Bioavailability of dissolved organic carbon in Icelandic glacial streams changes seasonally and with distance from the glacier

Glaciers impact carbon cycling in downstream ecosystems by releasing diverse and bioavailable dissolved organic carbon (DOC). However, our understanding of organic carbon (OC) dynamics in Icelandic glaciers remains limited, as most studies have focused on other glacial regions and often lack seasonal-scale resolution.

In this study, we investigate the bioavailability of glacial OC from Icelandic streams using incubation experiments. We sampled Virkisá on a seasonal scale (a total of 72 incubation experiments) and supplemented these samples with additional data from the glacial streams Skaftafellsá, Svínafellsá, Kvíárjökull, and Fjallsá for comparison. In the glacial stream Virkisá, DOC concentrations were highest in spring at the onset of the melt season (0.18 ± 0.11 mg/L) and lowest in autumn (0.08 ± 0.02 mg/L). Notably, we observed not only seasonal variability in DOC concentrations but also in the bioavailability of glacial OC. At the glacier outlet, DOC bioavailability was consistently negative throughout the year (-18.18%), indicating DOC production during incubation experiments. Similarly, negative BDOC values (ranging from -1.44% to -24.1%) were confirmed in four other glacier-fed streams during summer, discharging from the ice cap Öræfajökull. However, further downstream, incubation experiments revealed seasonal shifts: negative bioavailable DOC (BDOC) values in spring (-18.04% at 900 m from the glacier outlet) and positive values in summer (55.55% at the same site), likely reflecting increased biological activity and DOC consumption during summer.

Overall, BDOC values showed a positive correlation with distance from the glacier. At the furthest sampling point, 3000 m from the glacier outlet, BDOC averaged +8.21% in spring and 57.02% in summer. These findings challenge previous reports of high glacial OC bioavailability and underscore the need for a more in-depth understanding of the chemical and biological processes in glacier-fed streams, particularly at a seasonal scale—a factor often neglected in studies due to the difficult accessibility of glaciers during winter.