Additionally, both the species richness and macroinvertebrate
abundances showed different compositions at the sampling points
(entrances and exits of both treatments). The entrances had always
a greater richness and abundance of organisms of different trophic
levels and different guilds (although not always statistically significant).
This heterogeneity is primarily due to a greater structural
diversity with efficient trophic interactions (Bambaradeniya, 2000)
which is conditioned by the water quality (Azrina et al., 2006;
Barbosa et al., 2001; Castillo et al., 2000). However, the exits had
lower species richness and fewer trophic levels and guilds, indicating
lower habitat heterogeneity at these sampling points.