Nei
Kavaguichi
Leite, CENA/USP, nkleite@esalq.usp.br
(Presenting)
Alex
Vladimir
Krusche, CENA/USP, alex@cena.usp.br
Reynaldo
Luiz
Victória, CENA/USP, reyna@cena.usp.br
Maria Victoria
Ramos
Ballester, CENA/USP, vicky@cena.usp.br
Jeffrey
E.
Richey, University of Washington, jrichey@u.washington.edu
Land use changes that occurred in the state of Rondonia in the last decades transformed the landscape of this region. Mainly along the road BR-364, what was almost all forest before is now covered with extensive pastures, with impacts in the aquatic systems that are still not known. This work aims the identification of the relative importance of the natural and anthropogenic characteristics of the basin in the biogeochemistry of the Ji-Paraná river and its main tributaries, assuming that the chemistry of river waters reflects what happens in its basin. To achieve that, a comparative approach was used, in which differences between systems with distinct soils and land use/cover were analyzed, as well as seasonal variations during the period of this study (1999-2002). Regarding spatial variations, the lowest concentrations of ions were found in rivers draining areas with sandy and less fertile soils, whereas the highest ones were found in rivers located in the central part of the basin, draining soils with higher clay content and more fertile. In relation to seasonal variations, it was observed that rivers draining basins in which forests predominate show positive correlations between discharge and ionic content, while rivers draining basins with large areas of pastures, showed inverse correlations between discharge and concentrations of ions. These results indicate that the natural characteristics of these basins might be the main controlling factor of river biogeochemistry. However, variations in nutrient
concentrations, such as C, N and P, also indicate that human influences are already present in these systems.