Biological lability of dissolved organic carbon in stream water and contributing terrestrial sources

Abstract

Terrestrial sources of dissolved organic C (DOC) provide energy for stream microbial heterotrophs. Hydrologic conditions alter flow paths to the stream and influence terrestrial source quantity and quality. We used bioreactors to measure biodegradable DOC (BDOC) in water from soils, shallow wells, and spring seeps under baseflow conditions, overland flow during storms, and a stream during base flow and storm flow. Stream DOC concentrations ranged from 0.7 to 15.5 mg C/L and total BDOC ranged from 0.1 to 8.7 mg C/L. Under baseflow conditions, DOC was generally <2 mg C/L and BDOC was <0.7 mg C/L. BDOC made up 37.8% of DOC (8.2% labile and 29.6% semilabile constituents) and most DOC (62.2%) was recalcitrant. Storms increased DOC concentrations 6- to 12-fold and selectively mobilized BDOC increasing concentrations 8- to 27-fold. Labile and semilabile constituents increased 2-fold to 17.3% and 55.4%, respectively, so that storm flow DOC was largely biodegradable (73%). Terrestrial DOC concentrations declined from 2.15 mg C/L in soil water to 1.78 mg C/L in shallow wells to 1.26 mg C/L in spring seeps, and labile BDOC concentrations declined from 0.1 mg C/L in soil water to 0.04 mg C/L in shallow wells but increased to 0.13 mg C/L in spring seeps. Overland flow waters had elevated DOC (11.0 mg C/L) and labile BDOC (1.7 mg C/L) concentrations. Peak storm concentrations for 2 biological reactivity classes were coincident during 2 storms but divergent during another storm, suggesting temporal variation in hydrologic pathways. Our results demonstrate that storms deliver increased BDOC loads to a headwater stream and reveal the temporal variability of labile and semilabile BDOC constituents. Storms increase stream depth and velocity, resulting in increased uptake lengths for BDOC constituents, exporting them from the basin and probably providing a downstream subsidy.