The occurrences of drifting algal masses were mapped in the Archipelago Sea, Northern Baltic Sea, during the summers of 1996–1997 using an UW-video and SCUBA-diving. The study covers depths from the shore down to 50 meters. The algal masses were described and classified according to the thickness of the mass and possible occurrence of anoxia (smell of hydrogen sulfide, black colored sediment), i.e. indicators of the masses potential different ecological impacts. Three different types of drifting algal masses were distinguished: cover, mat, and mattress. The succession of the masses was studied during one growing season. First, the algae aggregate on the sea floor as a thin cover. Later they start to drift downwards along the sloping bottom, developing into partly anaerobic mats and further into totally anaerobic mattresses. The phenomenon was most severe in the outer archipelago of the SE corner of the study area. Factors contributing to the growth of ephemeral algae, which result in drifting algal mats, include high nutrient loading, good water transparency, and appropriate bottom substrate. During the study we became convinced that the UW-video system is a practical tool for studying underwater loose-lying algal masses.