of total phosphorous, total nitrogen and Secchi disk transparency Ribeirão das Lajes Reservoir was considered mesotrophic. The nitrogen and phosphorous content near the dam were influenced by water column conditions. In winter, the mixing of the water column influenced all the analyzed physical and chemical variables.

Key-Words: tropical reservoir, physical and chemical features, column mixing, water quality.