A Radiocarbon Inventory of Switzerland’s Lakes

The Radiocarbon Inventories of Switzerland (RICH) project aims to construct the first national-scale census of (radio)carbon across aquatic, terrestrial, and atmospheric reservoirs. Within the carbon cycle, inland waters play a crucial role with lakes integrating carbon from various sources within their catchments in addition to that fixed by local primary productivity. Here we will present radiocarbon measurements of water-column dissolved inorganic carbon (DIC), dissolved organic carbon (DOC), and particulate organic carbon (POC) from 15 lakes across Switzerland covering a range of sizes, elevations, and trophic states. In addition, a year of monthly water column measurements from Switzerland’s two largest lakes - Lake Constance and Lake Geneva - reveal seasonal trends resulting from changes in productivity and river inflow. Preliminary results show that the average radiocarbon signature of DIC in both Lake Constance and Lake Geneva is depleted in ¹⁴C relative to atmospheric CO₂, indicating a ca. 15-20% contribution from bedrock weathering (¹⁴C-dead carbon). The timeseries at Lake Constance builds on earlier measurements which have shown a decline in DI¹⁴C since the late 1960s due to decreasing concentrations of bomb radiocarbon in the atmosphere. DO¹⁴C values in Lake Constance are more enriched compared to DI¹⁴C, indicating the importance of terrestrial DOC sources. In contrast, DO¹⁴C values in Lake Geneva are similar to DI¹⁴C, consistent with lake primary productivity as the main source of DOC. Overall, variations in radiocarbon values between different lakes are much greater than seasonal variations observed in either Lake Constance or Lake Geneva. These results form the basis of a radiocarbon inventory of Swiss lakes and provide new insights into carbon cycling in these dynamic aquatic systems.