Spatio-temporal distribution of phytoplankton and zooplankton in lake Saint-Jean (Quebec), a hydroelectric reservoir

The lake Saint-Jean is a hydroelectric reservoir with an area of 1053 km2 and a mean depth of 6 m. Its rate of water renewal varies from four to five times per year, of which, two renewals happen during the spring season. From June to October, the photic layer of the lake is about 4 m and the thermocline is always deeper than 20 m. The mean surface water temperature is 16°C. Nutrient phosphate and nitrate levels are very low during the entire season. The mean concentration or chlorophyll a is 1.3 mg·m-3 with maxima of 3 to 4 mg·m-3. The correlation coefficient between Chl. a and the temperature, for all data, is 0.56 (p 20 μm) is composed of about twenty species but it is dominated by the diatoms Asterionella formosa and Tabellaria flocculosa. These two species account for over 90% of all phytoplankton during the study period (from June to October). Other species such as Melosira (Aulacoseira) distans, M. islandica, M. ambigua (at the beginning of June) and Anabaena flos aquae (by mid-July) were relatively important. The zooplankton is composed of 17 species grouped in 4 categories: Copepods, Cladocera, Rotifera and Protozoa. From mid-July, zooplankton is dominated by Polyarthra vulgaris and Codonella cratera; with a short life cycle, these two species form between 65 and 82% of all zooplanktonic individuals. In the first 5 m of the water column, the correlation coefficient between these two species, for all data, is about 0.55 (p