Are drifting algal mats conquering the bottom of the Archipelago Sea, SW Finland?

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 an d SCUBA-diving. The study covers depths from the shore down to 50 meters. T he algal masses were described and classified according to the thickness of the mass and possible occurrence of anoxia (smell of hydrogen sulfide, bla ck colored sediment), i.e. indicators of the masses potential different eco logical impacts. Three different types of drifting algal masses were distin guished: 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 m attresses. The phenomenon was most severe in the outer archipelago of the S E 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 stud y we became convinced that the UW-video system is a practical tool for stud ying underwater loose-lying algal masses.