0 1993, by the American Society of Limnology and Oceanography, Inc. Carbon dynamics and sources in the Paran River

The Parana River was sampled and analyzed for carbon and mineral concentrations at the Parana-Santa Fe (Argentina) cross-section (-600 km above the mouth) between March 198 1 and November 1984. Within this period, the Parani was affected by an extraordinary flood caused by the 1982 El Niiio/ Southern Oscillation (ENSO) climatic anomaly. Under such conditions, with a flow 75 % higher than the long-term mean, the river exported a total organic C (TOC) load of 8.43 Tg yr- l(3.0 g m-2 yr-I) of which 89 % was accounted for by dissolved organic C (DOC) and the remainder by particulate organic C (POC). Under normal hydrological conditions, however, the ParanP has a TOC load that is roughly half of that determined during the ENSO-triggered flood (4.43 Tg yr-I, 1.6 g m-2 yr-I), but with a very different DOC: POC ratio (63: 37). In July-August 1985 the Parani was sampled along a reach of over 1,300 km. At the time of sampling, the Paraguay River supplied-68 % of the POC load, 59 % of the labile particulate load (LPOC), and 7 1% of the DOC entering the middle reach of the Parani. Carbohydrates in LPOC appear to become more abundant than amino acids in the lower reaches, probably due a significant input from the floodplain of the ParanL. Understanding the temporal and spatial changes of the chemistry and the biology of major river systems- the biogeochemical dy-namics-is a significant step toward compre-hension of the circulation of matter and energy in natural systems. Many papers published ’ To whom correspondence should be addressed