Hydrobotanical and hydrobiological field work was carried out at Lake Ladoga in NW Russia, mostly at Impilahti Bay in the northern part of the lake. In a shallow medium dense stand of Elodea canadensis (1452 plants.m-2; mean 192 g.m-2 of dry weight) in Impilahti Bay, between 12:00 to 18:00 hours in August 1996, the water was by 1.0–1.4 °C warmer and its pH 1.05–1.2 higher than open water. In the stand, pH increased almost to 9.0. In the same stand, water become supersaturated with O2 to 134% at midday on a sunny August day, and to only 105% on a cloudy day. The daily pH and [O2] fluctuations within the medium dense E. canadensis stand in Impilahti Bay were much less than those measured in dense stands of this species, e.g., in shallow eutrophic Czech fishponds. Communities of littoral phytophilous zooplankton, living pelagically or slightly attached on the plants, formed in the macrophyte stands. The littoral phytophilous zooplankton complex was on average 4 times more abundant and had a 38 times greater biomass per water volume (0.26–164.2 g dry weight.m-3) than that in the open water near the macrophyte communities (0.05–4.91 g dry weight.m-3) or was 3 times more abundant and had on average a 10 times greater biomass, respectively, than that in the open water in the middle of the bay. This does not accord with theory, which predicts that ecotones have the highest biodiversity and productivity.