Photochemical and microbial consumption of dissolved organic carbon and dissolved oxygen in the Amazon River system

Abstract

Bacterial and photochemical mineralization of dissolved organic matter were investigated in the Amazon River system. Dissolved oxygen, dissolved organic carbon (DOC), and bacterial growth were measured during incubations conducted under natural sunlight and in the dark. Substrate addition experiments indicated that the relatively low rates of bacterial activity in Amazon River water were caused by C limitation. Experiments to determine the photoreactivity of this biologically refractory DOC revealed unusually high rates of photochemical consumption of DOC (~4.0 μM C h −1) and dissolved oxygen (~3.6 μM O 2 h −1) in Rio Negro surface waters. In additional experiments we observed that bacterial growth and respiration were not significantly stimulated or inhibited during periods of sunlight exposure. The molar ratio of DOC to O 2 consumed during photochemical processes was close to one (1.11–1.14) in all photooxidation experiments. Sunlight exposure over 27 h showed that at least 15% of Rio Negro DOM was photoreactive. The rate of photochemical consumption of DOC was approximately sevenfold greater than bacterial DOC utilization in Rio Negro surface waters; however, integrated over the entire water column microbial remineralization was the dominant process for oxygen and DOC consumption. Photomineralization of biologically refractory riverine DOM appears to be more important than previously believed and could be a major removal mechanism for terrestrially-derived DOM in the coastal ocean.