A mussel bed was sampled monthly at four intertidal levels (mid: 2.15; mid-low: 1.65; low: 1.2 and sublittoral fringe: 0.7 m from chart datum) from July 1979 to July 1980 at Pointe-Mitis in the St. Lawrence estuary. A strong spring reduction of abundance (both in density and biomass) suggested that the mussel bed was being degraded. Community perturbation was attributed to ice scour. Partial reestablishment of the mussel bed (all age classes) was observed during late spring and early summer and occurred mainly at the mid-low intertidal level. Changes in the size structure of the mussel bed with level suggest that the annual windstorm regime may be an important factor in the dynamics of the bed. Mean body mass decreased at the three lower shore levels but increased at the highest shore level. Overall, net secondary production (assessed by the increment-summation method) was negative due to the decrease in mean body mass. Annual production rates (kJ m-2 y-1) from the mid intertidal level to the sublittoral fringe were 1130, - 4072, - 4013 and - 3258, respectively, while P/B ratios (y-1) were 0.17, - 0.69, - 0.50 and - 0.45. The calculated production and the productivity (potential production) are compared and used to provide insight into the condition of the mussel bed.
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