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 determined by two independent approaches. First, Fe accumulation was calculated in cores collected from several sites in each lake by integrating Fe concentration profiles with sediment accumulation rates determined from Pb210 dating. These site-specific accumulation rates were corrected for sediment focussing so that whole-lake Fe accumulation values could be derived. Using this approach, recent whole-lake Fe accumulation in eight lakes ranged between approximately 750 and 4000 mg/m2 per year. Second, whole-lake Fe accumulation was estimated from lake mass budgets, which were measured over a maximum of 14 years. Accumulation measured using the mass balances ranged from 10 to 1330 mg/m2 per year. Comparison of the two approaches indicated that retentions calculated from the sediment cores were much greater than those estimated from the mass balances. The most likely explanation for this difference is that, in the two decades since the cores were collected, there has been a substantial decline in Fe retention (in mass units but not percent) in the study lakes, principally as a result of reduced inputs of Fe from the catchments.