Glacier-fed streams (GFS), as a special type of headwater streams, play an important role in the carbon cycle through the sequestration, transformation, transport, and mineralization of organic carbon. Moreover, dissolved organic carbon (DOC) impacts downstream life and is critically important for river food webs. DOC constitutes an important nutrition source for aquatic microorganisms which form the basis of lotic food webs. This thesis aimed to fill a knowledge gap on the bioavailability of DOC in glacier-fed streams during winter. Using water samples from 14 different GFS and 4 alpine streams without glacier influence, I investigated the bio-available fraction of the dissolved organic carbon (BDOC).