Prediction of reference phosphorus concentrations in Swedish lakes.

The relationship between total phosphorus (TP) and chemical, climatic, morphological, and geographic variables was examined for over a thousand reference lakes across Sweden. A significant relationship was found between TP and both absorbance of irradiance (at 420 nm, filtered) and altitude for all lakes. These two variables alone, however, were not able to adequately predict TP concentrations in naturally turbid lakes. A natural particulate matter factor (PM(n)) was developed as part of this study to incorporate the effect of natural suspended matter on lake phosphorus concentration. Variability in TP concentration was well explained with the addition of PM(n) to the model (R(2) = 0.71) even though conditions external and internal to the lakes varied greatly. The ability of the three parameter model to identify culturally eutrophic systems was then successfully tested using a data set of lakes with known anthropogenic phosphorus loads. Thus, the model allows for estimation of reference TP concentration and by extension can be used to identify when a reference concentration has been exceeded due to anthropogenic phosphorus loading. The model output also provides a realistic end point to which phosphorus concentrations should be reduced to achieve a natural trophic state in both clear water and naturally turbid lakes.