The diel variability of algal and bacterial chlorophyll and the physicochemical parameters (temperature, pH, dissolved oxygen, conductivity, transparency) in the vertical profile of three mid-forest humic lakes in Drawa National Park (northwest Poland) were analyzed in summer. The euphotic zone consisted of the smaller part of the water column in poly- and mesohumic lakes but almost the whole water column of the oligohumic lake. The 24-hour dynamics of chlorophyll