Publication Summary

This report investigates the use of satellite imagery to conduct statewide assessments of lake water quality. Various statistical tests were used to evaluate the relationships between lake trophic state indicators, morphometric measures, and satellite image spectral characteristics. A multivariate model was developed for three trophic state indicators.

The statewide census of trophic state indicators predicted from the multivariate model show little change in lake Secchi transparency values between 1991 and 2000. Predicted lake chlorophyll a concentrations had increased for low productivity lakes. Predicted total phosphorus concentrations in mesotrophic and eutrophic lakes increased from 1991 to 2000.

The state of Washington is required to periodically report on statewide conditions of lake water quality under both federal and state law. The Washington State Department of Ecology had to discontinue the lake monitoring program due to fiscal constraints, and at present, there is no statewide monitoring of lake water quality. To address these legal requirements, this report investigates the use of satellite imagery to conduct statewide assessments of lake water quality. Ground observations of three trophic state indicators from lakes were compiled from existing data sources. Data from the LandSat Thematic Mapper Satellite were compiled from 1991 to 2000. These data were used to conduct various statistical tests to evaluate the relationships between lake trophic state indicators, morphometric measures, and LandSat image spectral characteristics. A multivariate model was developed for each trophic state indicator. Model performance was evaluated with an independent data set.

The statewide census of trophic state indicators predicted from the empirical models show little change in lake Secchi transparency values between 1991 and 2000. Lake chlorophyll a concentrations had increased in observations from 2000 for those lakes with previously existing low productivity conditions. Total phosphorus concentrations in both mesotrophic and eutrophic lakes increased from 1991 to 2000. Results from the predicted census of each trophic state indicator showed only slight differences between most ecoregions. The census of predicted lake trophic states revealed that a small percentage of mesotrophic lakes had become eutrophic over the ten-year period. A relatively large percentage of oligotrophic lakes have become mesotrophic in the Coast Range and Cascades ecoregions. In addition, small mesotrophic lakes and highly convoluted lakes (i.e., high shoreline development index) show an increase in productivity over this same period.