ORIGINAL PAPER CO 2 and O 2 dynamics in human-impacted watersheds in the state of Sa ˜o Paulo, Brazil M. S. M. B. Saloma ˜o Æ J. J. Cole Æ C. A. Clemente Æ D. M. L. Silva Æ P. B. de Camargo Æ R. L. Victoria Æ L. A. Martinelli Received: 30 June 2007 / Accepted: 5 May 2008 / Published online: 28 May 2008 Ó Springer Science+Business Media B.V. 2008 Abstract We studied the spatial and temporal variation in O 2 and dissolved inorganic carbon (DIC) forms concentrations in ten subtropical water- sheds located in the state of Sa ˜o Paulo, Brazil, with different degrees of impact by urbanization and land- use changes. Additionally, we used stable carbon isotopic composition of DIC to explain observed patterns. We found that land-cover changes and watershed geology are the main drivers of DIC distribution. Land-cover/use changes influence the riverine DIC in two ways: by replacing the original Cerrado 3 (C3)-type forest vegetation by C4-type vegetation composed of grasses (pasture), and by sugarcane. Most domestic sewage is dumped untreated into rivers in the state of Sa ˜o Paulo. Consequently, in the most densely populated water- sheds, sewage is an important source of labile carbon and consequently of DIC to rivers. In terms of geology, although silicate weathering that produces kaolinite is the main type of weathering in the watersheds, the weathering of carbonate cements present in the geological formations of the western portion of the state of Sa ˜o Paulo are also an important source of DIC to rivers. Keywords Dissolved inorganic carbon  Dissolved oxygen  Respiration  Rivers  Watersheds Introduction Rivers link the terrestrial carbon (C) cycle with that of aquatic ecosystems and deliver approximately 0.5 Pg y -1 of terrestrially derived organic C to the ocean. This amount is, in fact, much smaller than the total amount of terrestrial C that enters rivers, because much of the input is metabolized during transport (Cole and Caraco 2001; Richey et al. 2002). Dissolved inorganic carbon (DIC) is one of the main components of the C cycle in rivers, and its sources may be linked to terrestrial and aquatic systems (Devol et al. 1987; Richey et al. 1990; Cameron et al. 1995; Graneli et al. 1996; Helie et al. 2002). In terrestrial systems, DIC may be generated through roots and microbial respiration in soils (Cameron et al. 1995) and by aluminum silicate weathering and dissolution of carbonate minerals (Jones and Mulholland 1998; Helie et al. 2002). In aquatic systems, decay of riverine organic matter is an additional source. Disturbed watersheds could also M. S. M. B. Saloma ˜o  D. M. L. Silva  P. B. de Camargo  R. L. Victoria  L. A. Martinelli (&) CENA, Av. Centena ´rio 303, 13416-000 Piracicaba, SP, Brazil e-mail: martinelli@cena.usp.br J. J. Cole Institute of Ecosystem Studies, 65 Sharon Turnpike, P.O. Box AB, Millbrook, NY 12545-0129, USA C. A. Clemente Departamento de Cie ˆncias do Solo, ESALQ, Av. Pa ´dua Dias 11, 13418-900 Piracicaba, SP, Brazil 123 Biogeochemistry (2008) 88:271–283 DOI 10.1007/s10533-008-9210-y