Efeitos De Diferentes Intervenções No Processo De Eutrofização Do Lago Paranoá (Brasília – Df)

Abstract

RESUMOO monitoramento temporal das variáveis de um corpo d'água possibilita o conhecimento de sua dinâmica e, por fi m, de seu manejo. O Lago Paranoá em Brasília, formado em 1959, passou por um longo período de eutrofi zação. Neste trabalho, usando dados de monitoramento de longa duração da CAESB, os efeitos da instalação de duas Estações de Tratamento de Esgoto (ETE) e de um evento de fl ushing foram avaliados para as variáveis Clorofi la-a, Fósforo Total e Transparência medidas entre 1976 e 2001. Utilizando-se da Análise de Intervenção foram mensuradas a efetividade de três intervenções: o início das operações das ETE (01/1993); o pleno funcionamento destas ETE (01/1996); e um evento de fl ushing (abertura abrupta das comportas, 11/1999). Os resultados mostram que: i) a 1 a intervenção não surtiu nenhum efeito; ii) a 2ª intervenção fez com que os níveis de Fósforo Total e Clorofi la-a fossem reduzidos signifi cativamente; iii) apenas o fl ushing (3ª intervenção), fez com que a transparência da água aumentasse para mais de um metro na maioria dos pontos, inferindo-se que apesar da redução do aporte de fósforo ao lago com as ETE (1ª e 2ª intervenções), a abundância restante de fi toplâncton na água tenha mantido um mecanismo de feedback com o fósforo que só foi interrompido com a abertura abrupta das comportas que eliminou o fi toplâncton da superfície e conduziu o lago à um novo patamar de produção primária. O programa de monitoramento da CAESB foi fundamental para o entendimento deste processo e o que justi fi ca a manutenção e a ampliação deste programa. Palavras-chave: intervenção, reservatório tropical, CAESB, monitoramento, Paranoá. ABSTRACT EFFECTS OF INTERVENTIONS ON THE EUTROPHICATION PROCESS OF PARANOÁ LAKE RESERVOIR OF BRASILIA, BRAZIL.Long-term environmental monitoring of aquatic ecosystems is essential to understand their dynamics and to the proposition of management practices. Paranoá Reservoir was created in 1959 and was subject to a long period of eutrophication. In this paper, data obtained by the monitoring program carried out by the "Companhia de Abastecimento e Saneamento de Brasília", between 1976 and 2001, were evaluated to measure the effects of three direct interventions on water quality (as measured by water transparency, P-total and chlorophyll-a concentrations): the activation of two water treatment plants (01/1993); the proper operation of these plants (01/1996), and a major fl ushing event (sudden release of water in 11/1999). The following results were obtained: i) the fi rst intervention had no effect on the mean values of the variables analyzed; ii) the chlorophyll-a and total phosphorus concentrations signifi cantly declined after the second intervention; iii) water transparency increased after the third intervention. An apparent mechanism of phosphorus feedback supported by phytoplankton was interrupted by the fl ushing and, after that intervention, the primary production of Paranoá Reservoir declined to a new level. The dataset obtained by the monitoring program was essential to...

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