Biogeochemical cycling of PCBs in lakes of variable trophic status: A paired-lake experiment

A paired whole-lake experiment was conducted on two remote, atmospherically driven lakes in the Experimental Lakes Area (ELA) to examine the stresses of trophic condition on air-water exchange and settling fluxes of polychlor inated biphenyls (PCBs). Lake 227 (L227) and Lake 110 (L110) are similar in volume, surface area, and watershed area but differ significantly in troph ic status. The two lakes have similar access to atmospheric PCBs but eutrop hic L227 may exhibit enhanced air to water exchange due to greater biotic u ptake of dissolved PCBs. Settling fluxes of PCB were significantly greater in eutrophic L227 (40 ng m(-2) d(-1) in 1993; 29 ng m(-2) d(-1) in 1994) th an in oligotrophic L110 (22 ng m(-2) d(-1) in 1993 and 17 ng m(-2) d(-1) in 1994). Dissolved Sigma PCB concentrations were not significantly different in the two lakes (similar to 0.3 ng liter(-1)). Greater Sigma PCB settling fluxes in L227 vs. L110 coupled with similar dissolved concentrations afte r June supports the hypothesis that air-water exchange supports the water c olumn PCB concentrations. Surprisingly, Sigma PCB fugacity gradients in bot h lakes indicated that net volatilization dominated during the entire ice-f ree period, requiring another source. A Sigma PCB mass budget in the epilim nion of each lake over the stratified period showed that the major PCB loss es were due to settling and volatilization. Sigma PCB settling losses were 2.4 times greater in L227 than in L110 due to higher settling particle flux es. The mass budgets suggest that the major PCB input to these remote lakes is from the watershed. Our results indicate the intimate environmental lin kage between atmospheric, land, and aquatic PCB reservoirs.