Effects of stream hydraulics and other environmental variables on density of Narapa bonettoi (Oligochaeta) in the Paraná River system

Abstract

Hydraulic and substratum conditions have been identified as two stream features which affect the benthic community composition, abundance and distribution. However, little attention has been given to the influence of hydraulic variables in large river beds. The aim of this study was to analyse the incidence of the near-bottom hydraulic conditions and other environmental variables on the density of Narapa bonettoi (Oligochaeta, Narapidae), a typical and dominant species found in sandy bed rivers of diverse hierarchy of the Paraná River basin. A large amount of existing and available benthic data were used. The highest correlations among N. bonettoi densities and environmental variables were obtained with the hydraulic variables of friction Reynolds number (RÃ) and shear stress (t 0 ); the substratum type (sand, silt and clay) and organic matter content. The results show that N. bonettoi density would be related with the hydraulic variables following a 'bell-shaped' tendency, e.g. with the friction Reynolds number, N. bonettoi would have a preference for transition values of turbulence ($40 < RÃ < $50), which is equivalent to t 0 between $0.6 and $0.8 kg m À2 . Densities show a slight decreasing tendency toward the maximum RÃ values, suggesting that the species does not support a completely developed turbulence (RÃ > $70). Significant but negative correlations were reported in the literature between benthic macroinvertebrates typical of smaller and steeper streams and RÃ values far beyond the threshold 70 value. N. bonettoi shows a clear preference for a completely sandy substratum and for sand particles sizes around 300 mm. Finally, considering the important role played by the hydraulic variables on the benthic community studied herein, the contagious distribution pattern of N. bonettoi, reported by other authors, would be principally linked with the interactions between current and sediment near the bottom rather than with the mobility of these organisms.

Eight years of monitoring aquatic Oligochaeta from the Baía and Ivinhema Rivers

“…According to Montanholi-Martins andTakeda (1998, 2001) and Takeda and Fujita (2004), the bed of the Ivinhema River is composed of pebble, gravel and sandy sediments. These two species are observed in sandy sediments of fluvial systems such as the Paraguay River (Ezcurra de Drago et al, 2004) and the Paraná River (Marchese and Ezcurra de Drago, 1992;MontanholiMartins and Takeda, 1998;Marchese et al, 2002;and Takeda and Fujita, 2004;Bletter et al, 2008). In the limnophase periods, higher densities of H. aedeochaeta and N. bonettoi were observed due to the decreased influence of water currents in the sandy sediment transport these species.…”

Using oligochaeta assemblages as an indicator of environmental changes

“…Most important variables were altitude and some environmental factors (conductivity, substrate type and temperature). Although the distribution of Oligochaeta is affected by a number of variables interacting mutually, this group is directly related to substrate type (Verdonschot, 2001;Bletter et al, 2008) and hydromorphological characteristics (Takeda et al, 2001;Bletter et al, 2008). The ability of Oligochaeta to grow and develop in lotic environments is a result of a series of complex interactions between hydrology, water quality and biotic factors (Verdonschot, 2001).…”

“…Oligochaeta species richness is directly related to food availability (Collado and Schmelz, 2001) and quality (Pasteris et al, 1994;Collado and Schmelz, 2001), substrate type (Verdonschot, 2001;Bletter et al, 2008), oxygen availability (Collado et al, 1999;Nijboer et al, 2004), and biological interactions (Darrigran et al, 1998). In addition, this group presents a low dispersion, and most of them are considered indicators of specific habitats (Verdonschot, 2001), of hydro-morphological character (Verdonschot, 2001), and of trophic state (Suriani et al, 2007).…”

Structural and functional responses of the oligochaete and aeolosomatid assemblage in lowland streams: a one-way-pollution-modelled ecosystem

“…Most fundamentally, an understanding of the feeding biology of aquatic oligochaetes in particular is necessary in order to make a correct interpretation of the various biologic processes in general (Rodríguez et al, 2001). Oligochaete species richness is directly related to food availability (Collado and Schmelz, 2001) and quality (Pasteris et al, 1994), substrate type (Bletter et al, 2008), oxygen availability (Nijboer et al, 2004), and biologic interactions (Darrigran et al, 1998). Within this context, the response of the oligochaete assemblage to organic contamination might consist in either a decrease or an increase in their richness and diversity at those sites with strong anthropic impact.…”