Efeitos da implantação da usina hidrelétrica Dona Francisca (RS, Brasil) sobre estágios imaturos de Odonata (Insecta)

Abstract

This study was carried out with the purpose to evaluate the short-term impact of a dam construction on the community of the immature Odonata through their analysis in lentic and lotic environments, before and after the impoundment of the reservoir. In the preimpoundment phase 17 stations were sampled, five in lentic environments (dams and ponds), and 12 in lotic environments (tributaries of the Rio Jacuí) between January and October 2000. In the post-impoundment sampling, five points inside the reservoir were studied (between May 2001 and October 2002) in lentic environments, and four points, in lotic environments (between November 2000 and May 2002). The lentic environments were sampled by using sieves, and the lotic ones, with a Surber sampler. Some abiotic factors measured were air and water temperatures, dissolved oxygen, pH, depth and, for lotic environments, the current velocity. The total number collected was 1298 specimens, attributed to eight families and 27 genera, of which 16 are new records for the State: Heteragrion, Oxystigma, Neuraeschna, Tibiogomphus, Phyllocycla, Desmogomphus, Cacoides, Archeogomphus, Anatya, Brechmorhoga, Libellula, Tramea, Cyanallagma, Forcepsioneura, Protoneura and Epipleoneura. The lentic habitat was better represented in abundance and richness in the pre-impoundment (681 individuals and 14 genera), with a drastic decrease in the post-impoundment (n = 41, S = 8), while the opposite occurs for lotic environments (n = 200 individuals and 14 genera versus 376 and 17, respectively). The lotic environments studied here showed greater richness than the lentic environments, presenting a greater heterogeneity of habitats, especially related to aquatic vegetation. For the lentic environments, the higher richness and abundance of odonatofauna were found in stable environments (ponds of greater depths and dimensions), with marginal and aquatic vegetation. In the post-impoundment phase, the fauna restoration occured mainly in points inside the reservoir where prevailed lotic conditions, as the mouth of the tributaries. For lotic environments, the sites of greatest abundance and richness were tributaries of the medium reaches (4th order), characterized by small size and shading of one of the margins by riparian vegetation. The points of 7th order, located in the Rio Jacuí (one of them downstream of the dam), showed the lowest values concerning the fauna, since these sites were strongly influenced by changes in water level of the U.H.E. Itaúba, located upstream. These results were enhanced after the impoundment of the reservoir of U.H.E Dona Francisca. Multivariate analysis showed that the effect of U.H.E. Dona Francisca construction on the immature stages of Odonata was not direct, interacting with the characteristics of each habitat, for both the abundance and the composition of the fauna.