Influence of migrating streambed and stream-groundwater interactions on organic matter breakdown and oxygen consumption

Streams and rivers contribute significantly to coarse particulate organic matter (CPOM) breakdown, and thus to the global carbon cycle. However, complex questions involving abiotic drivers and their interaction with bacterial biofilm remain unanswered due to the inherent complexity of natural systems. We conducted experiments to compare oxygen consumption and CPOM breakdown between mobile vs. stationary fraction of sediments during bedform migration and under losing and gaining flow conditions. For this purpose, we incubated cotton strips under controlled flow conditions in an experimental recirculating flume packed with natural stream sediments. Oxygen consumption was measured by planar optodes while bedform migration was quantified by time-lapse imaging and bottom tracking. Bacterial biomass and metabolic activity were quantified by flow cytometery and EcoPlate assay under aerobic and anaerobic conditions. Our results indicate that oxic conditions dominated the mobile and stationary fraction of the bed under losing conditions, while only the mobile fraction was oxygenated under gaining conditions. CPOM breakdown was much higher under gaining conditions than under losing conditions or in the surface water, with no clear differences between mobile and stationary fractions. Differences in microscopic structure, diversity and metabolic activity of the biofilms from each compartment are under evaluation and are expected to assist in explaining the observed variability in decay coefficients of the cotton strips. Findings reported here help to understand the interplay between abiotic constraints and biofilms ecology during the processing of CPOM in streambeds.