Relationships between water quality and phosphorus concentrations for lakes of the Puget Sound region, Washington

Abstract

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.