Usefulness of natural regions for lake management: analysis of variation among lakes in northwestern Wisconsin, USA

Lake management applications of a map of summer total phosphorus in approximately 3000 lakes in Minnesota, Wisconsin and Michigan were evaluated using data collected in springtime 1988 for 210 lakes in a northwestern area of Wisconsin that overlapped 4 regions of the phosphorus map area. Lake quality data was depicted spatially using a geographic information system with colour-coded dot marking, and the distribution of lakes in each phosphorus class supported the use of ecoregions for mapping. Phosphorus concentration distributions obtained from springtime data resembled those obtained from historical summer data for the entire region more closely than those from adjacent regions although springtime values were lower than summer values in eutrophic lakes. Spatial patterns of phosphorus concentrations were correlated with those of other variables indicating trophic state, and regional lake phosphorus boundaries required only 2 minor modifications. The strength of associations between catchment characteristics, lake morphology and lake phosphorus concentrations varied considerably between and within regions. Lake phosphorus maps could be valuable for local lake management and for basing lake quality expectations on lake quality patterns within natural ecoregions.