A comparison of phosphorus retention in lakes determined from mass balance and sediment core calculations

Phosphorus retention (TP_acc) was determined in 7 lakes in Ontario, Canada from mass balances and from the direct measurement of phosphorus accumulation in the sediments. Phosphorus retentions determined using the mass balance technique ranged between 20 ± 4.0 and 95 ± 23 mg/m²/yr and were slightly lower than those calculated by multiplying sediment total phosphorus (TP) concentrations by sediment accumulation rates (49 ± 14 to 148 ± 37 mg/m²/yr). The lower TP_acc values measured using the mass balance approach may result from the underestimation of external TP inputs. Alternatively, it is suggested that in the more acidic lakes, a decrease in TP concentrations in the lakewaters and/or a decrease in TP export from the catchment may have resulted in a decrease in TP_acc through time. Thus, relatively higher TP_acc values would be calculated using the sediment approach (since TP_acc values calculated from the sediment data are averaged over a greater number of years than those calculated from the mass balance data).Provided several cores are collected from the lake, the direct measurement of sediment TP and sediment accumulation rates for estimating TP_acc may be a suitable alternative to mass balances, especially in situations where it is not possible to measure all inputs of TP to the lake.