Bacterial carbon metabolism in the Amazon River system

Abstract

Bacterial abundance and production and community respiration were measured at several mainstem and tributary stations in November‐December 1988, April–May 1990, and August–September 1991 in a 1,950‐km reach of the Amazon River between Vargem Grande and Obidos, Brazil. Bacterial abundances averaged 1.1 × 109 cells liter‒1 in the mainstem during the three cruises, and rates of heterotrophic bacterial production and community respiration averaged 1.16 µg C liter‒1 h‒1 and 0.61 µM O2 h‒1, respectively. Bacterial production and respiration were carbon limited in the mainstem, indicating that the bulk of the relatively abundant particulate and dissolved organic matter was of limited bioavailability. Spatial variability in measured parameters was minimal even though tributary and mainstem stations included waters with distinct physical and chemical features. Blackwater and whitewater rivers supported similar bacterial abundances and rates of bacterial production and respiration. Strong seasonal patterns of variability were evident; maximal bacterial growth rates and minimal respiration rates were measured during high water stages of the hydrograph. Estimates of bacterial growth efficiencies ranged from 4 to 55%, with consistently higher values measured during high water. It appeared that enhanced bacterial growth efficiencies during periods of high water resulted from increased bioavailability of substrates derived from the extensive varzea (floodplain).