Radiocarbon evidence of river transport and food web uptake of old carbon in Lake Athabasca, Canada

Rivers are major conduits that connect terrestrial carbon cycling with downstream receiving environments. In large lakes, the utilization of river-transported carbon in biological production remains poorly constrained. Lake Athabasca, the eighth largest lake in Canada, receives inflow from the Athabasca River and a large boreal wetland, the Peace-Athabasca Delta. Radiocarbon (¹⁴C) was measured to trace the transport of old carbon into western Lake Athabasca and its influence on a modern lake food web by distinguishing the ages of river and lake carbon pools. The fraction of modern carbon in surface sediments showed river inputs of old organic carbon extended at least 50 km into the lake. Old carbon was incorporated into the food web via surface water plankton, and it subsidized ~ 20-50% of carbon in vertebrate consumers, specifically fish and aquatic-feeding birds. These measurements of old carbon showed extensive river influence in western Lake Athabasca (likely > 1500 km²) and high connectivity of river loads with biological uptake.