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The influence of algae bloom on microwave radar backscattering from surface wind waves and on gravity-capillary wave damping was studied experimentally in field experiment on the Gorky Water Reservour and in laboratory. It was obtained that the intensity of X-band radar backscattering decreased with phytoplankton concentration in the upper water layer. Samples of phytoplankton from the upper water layer and of biogenic film from the water surface were collected nearly simultaneously and co-located with radar probing of surface waves. Concentrations of main phytoplankton constituents as well as total algae concentration and their variability in the studied area were measured. Laboratory studies of damping of gravity-capillary waves on the water samples with removed film and on clean (without plankton) water covered with the biogenic films were carried out, and an effective water viscosity and the film elasticity values were retrieved. The increase of water viscosity and of film elasticity in the presence of algae were concluded to be the main physical reasons of enhanced wave damping, the contributions of the two effects in total wave damping and correspondingly in radar backscatter depression appeared to be comparable to each other. Since both the effective viscosity and the film elasticity depend on algae concentration the latter can be estimated from measurements of radar backscatter.