Iron Budgets in Temperate Lakes

Abstract

Iron mass balances for eight lakes (79 lake-years of data) on the Precambrian Shield were measured including catchment, atmospheric, and internal sources of iron and compared with literature data. In most cases, internal load from anoxic sediment surfaces was important. Net (R_ext) and gross (R_tot) Fe retentions were correlated with internal load and with morphometry and hydrology, respectively. An average annual settling velocity of Fe computed from the relationship of R_totwith the annual water load (q_s) ranged from 12 to 26 m/yr. A simple mass balance model, similar to that used successfully for phosphorus, was used to predict annual average Fe concentration. Although model predictions of average Fe concentration of the lake outflow (L_out/q_s) and observed concentrations (TFe) were significantly correlated, TFe was underestimated by the model in the Precambrian Shield lakes but overestimated in acid-stressed lakes in the Sudbury, Ontario region. Since the deviations were correlated with organic content of the water or sediment, dissolved organic carbon concentration (DOC), colour of the lake water, and loss on ignition of the surface sediments proved to be important secondary variables: the best predictive model was TFe = −43.9 + 0.81 L_out/q_s + 30.0 DOC.