Effects of increasing organic matter loading on the dissolved O2, free dissolved CO2 and respiration rates in the Piracicaba River basin, Southeast Brazil

Concentrations of biogenic gases (O2 and CO2) and respiration rates were measured at 7 sites along major tributaries of the Piracicaba River to address the effects of sewage loading on aquatic metabolism. Strictly aerobic conditions were observed at upstream sites (less polluted), with concentrations of O2 at or above and CO2 slightly above atmospheric equilibrium. Concentrations of biogenic gases were independent of variations in discharge and relatively constant during the sampling period. Respiration rates were significantly higher at upstream sites and CO2 concentrations were above atmospheric equilibrium. The concentrations of CO2 at downstream sites were inversely correlated with discharge. At the downstream sites (more polluted), anthropogenic inputs of labile organic matter (sewage) altered the aquatic metabolism by creating a less aerobic environment. At these sites, O2 concentrations were always below atmospheric equilibrium and directly related to discharge.