The Arazá-Inambari-Madre de Dios river system in southeastern Peru is a major headwater of the Amazon Basin that has been impacted by erosion and high suspended sediment loads from highway construction along its upper reaches and placer mining along its lower reaches. In addition, mercury from mining has bioaccumulated in fishes and a large part of the catchment will likely be impounded by a hydroelectric dam predicted to create the fifth largest reservoir in Latin America. We surveyed fishes, macroinvertebrates, epilithic algae and physicochemical parameters at 41 sites from the high puno of the Cordillera Vilcanota (4300 masl) to the Amazonian plains near Puerto Maldonado (200 masl). Our aim was to elucidate patterns in community structure along this gradient and to examine community responses to a parallel downstream increase in suspended solids and turbidity.
Results/Conclusions
Epilithic algal taxon richness (227 total species) decreased and assemblage composition shifted downstream, although neither benthic nor water-column chlorophyll-a concentrations changed with elevation. Benthic algal communities were dominated by diatoms (85%), with lesser fractions of Cyanophyta (9%), Chlorophyta (4%), and Streptophyta (2%). Macroinvertebrate taxon richness and abundance of most taxa declined downstream along the mainstem, although some scraper/collector taxa increased in abundance. Fishes, which only occurred below 2630 masl, increased in diversity and abundance downstream. Native fish communities above 1250 masl included only two insectivorous genera; an herbivorous genus entered the fish community at 1120 masl, and a wide range of other taxa (145 total species) occurred below 842 masl. There was an inverse relationship between density of fishes and density of macroinvertebrates, but not between either fish or macroinvertebrate density and benthic algae.
Turbidity and reactive phosphorous increased downstream within mainstem but not tributaries. Turbidity of up to 479 µS/cm was measured in the lower main river, however, we found no overall correlation between turbidity and either density or taxon richness of fishes or macroinvertebrates, even after correcting for elevation. Nevertheless, benthic algal biomass was negatively related to turbidity, elevation explained less variability in fish richness in mainstem (r2 = 0.48) versus tributary habitats (r2 = 0.70), and tributaries had significantly more diverse macroinvertebrate communities. Although anthropogenic impacts were not sufficient to completely disrupt natural elevational/longitudinal distribution patterns of fishes, macroinvertebrates and algae, we observed qualitative and quantitative effects on the biota at sites with high suspended sediment loads. Such effects are likely to be exacerbated by development projects currently proposed for the basin.