Seasonal variation in organic carbon and its bioavailability (Falljökull glacier, Iceland)

Predictions regarding the export of organic carbon (OC) linked to glacier runoff remain constrained. Conventional mass balance approaches, which calculate annual OC export based on singular sampling points, overlook potential diurnal and seasonal variations in OC dynamics. In our study, we address this gap by employing high temporal resolution to systematically explore the concentration and composition of glacier-derived OC. Moreover, we examine the bioavailability of OC in glacial discharge directly at the terminus. This comprehensive investigation aims to enable accurate predictions of future OC release resulting from glacier retreat. Our chosen study site is the temperate Icelandic glacier Falljökull, part of the Öræfajökull and Vatnajökull ice cap, selected for its year-round accessibility. Our findings reveal an average concentration of dissolved organic carbon (DOC) from the glacier of 0.14 mg L-1 based on 72 streamwater samples from the glacier terminus. Seasonal variations are evident with higher concentrations measured in winter (0.19 mg L-1) compared to summer (0.10 mg L-1). While the DOC concentration was relatively low during rain and glacial melt (0.12 mg L-1), snowmelt doubled the DOC concentration (0.20 mg L-1) indicating deposition as a source of glacial DOC. Furthermore, DOC concentration in glacial melt varied on a diurnal basis with peak values during early afternoon at highest discharges. The different weather events are reflected in the glacier discharge which could be shown by comparing the isotopic signature of ice, snow, and precipitation to the isotopic signature of the discharge water during designated events. Absorbance and fluorescence measurements pointed to a predominantly protein-like, labile composition of DOC in glacial runoff. However, using incubation experiments with glacial meltwater we often found DOC values to increase pointing to the production of OC. Preliminary results highlight the seasonal and diurnal variability of glacial OC concentration and composition and the need to further study glacial OC bioavailability.