BACTERIAL CARBON METABOLISM IN THE AMAZON RIVER SYSTEM

Bacterial abundance and production and community respiration were meas ured at several mainstem and tributary stations in November-December 1 988, April-May 1990, and August-September 1998 in a 1,950-km reach of the Amazon River between Vargem Grande and Obidos, Brazil. Bacterial a bundances averaged 1.1 x 10(9) cells liter(-1) in the mainstem during the three cruises, and rates of heterotrophic bacterial production and community respiration averaged 1.16 mu g C liter(-1) h(-1) and 0.61 m u M O-2 h(-1), respectively. Bacterial production and respiration were carbon limited in the mainstem, indicating that the bulk of the relat ively abundant particulate and dissolved organic matter was of limited bioavailability. Spatial variability in measured parameters was minim al even though tributary and mainstem stations included waters with di stinct physical and chemical features. Blackwater and whitewater river s supported similar bacterial abundances and rates of bacterial produc tion and respiration. Strong seasonal patterns of variability were evi dent; maximal bacterial growth rates and minimal respiration rates wer e measured during high water stages of the hydrograph. Estimates of ba cterial growth efficiencies ranged from 4 to 55%, with consistently hi gher values measured during high water. It appeared that enhanced bact erial growth efficiencies during periods of high water resulted from i ncreased bioavailability of substrates derived from the extensive varz ea (floodplain).