A light (PAR) extinction coefficient is calculated for the water column for the eleven primary lakes (Allequash, Big Muskellunge, Crystal, Sparkling, and Trout lakes, unnamed lakes 27-02 [Crystal Bog] and 12-15 [Trout Bog], Mendota, Monona, Wingra, and Fish) and two additional lakes near Madison, Wisconsin (Waubesa and Kegonsa). Data exists only for the Trout Lake-area lakes through 2018. Beginning in 2019, the Madison-area lakes were added. The fraction of surface light is computed at 0.25-m to 1-m depth intervals depending on the lake. The light (PAR) extinction coefficient is calculated by regressing ln(fraction of light(z)) on depth z. Sampling Frequency: fortnightly during ice-free season - every 6 weeks during ice-covered season Number of sites: 13