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Resumen:
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The effect of thermal discharges on the macrophytes was studied by SCUBA diving and dredging along permanent transects in the discharge area of the Olkiluoto power station in 1975, 1977, 1979 and 1982. Changes in the vegetation were examined with the aid of rank, heterogeneity and similarity values, based on coverages and the species composition. Since the two units of the power station came into use in 1978 and 1980, the main factors affecting the vegetation close to the cooling water outlet have been the temperature increase, the physical stress caused by the strongly flowing water, and a local nutrient supply. In the outer parts of the discharge area the most important factor seems to be the prolongation of the growing season resulting from the absence of ice in winter. Marked changes in the phytobenthos were recorded in the immediate vicinity of the cooling water outlet: Myriophyllum spicatum had formed a dense stand and most of the earlier species (e.g. charophytes) had disappeared. Other species tolerant of the strong warm water flow were Cladophora glomerata, Enteromorpha ahlneriana, Ectocarpus siliculosus and Potamogeton pectinatus. On some transects, Ruppia cirrhosa and Ceramium tenuicorne had increased. The total plant cover tended to increase and the heterogeneity along the depth gradient to decrease in the vegetation of the discharge area. On the whole, the zonation of the vegetation was weak during the study period.
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