Using sediment geochemical records to infer past lake water total phosphorus concentrations at a site with high internal P loading, Lake Søbygaard, Denmark

In lakes where phosphorus (P) supply is dominated by external loads, long-term mean lake water TP concentrations can be successfully reconstructed from sediment P profiles and dating using the SI-TP (Sediment Inferred lake water Total Phosphorus) mass balance model. However, it has not yet been shown that the model is applicable at lakes with high internal P loading, where sediment diagenesis releases P to the water column and degrades the sediment P record. Here we apply the SI-TP model to Lake Søbygaard in Denmark, a shallow lake with exceptionally high internal P loading for several decades since the external loading was reduced by 80-90% in 1982. Our aim was to test the impact of this sediment degradation on P record integrity. Using six published sediment P records collected over the period 1985 to 2004, we applied the model without site specific calibration. Degradation of the sediment P record by diagenesis was observed, and occurred at a rate consistent with theoretical models, offering a potential approach to correct for degradation Even neglecting degradation, the resulting sediment inferred lake water TP (SI-TP) record was found to be similar in magnitude and temporal trend to the corresponding monitored TP values. From this test of SI-TP model performance at Lake Søbygaard we conclude that the model has the potential to be more widely applied at lakes with high internal P loading.