Stream and interstitial water DOC of a gravel bar (Sitka stream, Czech Republic): characteristics, dynamics and presumable origin

Temporal changes in dissolved organic carbon (DOC) concentration, biodegradability, molecular weight and optical properties within both the stream and interstitial waters of a gravel bar were assayed in a small lowland stream. The mechanisms responsible for the observed quantitative and qualitative changes are discussed with reference to stream DOC (SDOC) and interstitial DOC (IDOC) sources. Both stream and interstitial waters showed similar temporal patterns of DOC concentrations, biodegradability and molecular weight of humic substances, but different temporal structural patterns of DOC molecules during the year. As was noticeable from annual UV-absorbance characteristics and from biodegradable DOC (BDOC) concentrations, SDOC contained more saturated chemical structures, was less bioreactive and was characterized by a lower molecular weight of humic substances, than IDOC. The character of SDOC suggests that it was composed of diagenetically old organic material which is resistant to microbial uptake and DOC from autochthonous production. The IDOC seemed to be diagenetically younger, and to originate more from allochthonous sources, particularly in the upwelling zone. Besides the difference between SDOC and IDOC quality, we have also found differences in IDOC quality between the downwelling and the upwelling zone, suggesting that decomposition of buried particulate organic matter (POM) was the main source of IDOC.