Effects of thermal effluents from the Bergum power station on the phytoplankton in the Bergumermeer

Summary The Bergum power station (600 MW) of the Friesian Provincial Electricity Board is situated at the northern shore of the Lake Bergum. The lake has a mean depth of 1.3 m and a surface area of 4.4 km². Its northern half is separated by a break-water into an intake area in the north-west and a discharge area in the north-east.The Lake Bergum is connected with other water bodies in the northern provinces of the Netherlands by four canals. The whole yaer various amounts of water enter Lake Bergum mainly from the western canal (Prinses Margrietkanaal) and to a lesser extent from the southern canal (De Lits). In wet seasons lake water flows off, mainly after passing the power station, to the northern canal (De Zwemmer); then the heated water (22 m³.sec_–1) does not enter the discharge area of the lake. When evapo-transpiration exceeds precipitation lake water flows off mainly to the eastern canal (Kolonelsdiep). In these relatively dry periods most of the heated water returns to the lake in the discharge area.We found that the mean increase in water temperature effected by the condensors of the power station was ca. 5°C; the maximum increase was 7.5°C. On average about 25% of the whole lake had a noticable higher (1°C) temperature than the intake water, only 6.5% was about 2°C above ambient temperatures.For about 3.5 years (1974–Sept. 1978) water samples for analysis of the chlorophyll concentrations of the different areas within the lake and the surrounding canals were taken every week during the growing season, and fortnightly during the winter period. The chlorophyll concentrations of the intake water were about 5% higher than those of the discharge water leaving the power station. Near the mouth of the northern canal in the discharge area still small, but significant lower chlorophyll concentrations were found. The southern half of the lake, in which practically no elevated water temperatures were found, had significant higher chlorophyll concentrations (10–15%) than the intake area. Water entering the lake from the western canal had significant (10–15%) lower chlorophyll concentrations than the intake area of the lake. Probably, relatively chlorophyll-poor canal water and chlorophyll-rich water from the southern lake area mix in the intake area. While the water passes the power station the chlorophyll concentrations decrease. In the discharge area of the lake the chlorophyll concentrations of the discharge water gradually increase again to values equal to those of the intake area.During the last 2 years of the research period oxygen production and consumption experiments were conducted almost every month. In each experiment light and dark botties containing intake and discharge water were suspended in water with both water temperatures. The light intensities during the incubation periods (2–3 hours) were chosen according to maximum production values. The incubations were started within one hour and/or one day after sampling. Directly after sampling gross productivity of the intake water incubated at discharge temperatures was about 1.5 times as high as at intake temperatures. The gross productivity of the discharge water was always somewhat lower than the gross productivity of the intake water incubated at corresponding temperatures. After one day this inhibiting effect of passage through the power station had increased, even when the discharge water had been cooled down to intake temperatures immediately after sampling.The oxygen consumption of the discharge water incubated at discharge temperatures as well as at intake temperatures was about 1.3 times the oxygen consumption of the intake water at intake temperatures. After one day the discharge water, which had stayed at discharge temperatures, consumed 1.6–1.7 times as much as the intake water incubated at intake temperatures. The oxygen consumption of the discharge water which had been cooled down to intake temperatures directly after sampling, was after one day still 1.3 times the oxygen consumption of the intake water at intake temperatures.This research was financially supported by the Ministerie van Volksgezondheld en Millieuhygiëne (Ministry of Public Health and the Environment). An extensive report (in Dutch) will be published this year.