Seasonally Dynamic Dissolved Carbon Cycling in a Large Hard Water Lake

Abstract 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 ( 14 C) a particularly powerful tool due to the large range in 14 C characteristics among carbon sources. Here, we present 14 C 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 14 C‐depleted relative to the atmosphere primarily due to the dissolution of carbonate rocks in the lake's catchment. Variability in DI 14 C is largely tied to the Rhône River inflow, where DI 14 C values were also found to vary seasonally. DOC has a 14 C signature similar to that of DIC, reflecting the fact that much of the lake DOC pool is autochthonous. However, more 14 C‐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.