Seasonal variability of dissolved organic carbon in a Mediterranean stream

The seasonal variability of dissolved organic carbon (DOC) flux in a Mediterranean stream subjected to discharges of wide range of intensities and variable dry period was studied as a function of the hydrologic conditions, and the relationship between surface and subsurface (hyporheic and groundwater) DOC concentration. DOC concentration in stream water (2.6 mg l(-1) +/- 1.5 SD) was higher than groundwater (1.3 mg l(-1) +/- 1.2 SD) and lower than hyporheic water (3.8 mg l(-1) +/- 1.7 SD), suggesting that, at baseflow, stream DOC concentration increases when groundwater discharges through the hyporheic zone. Storms contributed to 39% of annual water export and to 52% of the total annual DOC export (220 kg km(-2)). A positive relationship was observed between Discharge (Q) and stream DOC concentration. Discharge explained only 40% of the annual variance in stream DOC, but explained up to 93% of the variance within floods. The rate of stream DOC changes with discharge change during storms (dDOC/dQ), ranged between 0 and 0.0045 C mg l(-1) s l(-1), with minimum values during Spring and Summer, and maxima values in Fall and Winter. These dynamics suggest that storm inputs of terrigenous DOC vary between seasons. During floods in the dormant season, DOC recession curves were always steeper than discharge decline, suggesting short flushing of DOC from the leaching of fresh detritus stored in the riparian zone.