Comparison of iron accumulation in lakes using sediment core and mass balance calculations

The accumulation of iron (Fe) in several lakes in Ontario, Canada was deter mined by two independent approaches. First, Fe accumulation was calculated in cores collected from several sites in each lake by integrating Fe concen tration profiles with sediment accumulation rates determined from Pb-210 da ting. These site-specific accumulation rates were corrected for sediment fo cussing so that whole-lake Fe accumulation values could be derived. Using t his approach, recent whore-lake Fe accumulation in eight lakes ranged betwe en approximately 750 and 4000 mg/m(2) per year. Second, whole-lake Fe accum ulation was estimated from lake mass budgets, which were measured over a ma ximum of 14 years. Accumulation measured using the mass balances ranged fro m 10 to 1330 mg/m(2) per year. Comparison of the two approaches indicated t hat retentions calculated from the sediment cores were much greater than th ose estimated from the mass balances. The most likely explanation for this difference is that, in the two decades since the cores were collected, ther e has been a substantial decline in Fe retention (in mass units but not per cent) in the study lakes, principally as a result of reduced inputs of Fe f rom the catchments. (C) 2001 Elsevier Science B.V. All rights reserved.