Distribution spatio-temporelle du phytoplancton et du zooplancton dans le lac Saint-Jean (Québec), un réservoir hydroélectrique Spatio-temporal distribution of phytoplankton and zooplankton in lake Saint-Jean (Quebec), a hydroelectric reservoir

Résumé

Le Lac Saint-Jean est un réservoir hydroélectrique ayant une superficie de 1 053 km² et une profondeur moyenne de 6 m. Ses eaux se renouvellent de quatre à cinq fois par année, dont deux fois durant la saison printanière. Entre juin et octobre, l'épaisseur de la couche photique du lac est environ de 4 m et la thermocline est presque toujours plus profonde que 20 m avec une température moyenne saisonnière de 16°C. Les éléments nutritifs (phosphates et nitrates) demeurent à un niveau très bas durant toute la saison. Le taux moyen de chlorophylle a est de 1,3 mg m^-3 avec des valeurs maximales de 3 à 4 mg m^-3. Le coefficient de corrélation entre la chl. a et la température, pour l'ensemble des données du lac, est de 0,56 (p<0,01).

La communauté phytoplanctonique (>20 µm) compte environ vingt espèces mais la dominance est assurée seulement par les Diatomées Asterionella formosa et Tabellaria flocculosa. Ces deux espèces représentent plus de 90 % de tous les organismes entre juin et octobre. D'autres espèces comme Melosira (Aulacoseira) distans, M. islandica et M. ambigua ne sont importantes qu'au début de juin tandis qu'Anabaena flos-aquae ne l'est qu'à la mi-juillet. La communauté zooplanctonique regroupe 17 espèces réparties en 4 catégories : Copépodes, Cladocères, Rotifères et Protozoaires. À partir de la mi-juillet, le zooplancton est surtout dominé par le Rotifère Polyarthra vulgaris et par le Protozoaire Codonella cratera. Ayant des cycles vitaux relativement courts, ces deux espèces forment à elles seules entre 65 et 82% de tous les individus. Dans les 5 premiers mètres de la colonne d'eau, le coefficient de corrélation entre la densité de ces deux espèces est de 0,55, (p<0,01). La variabilité spatio-temporelle des biomasses et la distribution des organismes planctoniques du lac sont influencées par l'hydrodynamique de l'écosystème, particulièrement par les variations saisonnières du débit qui influent sur les nutriments, la température et également par le brassage intense qui maintient la thermocline à un niveau au-dessous de 20 m.

Mots-clés:

• Lac Saint-Jean,
• Réservoir hydroélectrique,
• Phytoplancton,
• Zooplancton

Abstract

The lake Saint-Jean is well known to fishermen and renowed for its freshwater salmon (Salmo salar). Of its 21 tributaries, three provide about 75% of its water; they are the Peribonka, the Mistassini and the Ashuapmushuan rivers. Although it is the fifth largest lake in the province of Quebec, its water is renewed four to five times a year; about a quarter of its 1 053 km² area is only 3 m deep, with a maximum depth of 63 m in the middle. This kind of shallow lake is very sensitive to winds.

The present study looked at the spatial and temporal variability of phytoplankton and zooplankton; ten stations were distributed over the whole lake (Figure 1 and Table 1) and water samples were taken at different depths (1, 2, 5, 10, 15 and 20 m when possible) from June to October 1997. Several physicochemical parameters were measured; phytoplankton and zooplankton samples were collected as well and the main species were identified and counted. The mean surface water temperature was 16° C compared to the average of the 1, 2 and 5 m data of 15°C and the deep layer (10, 15 and 20 m) of 13°C; from mid-August to mid-October, the water temperature is the same at all depths because of very good mixing due to the strong winds. Referring to the same layers, the mean values of chlorophyll a for the study period were 1.6, 1.3 and 0.6 mg m-3. For the early June sampling, values of chlorophyll a varied from 0.6 to 1.9 mg m?3. As for July and August, values between 3 and 4 mg m^-3 were recorded; the overall mean of chlorophyll a was 1.3 mg m^-3. The correlation coefficient between chlorophyll a and the temperature, for all data, was 0.56 (p<0.01). As stated before, a very good mixing of the upper water layer is encountered, bringing the phytoplankton to depths below the photic layer evaluated at about 4 m; such conditions of light limitation do not foster a good photosynthetic activity.

Nitrates and orthophosphates were always low during the study, respectively lower than 0.8 mg L^-1 and between 0.1 and 0.2 mg L^-1. Linear or exponential regression (Figure 5) between the sum of nitrites and nitrates and chlorophyll a gave correlation coefficients of -0.48 or -0.52 (p<0.01), suggesting a decrease in inorganic nitrogen caused by photosynthetic activity. Of the 26 species of phytoplankton identified with dimensions above 20 µm (Table 2), only two dominated during the study period; these were the diatoms Asterionella formosa and Tabellaria flocculosa with an occurrence between 72 and 87% for the former and between 11 and 24% for the latter. The geographical variability is presented (Figure 6A) along with the seasonal and vertical variability. Such distributions likely result from the combination of the inflow from the Peribonka river, the Coriolis effect and the outflow to the Grande-Decharge river.

The main zooplankton pesent in the lake have a short life cycle; two of the 17 species identified were encountered most of the time, the rotifer Polyarthra vulgaris and the protozoan Codonella cratera accounting for 10 to 29% and 36 to 72% of those species identified (July to October). The correlation coefficient between these two species was 0.55 (p<0.01) in the first 5 meters of the water column (Figure 8). The reciprocal diversity index (Simpson) varied between 0.63 and 0.97 for the zooplankton community, reflecting the dominance of only few species; such an index was not calculated for the phytoplankton because the nanoplankton (<20 µm) was not sampled and is often very important in such systems. However, the dominance of the protozoae and the rotifera, organisms with rather short life cycles (few hours to few days), might have some relation to the hydrodynamics of the ecosystem.

With a thermocline always deeper than the photic layer, the water column is mixed intensively and the phytoplankton is often at depth with little or no light for photosynthetic activity. Based on the low nitrate and chlorophyll a levels, we consider the lake Saint-Jean as oligotrophic. This project was the first systematic study of planktonic populations in the lake and was just a part of a larger study about the distribution and feeding of rainbow smelt (Osmerus mordax) larvae. This will be one of the main steps to understand trophic relationships in this ecosystem famous for sport fishing of landlocked Atlantic salmon (Salmo salar).

Keywords:

• Lake Saint-Jean,
• Hydroelectric reservoir,
• Phytoplankton,
• Zooplankton