Seasonally Dynamic Dissolved Carbon Cycling in a Large Hard Water Lake

Inland waters play a crucial role in the global carbon cycle, with lakes integrating carbon from various sources within their catchment in addition to that fixed by local primary productivity. Isotopic measurements of carbon pools can differentiate contributions from these sources, with natural abundance radiocarbon (¹⁴C) a particularly powerful tool due to the large range in ¹⁴C characteristics among carbon sources. Here, we present ¹⁴C measurements of dissolved inorganic carbon (DIC) and dissolved organic carbon (DOC) from monthly water column samplings over the course of a year in Lake Geneva, a large oligotrophic hard water perialpine lake in Western Europe. We find that DIC in the lake is significantly ¹⁴C-depleted relative to the atmosphere primarily due to the dissolution of carbonate rocks in the lake's catchment. Variability in DI¹⁴C is largely tied to the Rhône River inflow, where DI¹⁴C values were also found to vary seasonally. DOC has a ¹⁴C signature similar to that of DIC, reflecting the fact that much of the lake DOC pool is autochthonous. However, more ¹⁴C-depleted DOC was observed in July and tied to increased river discharge from snow and glacier melt within the upper Rhône River basin. These observations shed light on carbon sources and dynamics within Lake Geneva and its alpine catchment and highlight the importance of preaged dissolved carbon inputs to the largest natural lake in Western Europe.