Plankton, Chemistry, and Physics of Lakes in the Churchill Falls Region of Labrador

Abstract

Of 10 lakes and a reservoir in the Churchill Falls region of western Labrador studied in 1970–71, nine lakes were 12 m or less deep, humic, thermally unstratified, and had Secchi disc transparencies between 3.5 and 8.25 m. Maximum surface temperatures of 16–17 C in the nine lakes were generally reached by mid-August. Michikamau Lake (area 1980 km2, depth 84 m) rarely exceeded 12 C and had a Secchi disc transparency of 13 m. Conductivities and alkalinities (as CaCO3) ranged from about 13 μmho/cm and less than 6.5 mg/liter, for lakes draining granite, gneiss, or quartz bedrock, to about 30 and 15, for those draining gabbro or slate. Maximum live phytoplankton biomasses in 1970 ranged from 70 mg/m3, in Michikamau Lake, to 739 mg/m3, in Ossokmanuan Reservoir, and, in general, there was a positive correlation with conductivity. Diatoms were the most abundant group, in particular Asterionella formosa, Tabellaria fenestrata, and Rhizosolenia eriensis; however, flagellated Chrysophyta, mainly species of Dinobryon, were more abundant in the reservoir than in the natural lakes. Predominant zooplankters included Diaptomus minutus, Cyclops scutifer, Holopedium gibberum, Daphnia longiremis, D. galeata mendotae, and Bosmina coregoni. Maximum seasonal standing crops ranged from 1600 to 8910 individuals/m3, with the highest densities being found in the reservoir. The chemical and biological data suggested that the reservoir 8 yr after filling, showed little evidence of trophic upsurge and was possibly entering a period of trophic depression.