Lake catchment characteristics and external P load — cultivated area/lake area ratio as a tool for evaluating the risk of eutrophication from land use information

The effects of land use on external P load (LP), critical P load (LC) and on LP/LC of 27 lakes in Finland were examined with regression analyses. Catchment area to lake area ratio (AC/AL), cultivated catchment area to lake area ratio (AF/AL) and the percentage of cultivated land in the catchment (F%) were used as independent variables. We studied whether these parameters can be used to estimate the risk of exceeding the critical P load (LP/LC > 1). All three parameters had a positive effect on LP and LP/LC. Compared with AC/AL and F%, AF/AL had a higher coefficient of determination in predicting LP and LP/LC. This was because AC/AL does not include land use variations in catchments and F% does not include information on catchment size, while AF/AL includes information on both. Critical P load was exceeded when AF/AL > 1.63. AF/AL can be a useful and simple tool for evaluating the risk of lake eutrophication.