Articulos

Hydrobiologia 493: 115–130, 2003. © 2003 Kluwer Academic Publishers. Printed in the Netherlands.

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Limnological features in Tapacur´ reservoir (northeast Brazil) a during a severe drought
Marc Bouvy1 , Silvia M. Nascimento2, Renato J.R. Molica3 , Andrea Ferreira4 , Vera Huszar4 & Sandra M.F.O. Azevedo5
IRD, Bel Air, UR FLAG, BP 1386, Dakar, Senegal E-mail: bouvy@dakar.ird.sn 2Southampton Oceanography Centre, University of Southampton, European Way, Southampton S014 3ZH, U.K. 3 Instituto Tecnol´ gico do Estado de Pernambuco (ITEP), Recife/PE, Brazil o 4 Laborat´ rio de Ficologia, Depto de Botânica, Museu Nacional, UFRJ, Quinta da Boa Vista, São Crist´ vão, o o RJ, 20.940-040, Brazil 5 Laborat´ rio de Ecofisiologia e Toxicologia de Cianobact´ rias, NPPN, CCS, UFRJ, Ilha do Fundão,o e RJ, 21.941-590, Brazil
Received 25 April 2002; in revised form 3 December 2002; accepted 29 December 2002
1 Centre

Key words: stratification, particulate matter, El Niño event, reservoir, northeast Brazil

Abstract The drastic interactions of weather as El Niño events with catchment and hydrological processes can cause unexpected changes in physical, chemical and biological propertiesof freshwater aquatic ecosystems. The severe drought during 1998–1999 in the northeastern region of Brazil induced ecological changes in numerous reservoirs as in Tapacurá reservoir, one of the biggest drinking-water suppliers in Pernambuco state. Investigations were based on monthly sampling over 2 years (May 1998–May 2000) conducted at 3 representative stations with 3 sampled depths through thewater column (0.5 m, middle and 0.5 m above the bottom). Temporal changes in ecological processes, especially stratification, were driven by two major precipitation patterns, with an initial marked dry period (period 1) followed by a rainy season (period 2). Dissolved oxygen and pH variations, higher conductivity and alkalinity values, higher concentrations of particulate organic material (carbon,nitrogen and phosphorus) and higher levels of algal biomass (chlorophyll a) characterized the dry period (May 1998–May 1999). During this phase of low water level when the reservoir storage capacity reached a minimum of 3.9%, the concentrations of chlorophyll a gradually increased with a cyanobacterial bloom (Cylindrospermopsis raciborskii) noted in April 1999. The decline in chlorophyll a andparticulate organic matter were observed as a result of the first rains in May– June 1999, with the drastic changes of quality of matter (higher particulate C/N ratio). After a phase characterized by the entire water column turning anoxic, a second phase in the stratification process could be identified from June 1999 with the pronounced rainfalls accompanied by an overturn event. Annual rainfall deficitand lack of reservoir water renewal in 1998–1999 linked to the 1997 El Niño consequences were important determinants of high eutrophication levels and drastic ecological modifications in Tapacurá reservoir. Introduction Changes in climate to drier conditions alter the hydrologic properties of individual lakes and can disrupt lake properties such as thermal structure and light regime, which directlyinfluence aquatic organisms (Carpenter et al., 1992). Ecologists are becoming increasingly aware that large scale climatic fluctuations, such as the El Niño-Southern Oscillation (ENSO) can influence interannual variability in ecological processes in lakes (Gerten & Adrian, 2000). Changes within an ecosystem can show the variability caused

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Figure 1. Map of Pernambuco state (Brazil),location of the Tapacur´ reservoir. The lower map shows the locations of the three sampling a stations.

by the abiotic environment or by dynamic processes within the biotic system (Reynolds, 1984). For phytoplankton, besides chemical factors (mostly nutrients), physical external factors (light, temperature, mixing regime) are of major importance for conditioning the structure of phytoplankton...