Some aspects of the biology of Lake Trawsfynydd; a Power Station cooling pond

Abstract

1. The geography and climate of N. Wales in the neighbourhood of Lake Trawsfynydd is described. 2. The main inflows to the lake and the lake water contain few dissolved salts, have an acid pH and are stained brown with humic acids. 3. The lake has a surface area of 5.31 × 106 m2, a volume of 27.6 × 106 m3 and a mean depth of 5.2 m. 4. Hydroelectric drawdown causes the lake level to fluctuate ± 1.5 m about the mean level. These fluctuations are the most probable cause of the absence of marginal vegetation and the dearth of littoral fauna, but do not appear to cause any changes in the zooplankton. 5. The flow rates round the lake caused by power station operation are shown to be too low to affect the distribution of the zooplankton, but help in maintaining a fairly even horizontal distribution of phytoplankton. 6. No permanent thermal stratification developed during the summer either before or after the power station started to discharge warm water. 7. The water temperature was increased by 6–7°C in summer and 10–12°C in winter causing a horizontal temperature gradient in the lake with an annual average difference of 5°C between the warm and cold sides of the lake. 8. Ten species of crustacea were found in the zooplankton samples of which only six are true members of the limnoplankton. The same species were found in both the warm lagoons and the cool main lake. 9. The cycles of abundance and the numbers of Bosmina obtusirostris and Daphnia longispina were similar in both the warm and cold parts of the lake. 10. The numbers of Diaphanosoma brachyurum and Ceriodaphnia quadrangula were larger in the cool main lake than in the warm lagoons, but the timing of the period of maximum population density was the same in both parts of the lake. 11. There were two main generations per year in the life cycle of Cyclops abyssorum in the warm and cold sides of the lake. 12. No changes in the composition, abundance or in the timing of the periods of increase and decrease in the zooplankton have been recorded which can be directly attributed to the discharge of warmed water from the power station into the lake at Trawsfynydd.