Source-driven variability in dissolved organic carbon across a proglacial floodplain as a space- for time analogues of future carbon dynamics

Proglacial floodplains are highly heterogeneous braided river systems in which glacier melt, groundwater, and snowmelt-fed tributaries interact over meter-to-tens-of-meters scales. This pronounced physicochemical heterogeneity among water sources generates contrasting hydrological regimes, benthic microbial communities, and water chemistry, resulting in strong spatial variability in biogeochemical processes. Confluences within these networks connect channels with distinct source signatures and microbial assemblages and may function as biogeochemical hotspots that disproportionately influence organic matter processing at the network scale. As rapid glacier retreat alters the relative contributions of meltwater, groundwater, and subglacial flows, proglacial floodplains offer valuable space-for-time analogues to investigate future shifts in carbon dynamics in alpine catchments.

Here, we investigated dissolved organic carbon (DOC) dynamics in a proglacial floodplain dominated by three contrasting water sources: a clean-ice glacier, a talus/rock glacier, and a groundwater spring. We hypothesized a transition from conservative transport or DOC consumption in glacier-fed streams to DOC production in streams influenced by talus/rock glacier and groundwater inputs, driven by differences in physicochemical conditions (e.g., turbidity, nutrients, temperature) and associated biological activity. Additionally, we aimed to quantify the net carbon balance of the floodplain at the system scale.

We sampled the three main water sources and 14 nodes across the braided network, with particular emphasis on major confluences. End-member mixing analysis (both EMMA/EEMMA) was applied to quantify source contributions, and differences between observed and expected DOC concentrations were evaluated. We used the percent differences between measured and predicted values to determine whether a stream segment functions as a DOC sink or source. Daily DOC loads were calculated at the floodplain outlet to assess net system functioning.

Our results revealed pronounced spatial variability in carbon dynamics associated with dominant water sources. Clean-ice glacier-dominated nodes were characterized by high discharge, elevated turbidity, and turbulent flow, and generally acted as DOC sinks. In contrast, nodes influenced by talus/rock glacier and groundwater inputs exhibited hydrological stability and functioned as DOC sources. Temporally, sink-source behavior shifted between early and late melt season conditions. Despite pronounced spatial and temporal variability, seasonal net DOC load at the outlet was close to zero, indicating that carbon behaved conservatively at the floodplain scale and reflecting the offsetting contributions of coexisting sink and source streams within the floodplain. Taken together, our results suggest that continued glacier retreat will promote a transition toward more hydrologically stable channels with enhanced carbon production. Such a shift is expected to reduce the prevalence of DOC sink behavior and increase the role of proglacial river networks as net carbon sources, with important implications for downstream carbon exports in future alpine catchments.