Seasonal variation and interaction of photodegradation and microbial metabolism of DOC in black water Amazonian ecosystems

Abstract

Photodegradation of dissolved organic carbon (DOC) can generate labile substrates readily available for microbial consumption, thus increasing DOC removal, especially in fresh - water humic ecosystems. While a few studies have evaluated the effects of sunlight on DOC removal and CO2 production in aquatic environments, none have investigated the seasonal variation and interaction of photodegradation and microbial metabolism of DOC in a large tropical black-water river system. We present the results of experiments designed to evaluate the rates of photodegradation and subsequent microbial metabolism of DOC in the Negro River and an associated floodplain lake (Lake Tupe) in the central Brazilian Amazon Basin. Water samples collected in both environments at different phases of the river hydrological cycle were filtered and exposed to natural sunlight to estimate photodegradation; they were then inoculated with natural bacteria and incubated in the dark to evaluate bacterial metabolism. Changes in incident solar radiation and in DOC concentration and quality throughout the hydrological cycle directly affected the DOC photodegradation rates and microbial metabolism. Total DOC mineralization (photodegradation plus bacterial consumption) was more intense in the falling water period. DOC photodegradation generally stimulated further microbial DOC degradation, enhancing total DOC removal in samples exposed to solar radiation in both ecosystems. While direct photodegradation represented only a small part of the total DOC mineralization (6.7% in the high water period in the Negro River), the combined effect of photodegradation and stimulus of bacterial metabolism could account for a significant part of the CO2 production in Amazonian black water ecosystems. © Inter-Research 2013.