PHOSPHORUS LOADING TO INLAND LAKES

Understanding the cycling of phosphorus in the environment is important to reduce contamination and possible eutrophication of inland lakes. Lake sediment cores can record historical and current loading trends but, due to diagenetic reworking of phosphorus in lake sediment it is debatable whether environmental records accurately capture historical loading. We questioned whether paleo-phosphorus cycling could be interpreted from lake sediments under certain conditions. Therefore, we measured total phosphorus concentrations in dated sediment cores from 17 inland lakes of Michigan. Using a multi-element approach, three distinct loading trends of phosphorus were evident. First, phosphorus concentration and accumulation rate profiles that were similar to terrestrial elements suggested a largely watershed source. Secondly, phosphorus concentrations that followed sediment iron suggested a diagenetic reworking of phosphorus in the sediment. This was confirmed using porewater metal concentration profiles. Lastly, phosphorus and calcium concentration and accumulation rate profiles that were similar indicated an in-lake process. Changes in land use/land cover (i.e. deforestation) resulted in large scale changes in phosphorus loading to most lakes. However, in some lakes the sediment phosphorus record was unaffected by land use change. In these cases in-lake processes appeared to have influence on the sediment record, specifically calcium carbonate precipitation. These results suggest that the sediment phosphorus record can be interpreted and when combined with the multi-element approach can provide significant insight into loading of phosphorus to inland lakes.