Environmental factors and benthic Oligochaeta (Annelida, Clitellata) assemblages in a stretch of the Upper São Francisco River (Minas Gerais State, Brazil).

The Oligochaeta forms an important part of the macroinvertebrates inhabiting sediments of lotic ecosystems. It has an important role in the cycling of matter and energy transfer in these environments. The aim of this study is to analyse limnological variables, their influence on the structure and diversity of benthic oligochaete taxocenosis in a stretch of the Upper São Francisco River and its tributary the Piumhi River. Samples were taken in two climatic periods, the dry season in October 2006 and 2007 and the rainy season in March 2007 and 2008 at three points along the Piumhi River and six points along the São Francisco River. The sediment of the São Francisco consisted predominantly of sand and clay, whereas the sediment of the Piumhi was mainly sandy. Six species of oligochaete occurred in the Piumhi River while seven were found in the São Francisco. Of these, Pristina synclites Stephenson, 1925, Pristina americana Cernosvitov, 1937, Bothrioneurum sp. Stolc, 1888 and Limnodrilus hoffmeisteri Claparede, 1862 occurred in the sediment of both rivers. L. hoffmeisteri showed the highest numerical abundance in the Piumhi River and Brinkhurstia americana (Brinkhurst, 1964) and L. neotropicus Cernosvitov, 1939 were the most abundant species in the São Francisco River. The highest oligochaete density was recorded in the Piumhi during the dry seasons. Canonical correspondence analyses (CCA) of sediment characteristics explained most of the data variability and the association of the presence of oligochaete species in the Piumhi and São Francisco Rivers with the limnological variables (grain size composition and total nitrogen and phosphorus concentrations in the sediment).