Seasonal Patterns of Chlorophyll a and Secchi Disk Transparency in Lakes of East-Central Minnesota: Implications for Design of Ground- and Satellite-Based Monitoring Programs

ABSTRACT A data base consisting of growing-season measurements of chlorophyll a (chla) and Secchi disk transparency (SD) on lakes in the seven-county metropolitan area of the Twin Cities (Minneapolis and St. Paul, MN) was analyzed statistically to determine the consistency of seasonal patterns among lakes in the region. The data base included 370 lake-years of measurements on 145 lakes obtained between 1986 and 1997. Distinct patterns were found for both trophic variables, with maximum chla and minimum SD occurring from late July through mid-September in most lakes. Lake geometry, air temperature, and precipitation had little effect on seasonal patterns. Strong relationships (r2 values of 0.91 and 0.84) were found between average chla and maximum chla and between average SD and minimum SD for the growing season. The relationships were not affected by trophic state, lake geometry, precipitation, or temperature classes. Numbers of observations needed to estimate the growing season mean with specified relative errors were calculated for both chla and SD. Given the limitations on the frequency of data collection by both conventional means and satellite imagery, an index period of late July to mid September was found to be the optimal time to measure trophic conditions in TCMA lakes. During this period, lakes behave similarly, and in-lake variability is at a minimum for SD. Regression relationships to estimate growing season mean from 1–3 observations during this period have r2 values of 0.73–0.90 for chla and 0.83–0.90 for SD. Application of index-period lake monitoring by satellite imagery or conventional means has the potential to significantly increase the number of lakes assessed each year with minimal additional cost and effort.