Overwintering of benthic vegetation outside the Olkiluoto nuclear power station, west coast of Finland

The effect of thermal discharges on the overwintering of the phytobenthos was studied at five permanent sampling sites outside the Olkiluoto nuclear power station (Finnish coast of the Bothnian Sea) in 1985—1986. The increased illumination in the ice-free area in winter and the direct influence of the temperature rise caused a significant advance in the start of the growing period in many species and excessive occurrences of diatoms. Submerged green shoots of Phragmites australis began to grow in the discharge channel of the power station in January, and by early June a well-developed stand had formed with new aerial stems. From February to April stones and vascular plants were covered by diatoms; Melosira moniliformis and M. lineata dominated in the discharge channel, and Berkeleya rutilans in the middle and outer parts of the discharge area. The maximum biomass of Cladophora glomerata was recorded in the discharge channel in late April. In the ice-free area, the above-ground parts of Potamogeton pectinatus remained green over the winter, without any clear inactive phase. The above-ground biomass of a Myriophyllum spicatum stand in the vicinity of the discharge channel declined drastically in late autumn, but after the winter the stand regenerated.