Fate and distribution in sediments of carbon- 14 added to Canadian Shield lakes of differing trophic state

C was added to three lake basins of differing trophic state at the Experimental Lakes Area in northwestern Ontario in 1976 and 1978 to study primary production and gas exchange with the atmosphere. This study was conducted to determine the long-term fate of 14C and to use its observed distribution in sediments to evaluate the predictions of the mud depositional boundary depth (MDBD) model. The eutrophic north basin of Lake 226 initially retained 29% of the added 14C, although mineralization reduced this to ~20% within 2 yr. Sediment focusing redistributed 14C within the basin for 2-7 yr following the addition. After 11 yr, Lake 226N retained - 18% of the original 14C, with 13% of that remaining in erosional areas of sediment and 87% in transitional or depositional areas. Although the mesotrophic south basin of Lake 226 and oligotrophic Lake 224 retained less (each -7%) of the initial 14C additions, its distribution was similar to that in Lake 226N. The MDBD model, which describes sediment sites as erosional, transitional, or depo- sitional based on site exposure, water depth, and sediment slope, correctly classifies the sediments of these lakes.