Application of Landsat imagery to regional-scale assessments of lake clarity

A procedure that uses Landsat imagery to estimate Secchi disk transparency (SDT) of lakes was developed and applied to similar to 500 lakes with surface areas > 10ha in the seven-county metropolitan area of Minneapolis and St. Paul, MN, USA, to assess spatial patterns and temporal trends in lake clarity. Thirteen Landsat MSS and TM images over the period 1973-1998 were used for the analysis. Satellite brightness values from lake surfaces were calibrated against available historical data on SDT (n = similar to 20-40) measured nearly contemporaneously with the acquisition date of each image. Calibration regression equations for the late-summer TM images had a range of r(2) from 0.72 to 0.93. Regression analysis for three late-summer MSS images yielded r(2) values ranging from 0.60 to 0.79. Results indicate that a single late-summer image yields a reliable estimate of regional lake clarity and reasonably accurate estimates of SDT for individual lakes. An analysis of seasonal patterns on a large lake water-quality database was used to develop a model that adjusts synoptic satellite SDT estimates from different dates to a common reference, making them more comparable from year-to-year. Analysis of long-term trends shows that in spite of the large land-use changes within the region over the study period, only 49 (about 10%) of assessed lakes in the region showed significant temporal trends in SDT over the period, and more lakes had increasing SDT (34) than decreasing SDT (15). (C) 2002 Elsevier Science Ltd. All rights reserved.