Understanding variation in trophic status of lakes on the Boreal Plain: A 20 year retrospective using Landsat TM imagery

There is a critical need for spatially and temporally extensive information on the trophic status of lakes to assist in scientifically sound forest management decisions. To meet this need, this study examined the utility of Landsat TM imagery in deriving indicators of trophic status in remote and relatively undisturbed lakes on the Boreal Plain of northern Alberta. Based on data collected during a survey of lakes in 2001, normalized exoatmospheric reflectance values of the red band explained 68, 82, and 47% of the variance in chlorophyll a, turbidity, and Secchi disk depth, respectively. To understand the natural variation in trophic status in the lakes, we applied the linear regression equations to images collected during late summer (i.e., August) for 18 out of 20 years from 1984 to 2003 and performed a two-way analysis of variance to decompose the total variation into space, time, and space × time interaction factors. We found that temporal factors accounted for 10% and spatial factors accounted for 50% of the total variation in trophic status, with a minimum of 10 years and 20 lakes needed to reach stability in the contributions of these factors. This study suggests that regional factors that are external to the lake explained the majority (60%) of the variation in trophic status of the lakes.