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In estuaries, the intensity of vertical mixing due to tides varies over a broad range of time scales. In the St. Lawrence Estuary, Canada, increased stability of the water column has 2 effects on the phytoplankton: (1) a physiological effect in the form of a light-dependent increase in the photosynthetic capacity; (2) a population response in the form of an increase in chlrophyll a variability. Phytoplankton cells must be able to adjust their metabolic activities to highly fluctuating environmental conditions. Previous experimental studies demonstrate that they can adjust rapidly to changing light intensities through variations in chlorophyll content, thus allowing utilization of available light with maximum efficiency (Owens et al., 1978; Riper et al., 1979; Falkowski, 1980; Harris, 1980). If physiological adjustment by the phytoplankton is faster than changes in environmental conditions, the cells continuously adjust their metabolic activities to the new conditions (Vincent, 1980); if not, the cells can only adjust to mean environmental conditions (Savidge, 1979; Falkowski, 1980). An example of such a physiological adjustment is that of the photosynthetic capacity of phytoplankton which should respond to fluctuating environmental conditions, since it is influenced by light conditions to which the cells were previously exposed (Prezelin et al., 1977; Prezelin and Sweeney, 1977, 1978). In estuaries, the intensity of vertical mixing due to the tides varies over a broad range of frequencies: monthly (M,), fortnightly (Mf) and semidiurnal (M2) tidal harmonics, and slack waters. Vertical mixing not only affects nutrient and oxygen redistribution in the ' Contribution to the program of GIROQ (Groupe interuniversitaire de recherches ocCanographiques du QuBbec) " Centre Champlain des Sciences de la mer, Sciences et lev6s ockaniques, C.P. 15 500, 901 Cap Diamant, Qukbec, Canada G1K 7Y7 O Inter-Research/Printed in F. R. Germany water column (Webb and D'Elia, 1980), but also the distribution of plankton organisms in the mixed layer. Since tidally-induced vertical mixing is highly prevalent in estuaries, they provide a unique experimental opportunity to study the effects of vertical mixing on phytoplankton. In the St. Lawrence Estuary, Quebec, a research program (ECOVARIATE) was conducted on estuarine variability between 1975 and 1977. Results on phytoplankton were pubhshed by Fortier et al. (1978), Frechette and Legendre (1978), Demers et al. (1979), Demers and Legendre (1979), Fortier and Legendre (1979), Lafleur et al. (1979), Roy and Legendre (1980), Demers and Legendre (1981), and Frechette and Legendre (in press). In the present study, the influence of varying degrees of vertical mixing on estuarine phytoplankton is examined, both at individual (physiological) and population (distributional) levels, over a 2-month period. During summer 1975, sampling was conducted at an anchor station (1 m depth) in the Middle St. Lawrence Estuary (the portion between the Saguenay Fjord and Quebec City). This station was occupied during 14 to 18 June, 5 to 12 July, and 1 to 8 August. Hourly measurements were made of chlorophyll a concentration per volume (B), photosynthetic capacity (under saturating light intensity of 375 ~ E i n m-2 S' , in an Hawaiian-type incubator: Doty and Oguri, 1959) normalized for chlorophyll a (E,,), and the vertical density structure of the water column. For more informations about the Methods, see Demers and Legendre (1979, 1981). The difference between the densities at 5 m and 35 m depths (Aa,) was used as an index of the vertical stability, in order to include the whole mixed layer. Under conditions of intense vertical mixing (spring tides), Demers and Legendre (1979) have found circadian variations in the photosynthetic capacity of Mar. Ecol. Prog. Ser. 7: 337-340, 1982 St. Lawrence Estuary phytoplankton. The intense vertical mixing causes all the cells in the mixed layer to experience similar average light conditions (Demers and Legendre, 1981). In order to eliminate these 24-h variations from the long-term trend studied here, the photosynthetic capacity was integrated over periods of 24 h. In addition, mean values of Ao, over 24 h were also computed. A linear increase of daily photosynthetic capacity was found with increasing mean daily density gradient (Fig. 1A). T h ~ s linear relationship is independent of the sampling dates, since the various conditions of stability were encountered at different times during the 2month sampling period. There is a n apparent linear relationship of both chlorophyll a concentrations and photosynthetic capacity with the density gradient (Table 1). However, when partial correlations are comTable 1. Coefficients of linear correlation (upper right, v = 13) and of partial correlation (lower left, v = 12) between daily photosynthetic capacity, daily mean chlorophyll a concentrations, and daily mean density gradient (index of vertical stability of the water column)

Water Column Stability and Photosynthetic Capacity of Estuarine Phytoplankton: Long-Term Relationships