Abstract
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 a and bacteriochlorophyll d+e in poly- and mesohumic lake was significantly smaller than that in the polyhumic lake. The maximum concentrations of chlorophyll a and bacteriochlorophyll d+e were observed near the bottom border or below the euphotic zone in waters with oxygen deficits and containing sulfur hydrogen. The main producers of chlorophyll in meso- and oligohumic lake were green anaerobic sulphuric bacteria.