Prediction of Egeria najas and Egeria densa occurrence in a large subtropical reservoir (Itaipu Reservoir, Brazil-Paraguay) Luis Mauricio Bini a, * , Sidinei Magela Thomaz b a Departamento de Biologia Geral, ICB, Universidade Federal de Goia ´s, CP 131, CEP 74001-970 Goia ˆnia, GO, Brazil b Universidade Estadual de Maringa ´, Nupelia, PEA-UEM, Av. Colombo, 5790, CEP 87020-900 Maringa ´, PR, Brazil Received 3 September 2003; received in revised form 10 May 2005; accepted 7 June 2005 Abstract Incidence data of two native submerged aquatic macrophytes (Egeria najas Planch. and Egeria densa Planch.) were obtained in eight arms of a large (1350 km 2 ) subtropical reservoir (Itaipu Binacional Reservoir, Brazil-Paraguay). Environmental variables were measured simultaneously. Two large-scale surveys in the same localities identified by a global positioning system were carried out in April 1999 (n = 235) and January 2001 (n = 230). Logistic regressions were used to test the effect of environmental variables on the likelihood of E. najas and E. densa presence or absence. The two species were found under different environmental conditions: conductivity, light attenuation coefficient (k) and fetch were, in this order, the most important environmental variables in predicting the probability of occurrence of E. najas, whereas light attenuation coefficient was the main predictor of the probability of occurrence of E. densa. Thus, both species were negatively affected by the light attenuation coefficient. However, this effect was stronger in E. densa. The small area occupied by these species may be accounted for by the permanent high turbidity of Itaipu Reservoir. Additionally, the dominance of E. najas over of E. densa can be explained by the probably higher light requirements of E. densa. In other reservoirs worldwide, with higher water transparency, the opposite is frequently true. Between 1999 and 2001, an episodic water-level drawdown (5 m) caused the disappearance of submerged vegetation. www.elsevier.com/locate/aquabot Aquatic Botany 83 (2005) 227–238 * Corresponding author. Fax: +55 62 521 11 90. E-mail addresses: bini@icb.ufg.br (L.M. Bini), smthomaz@nupelia.uem.br (S.M. Thomaz). 0304-3770/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.aquabot.2005.06.010