Predictive regression relationships between concentrations of total phosphorus (P) in near-surface waters (1m) and water-quality characteristics were developed for lakes in the Puget Sound region. Nitrogen-to-phosphorus ratios support the strategy of emphasizing total P for lake eutrophication assessment within the range of lake conditions represented. Relationships based on summer total P concentrations, rather than winter-spring total P, led to more successful prediction of mean summertime chlorophyll a concentrations and Secchi-disc transparencies. The 78 lakes evaluated in the study had an average mean concentration of total phosphorus in near-surface waters of 22 ?g/L during the summer with mean concentrations for individual lakes ranging from 7 to 76 ?g/L. For regression relationships based on summer total phosphorus concentrations, the standard error of predicted Secchi-disc transparencies is ?0.9 meters for phosphorus concentrations of 15 ?g/L or greater, and the standard error of predicted chlorophyll a concentrations is ?5 ?g/L. Predictions for the range of conditions evaluated favorably agreed with predictions from similar relationships developed by other researchers for different lakes. To supplement estimates of transparency and chlorophyll a from regression relationships, a broader system of water-quality groups, including recreational suitability and fish habitat quality, was developed from published information and our experience in the region.
Additional Publication Details
USGS Numbered Series
Relationships between water quality and phosphorus concentrations for lakes of the Puget Sound region, Washington