Abstract:

Some biological shifts are predicted to occur when a stream of small order enters a mainstream. Channel hydraulics, quantitative geomorphology of the watershed, sestonic components and other factors influencing the structure and composition of aquatic communities produce a punctuate gradient in the longitudinal distribution of macroinvertebrates. A sampling program was conducted to determine the effects of a third - order stream tributary upon the longitudinal gradient of benthic macroinvertebrates in a fifth- order stream. Nine Surber samples were taken at each of four sampling sites located on El Durazno river and El Cajón creek on the Calamuchita valley in Córdoba Province. Two sites were 30 m upstream from the linkage in each tributary, another one was right down the linkage, and the other was 1000 m down stream from the linkage. Similarity within sites was relatively high (0.87). However there were significant differences among sites in relation to density of certain invertebrate taza and to the relative contribution of functional feeding groups. In all sites, the predominant functional feeding group was the scrapers; filtering and gathering collectors, shredders and predators differed between habitats suggesting a tributary effect on the functional gradient of the receptor. The tributary affected the abundance patterns and increased the density at the confluence, but had little effect on species richness. One thousand meters downstream from the tributary-affected site, the receptor recovered its anterior state. This short distance suggests high resilience of this system.