During collection of pollutants having a density lower than that of water and carried by a surface layer of a body of water, water of the surface layer is caused to flow into and through a collection vessel having a separation compartment with a top wall, pollutants entrained by the inflowing surface layer water are allowed to collect gravimetrically as a supernatant layer carried beneath the top wall of the separation compartment on water in the separation compartment, and changes of the weight of the collection vessel in the body of water are monitored. Intake and discharge phases may be initiated and terminated to in response to the said weight reaching predetermined values.