The quality of DOC is important to in-stream heterotrophic production, and the amount of biodegradable DOC (BDOC) regulates both microbial biomass as well as nutrient cycling. For example, carbon quality influences nitrogen transformations because heterotrophic denitrification requires a carbon source and autotrophic nitrifiers must compete for ammonium with heterotrophic bacteria that require carbon. Stream DOC is comprised of a mixture of molecules that vary in their biological lability resulting in a pool of DOC with a continuous array of lability classes. Variations in BDOC occur along flow paths, and the activation of different flow paths during storms influences the lability of DOC delivered to the stream. Our objective was to determine how changes in flow paths during storms influence in-stream BDOC. We determined storm BDOC and separated the BDOC into two different biological lability classes using plug-flow bioreactors colonized by microorganisms in stream water. The removal of DOC within the bioreactors is dominated by biological activity, and BDOC is calculated as the difference between the inflow and outflow DOC concentration. The most biologically labile DOC molecules are rapidly metabolized over short bioreactor retention times while semi-labile DOC is metabolized after longer exposures. Storm samples were analyzed for BDOC using 2 bioreactors with different empty bed contact times (EBCT) which allowed for the separation of the BDOC into labile and semi-labile classes based EBCT.
Results/Conclusions
DOC consistently increased during storms with the peak concentration more than 10-fold higher than the pre-storm baseflow concentration. The size of both DOC lability classes increased proportionally during storms compared to baseflow with 5.10 to 19.30% of the DOC in storm flow classified as labile and 27.73 to 52.91% as semi-labile. The most labile class of DOC made up on average 24.28±0.02% of the total BDOC present in stream water. The temporal pattern was dynamic with timing of the %BDOC peaks compared to the discharge peak differing between the two lability classes. The increase in BDOC observed during storm flow suggests that changes in terrestrial flow paths during storms mix hydrologic sources pools with varying BDOC concentrations. Because storm flows bypass soil control mechanisms, there are dynamic changes in BDOC in stream water during storms, and an understanding of the relationship between water flux and C flux will help to determine energy flow across the terrestrial-aquatic interface.