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Resumen:
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In 1972-1982 phytoplankton was studied by the methods of Lorenzen and Utermöhl in the sea area that since 1978 has received cooling water from the Olkiluoto nuclear power station. Throughout the study period, the species composition was typical of oligotrophic coastal waters of the Bothnian Sea. The diatom Chaetoceros wighamii usually predominated in spring and flagellates prevailed in summer. The chlorophyll a concentration and phytoplankton biomass (fresh weight) increased in the late seventies and early eighties, due to the general increase of nutrients in the Baltic Sea. The thermal discharges advanced the spring biomass peak in the discharge area by $\le2$ weeks, but did not otherwise change the seasonal rhythm. The discharges seemed to slightly increase the abundance of the diatom Rhizosolenia minima, and also that of diatoms derived from the benthos. However, the temperature rise did not markedly affect the total phytoplankton quantities in the discharge area. Nutrients and salinity were the most significant factors explaining changes in the chlorophyll a concentration and total biomass.
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