David J Holtschlag

This report describes an investigation of visible light reflectances from Torch Lake in Antrim County, Michigan. The oligotrophic lake is the largest inland lake in Michigan by volume and the second largest by surface area. Local residents have expressed concern that a recent, on-going proliferation of golden-brown algae may be impacting the water quality and aesthetics of the lake. This report examines the seasonal and long-term variations in reflectance intensities of visible blue, green, and red light bands measured by the Landsat series of satellites from 1984 through 2023. To provide a basis for assessing possible changes, a comparable analysis was conducted for two regions having water depths greater than 100 m in Grand Traverse Bay, which lie just west of Torch Lake. Results for Torch Lake indicate that the seasonal response for the more photosynthetically active blue and green bands were similar to each other, both having a secondary peak in late August. The amplitudes of these seasonal responses were greater for water depths greater than 50 feet. In contrast, the red band for Torch Lake had a minimum in late August, which is comparable with the late August minimums in all three bands in Grand Traverse Bay. Minor seasonal differences were detected between water depths in the red band. The local levels for the blue and green bands of Torch Lake generally follow a sigmoid curve, which maintain a plateau of positive values until about 2010. After 2010, levels decline precipitously, especially in the blue band, before leveling off in 2020. Amplitudes of these trends vary more at shallower water depths. Local levels in the red band of Torch Lake and all bands of Grand Traverse Bay tend to decrease sinusodially over time. Differences between seasonal and trend responses between bands and regions of interest may indicate differences in underlying physical, chemical, or biological processes.