Effects of increasing organic matter loading on the dissolved O-2, free dissolved CO2 and respiration rates in the Piracicaba River basin, southeast Brazil

Concentrations of biogenic gases (O-2 and CO2) and respiration rates were m easured at 7 sites along major tributaries of the Piracicaba River to addre ss the effects of sewage loading on aquatic metabolism. Strictly aerobic co nditions were observed at upstream sites (less polluted), with concentratio ns of O-2 at or above and CO2 slightly above atmospheric equilibrium. Conce ntrations of biogenic gases were independent of variations in discharge and relatively constant during the sampling period. Respiration rates were sig nificantly higher at upstream sites and CO2 concentrations were above atmos pheric equilibrium. The concentrations of CO2 at downstream sites were inve rsely correlated with discharge. At the downstream sites (more polluted), a nthropogenic inputs of labile organic matter (sewage) altered the aquatic m etabolism by creating a less aerobic environment. At these sites, O-2 conce ntrations were always below atmospheric equilibrium and directly related to discharge. (C) 1999 Elsevier Science Ltd. All rights reserved.