In late June 1983, Secchi depth d was 3.5 m and the vertical attenuation coefficient K was ≤ 0.5 m−1 in Otisco Lake. By 2 August, near the peak of a “whiting,” the values were 1.4 m and 0.64 m−1, and calcium‐containing, acidlabile particles >4 µm comprised ∼ 65% of total analyzed inorganic particles. From measured values of K and of the reflectance ratio R, the volume‐scattering coefficient b was calculated to be 2.88 m−1 and the volume absorption coefficient a, 0.31 m−1. Turbidity measurements confirmed the value of b and indicated ∼ 60% of the value was due to CaCO3. Of the value of a, water, Gelbstoff, and chlorophyll‐bearing particles contributed 54, 25, and 21%, respectively. The observed changes in d and K, from June to August, are theoretically consistent with an approximate doubling of b (due to CaCO3), with little or no change of a.