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Mussels secrete byssal threads regularly to the substratum, and the strength of these threads can fluctuate with time. The present study examined weekly variations in the attachment strength of 2 yr old cultured mussels Mytilus edulis on submerged longlines in a semi-enclosed lagoon from late May to mid-October. Some possible factors influencing attachment strength were investigated: environmental factors (temperature, food availability, wind velocity and hydrodynamic conditions such as current velocity, turbulence and wave height) and reproductive condition were measured concurrently. Attachment strength was measured directly on cultured mussels using a dynamometer. Attachment strength varied 2-fold from summer to fall, a difference related not only to the number of byssal threads but also to their individual strengths. The hierarchical influence of each factor on attachment strength is discussed. Our results suggest that spawning seemed to be correlated with an important decrease (-32%) in attachment strength, and that water temperature (negative relationship) and turbulence (positive relationship) were the most important factors explaining the variation in mussel attachment strength. In contrast to previous studies carried out in intertidal zones, no trade-offs were observed between reproduction and attachment strength.

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The freshwater discharge at the head of the St. Lawrence Estuary near Québec city in eastern Canada, was monitored at monthly timescales from 1955 to 1988 by the Quebec Department of Environment and Fauna (DEF). Since 1988, these estimates have been discontinued. Using the 1962discontinued. Using the -1988 data, two models are developed to estimate the freshwater discharge at the head of the Estuary. The first is a regression model which estimates the discharge at monthly timescales using sea level data available at Neuville near Québec city. A second order polynomial fits the data with a correlation coefficient of R = 0.93. The second model is a one-dimensional numerical model which estimates the hourly discharge using hourly sea level data available at Neuville and at Lauzon as upstream and downstream boundary conditions respectively. The numerical model is calibrated with current measurements for the tidal variability, and with the DEF's estimates for monthly means. The linear least-squares fits between the monthly-averaged numerical model's results and the DEF's estimates is R = 0.91. Although slightly lower than the regression model for monthly means, the numerical model covers a much wider range of timescales, and can be used in real-time. Analysis of long term hourly river discharge shows that the fortnightly variation of river discharge represents roughly 7% of the annual mean. ||||| des moindres carrés, entre les estimés du MEF et ceux effectués avec l'aide du modèle numérique est de R = 0,91. Bien que légèrement inférieur pour les moyennes mensuelles que les estimations du modèle régressif, le modèle numérique permet de couvrir une plus grande variabilité d'échelle temporelle, et peut être utilisé en temps réel. L'analyse à long terme du débit fluvial horaire montre que la variation bimensuelle représente approximativement 7 % de la moyenne annuelle.

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