Improved reference values for phosphorus in Swedish agricultural lakes - Lake sediment records of historical concentrations in Sweden and near-by countries

This report reviews the paleolimnological methods that can be used to reconstruct past lake-water total phosphorus (TotP) in Swedish agricultural lakes, including available data sets and data set management techniques. It also summarizes the results of five studies of eleven Swedish lakes and results from similar studies in near-by countries.The results highlight the complexity of the long-term effects of human land use. Firstly, the onset of agriculturally driven TotP increase varies from the medieval period to the 20th century, with some lakes apparently unaffected (with respect to TotP at least) by agriculture in their catchments. Secondly, the impact of some preindustrial agriculture was significant. If we assume a reference condition with no human impact, some lakes were likely at a moderate or worse ecological status already before 1850 CE due to preindustrial agriculture. However, the number of Swedish lakes with TotP-reconstructions is low compared to some near-by countries and these results are anecdotal.We suggest that further studies are carried out, using short sediment cores (i.e. ~50 cm) on a larger number of lakes. While short cores would only cover the last few hundred years, they would provide more data on preindustrial TotP levels and the development through the industrial period which could be the basis of more realistic reference values for phosphorous in an agricultural landscape